Digestive tube organs
1. General features of digestive system.
2. Oral cavity. Lips, cheeks, soft and dark palatine, their structure and functions.
3. The tongue, tissues compounds, structural peculiarities of the upper, lower and back surfaces.
4. Morphofunctional characteristic of the tongue papillae.
5. Taste bud structure and functions.
6. Sources of origin, structure and tissues compounds of the tooth.
7. Enamel, dentin and cementum hystologic structure and composition.
8. Palp and periodontal ligament structure and functions.
9. Tooth development. Permanent and deciduous teeth.
10. General morphofunctional characteristic of large salivary glands and their classification.
11. Structural peculiarities of the parotid, submandibular and sublingual glands secretory portions (acini).
12. Main microscopic and ultrastructural signs of muco– and serocytes.
13. Large salivary glands excretory ducts.
14. Salivary glands excretory products and hormones.
15. Morphogenesis and regeneration of the salivary glands.
16. Salivary glands aging.
17. General structure of the digestive tube. Pharynx wall histologic structure.
18. Oesophageal mucosa and submucosa structure.
19. Oesophageal glands disposition, structure and functions.
20. Peculiarities of the muscular tunica in the different part of oesophagus.
21. Lymph-epithelial rink of Pirogov-Valdaer compounds and functions.
22. Structure and functions of the palatine tonsil.
23. General features of stomach structure: portions and layers.
24. Stomach mucosa peculiaruties.
25. Stomach glands types,disposition and cell compounds.
26. Proper glands of the stomach: cells, functions.
27. Cardial and pyloric glands of the stomach: cells, functions.
28. Stomach muscular and serous tunics.
29. Development and tissues compounds of the small intestine wall.
30. Peculiarities of the small intestine relief. System villus-crypt.
31. Morphofunctional characteristic of the simple columnar brushed epithelium of the villi and crypts.
32. Ultrastructure and functions of different enterocytes.
33. Small intestine submucosa. Duodenal glands structure and functions.
34. Lymphoid follicles in small intestine, disposition and functions.
35. Muscular and external tunics of the small intestine.
36. Absorption histophysiology in small intestine.
37. The sources of the digestive tube middle and posterior portion embryonic development.
38. Large intestine anatomical portion and wall tunics structure.
39. Appendix structure and functions.
40. Rectum portions and functional peculiarities.
41. Histophysiology of the large intestine
About the Digestive System
Almost all animals have a tube-type digestive system in which food enters the mouth, passes through a long tube, and exits as feces (poop) through the anus. The smooth muscle in the walls of the tube-shaped digestive organs rhythmically and efficiently moves the food through the system, where it is broken down into tiny absorbable atoms and molecules.
During the process of absorption, nutrients that come from the food (including carbohydrates, proteins, fats, vitamins, and minerals) pass through channels in the intestinal wall and into the bloodstream. The blood works to distribute these nutrients to the rest of the body. The waste parts of food that the body can’t use are passed out of the body as feces.
Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.
How Digestion Works
The digestive system is made up of the alimentary canal (also called the digestive tract) and the other abdominal organs that play a part in digestion, such as the liver and pancreas. The alimentary canal is the long tube of organs — including the esophagus, stomach, and intestines — that runs from the mouth to the anus. An adult’s digestive tract is about
Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty meal, our salivary glands, which are located under the tongue and near the lower jaw, begin producing saliva. This flow of saliva is set in motion by a brain reflex that’s triggered when we sense food or think about eating. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.
As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called amylase, which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.
Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. The pharynx, a passageway for food and air, is about
From the throat, food travels down a muscular tube in the chest called the esophagus. Waves of muscle contractions called peristalsis force food down through the esophagus to the stomach. A persoormally isn’t aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.
At the end of the esophagus, a muscular ring or valve called a sphincter allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with acids and enzymes, breaking it into much smaller, digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about
Most substances in the food we eat need further digestion and must travel into the intestine before being absorbed. When it’s empty, an adult’s stomach has a volume of one fifth of a cup (
By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme. A walnut-sized muscular valve at the outlet of the stomach called the pylorus keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.
The small intestine is made up of three parts:
1. the duodenum, the C-shaped first part
2. the jejunum, the coiled midsection
3. the ileum, the final section that leads into the large intestine
The inner wall of the small intestine is covered with millions of microscopic, finger-like projections called villi. The villi are the vehicles through which nutrients can be absorbed into the body.
The liver (located under the rib cage in the right upper part of the abdomen), the gallbladder (hidden just below the liver), and the pancreas (beneath the stomach) are not part of the alimentary canal, but these organs are essential to digestion.
The liver produces bile, which helps the body absorb fat. Bile is stored in the gallbladder until it is needed. The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also makes a substance that neutralizes stomach acid. These enzymes and bile travel through special channels (called ducts) directly into the small intestine, where they help to break down food. The liver also plays a major role in the handling and processing of nutrients, which are carried to the liver in the blood from the small intestine.
From the small intestine, undigested food (and some water) travels to the large intestine through a muscular ring or valve that prevents food from returning to the small intestine. By the time food reaches the large intestine, the work of absorbing nutrients is nearly finished. The large intestine’s main function is to remove water from the undigested matter and form solid waste that can be excreted. The large intestine is made up of these three parts:
1. The cecum is a pouch at the beginning of the large intestine that joins the small intestine to the large intestine. This transition area expands in diameter, allowing food to travel from the small intestine to the large. The appendix, a small, hollow, finger-like pouch, hangs at the end of the cecum. Doctors believe the appendix is left over from a previous time in human evolution. It no longer appears to be useful to the digestive process.
2. The colon extends from the cecum up the right side of the abdomen, across the upper abdomen, and then down the left side of the abdomen, finally connecting to the rectum. The colon has three parts: the ascending colon; the transverse colon, which absorb fluids and salts; and the descending colon, which holds the resulting waste. Bacteria in the colon help to digest the remaining food products.
3. The rectum is where feces are stored until they leave the digestive system through the anus as a bowel movement.
The digestive system consists of the digestive tract and its associated glands. Its functions are to obtain from ingested food the metabolites necessary for the growth and energy needs of the body. Before stored or used as energy, food is degested and transformed into small molecules that can be easily absorbed through the lining of the digestive tract. However, a barrier between the environment and the internal milieu of the body must be maintained. The first step in the comlex process known as digestion occurs in the mouth, where food is ground into smaller pieces by mastification and moistened by saliva, which also initiates the digestion of carbohydrates. Digestion continues in the stomach and small intestine, the food – transformed into basic components (aminoacids, monosaccharides, glycerides, etc) – is absorbed. Water absorption occurs in the large intestine, and as a consequence the undisgested contents become semisolid.
The digestive process commences in the oral cavity with the ingestion, fragmentation and moistening of food but, in addition to its digestive role, the oral cavity is involved in speech, facial expression, sensory reception and breathing. The major structures of the oral cavity, the lips, teeth, tongue, oral mucosa and the associated salivary glands, participate in all these functions. Mastication is the process by which ingested food 1% made suitable for swallowing. Chewing not only involves coordinated movements of the mandible and the cutting and granding action of the teeth but also activity of the lips and tongue, which continually redirect food between the occlusal surfaces of the teeth. The watery component of saliva moistens and lubricates the masticatory process whilst salivary mucus helps to bind the food bolus ready for swallowing. The entire oral cavity is lined by a protective mucous membrane, the oral mucosa, which contains many sensory receptors, including the taste receptors of the tongue.
The epithelium of the oral mucosa is of the stratified squamous type which tends to be keratinised in areas subject to considerable friction such as the palate. The oral epithelium is supported by dense collagenous tissue, the lamina propria. The roof of the mouth consists of the hard and soft palates, both covered with the same type of stratified epithelium. In highly mobile areas such as the soft palate and floor of the mouth, the lamina propria is connected to the underlying muscle by loose submucosal supporting tissue. In contrast, in areas where the oral mucosa overlies bone, such as the hard palate and tooth-bearing ridges, the lamina propria is tightly bound to the periosteum by a relatively thin dense fibrous submucosa. Throughout the oral mucosa numerous small accessory salivary glands of both serous and mucous types are distributed in the submucosa.
The palatine uvula is a small conical process that extends downward from the center of the lower border of the soft palate. It has a core of muscle and areolar connective tissue covered by typical oral mucosa.\
Mouth (An Overview)
The function of the mouth and its associated structures is to form a receptacle for food, to begin mechanical digestion through chewing (mastication), to swallow food, and to form words in speech. It can also assist the respiratory system in the passage of air.
LIPS. The external surface of the lip is covered by hairy skin which passes through a transition zone to merge with the oral mucosa of the inner surface. The transition zone constitutes the free vermilion border of the lip, and derives its colour from the richly vascular dermis which here has only a thin, lightly keratinised epidermal covering. The free border is highly sensitive due to its rich sensory innervatlon. Sinse the vermilion border is devoid of sweat and sebaceous glands, in requires continuous moistening by saliva to prevent cracking. The oral mucosa covering the inner surface of the lip has a thick stratified sguamous epithelium and the underlying submucosa contains numerous accessory salivary glands of serous, mucous and mixed sero-mucous types.
The Tooth
A tooth is a hard structure, set in the upper or lower jaw, that is used for chewing food. Teeth also give shape to the face and aid in the process of speaking clearly. The enamel that covers the crown (the part above the gum) in each tooth can be broken down by acids produced by the mouth for digestive purposes. This process is called “decay”. To prevent decay, good oral hygiene, consisting of daily brushing and flossing, is necessary. The hardest substance in the human body is one of the four kinds of tissue which make up the tooth. It is enamel and covers the crown (area above the gum line) of the tooth. A bony material called “cementum” covers the root, which fits into the jaw socket and is joined to it with membranes. “Dentin” is found under the enamel and the cementum, and this material forms the largest part of the tooth. At the heart of each tooth is living “pulp,” which contains nerves, connective tissues, blood vessels and lymphatics. When a person gets a toothache, the pulp is what hurts.
TEETH. Each tooth may be grossly divided into three segments, the crown, neck and the root: the crown is that portion which projects into the oral cavity and is protected by a layer of highly mineralised enamel which coversit entirely. The bulk of the tooth is made up of dentine, a mineralised tissue which has a similar chemical composition to bone. The dentine has a central pulp cavity containing the dental pulp which consists of specialised supporting tissue containing many sensory nerve fibers.
Neck of the tooth is its short part between the crown and neck.
The tooth root is embedded in a bony redge in the faw called the alveolar ridge: the tooth socket is known as the alveolus and, at the lip or cheek aspect of the alveolus, the bony plate is generally thinner than at the tongue or palatal aspect. The root of the tooth is invested by a thin layer of cementum which is connected to the bone of the socket by a thin fibrous layer called the periodontal ligament or periodontal membrane. The oral mucosa covering the upper part of the alveolar ridge is called the gingiva and at the junction of the crown and root protective cuff around the tooth. The potential space between the gingival cuff and the enamel of the crown is called the ginglval crevice. All of the tissues which surround and support the tooth are collectively known as the periodontiym.
The dentine which forms the bulk of the crown and root is composed of a calcified organic matrix similar to that of bone. The inorganic component (70 %) constitutes a somewhat larger proportion of the matrix of dentine than that of bone and exists mainly in the form of hydroxiapatite crystals. From the pulp cavity, minute parallel tubules, called dentine tubules, radiate to the periphery of the dentine: in longitudinal sections of teeth, the tubules appear to follow an S-shaped course.
The crown of the tooth is covered by enamel, an extremely hard, translucent substance composed of parallel rods or prisms of highly calcified material cemented together by an almost equally calcified interprismatic material. The root is invested by a thin layer of cementum which is generally thicker towards the apex of the root.
The morphological form of the tooth crown and roots varies considerably in different parts of the mouth; nevertheless the basic arrangement of the dental tissues is the same in all teeth. In humans, the primary (deciduous) dentition consists of 20 teeth comprising two incisors, one canine and two molars in each quadrant. These begin to be formed at the age of 6 weeks during fetal development and erupt between the ages of 8 and 30 months after birth. Between the ages of 6 and 12 years, the deciduous teeth are succeeded by permanent teeth, namely two incisors, one canine and two premolars in each arch. Distal to these will develop three permanent molars having no primary precursors: the first permanent molar erupts at age 6, the second at age 12 and the third (wisdom tooth) at age 17 to 21 years The points found on the posterior teeth are known as cusps.
Odontoblasts and dentine. Dentlne, the dense calcified tissue which forms the bulk of the tooth, is broadly similar to bone composition but is more highly mineralised and thus much harder than bone. The cells responsible for dentine formation, the odontoblasts, di ferentiateas a single layer of tall columnar cells on the surface of the dental papilla apposed to the ameloblast layer of the enamel organ. The odontoblasts initiate tooth formation by deposition af organic dentine matrix between the odontoblastic and ameloblastic layers: calclficatlon of this dentine matrix then induces enamel formation by ameloblasts. Dentine formation proceeds by continuing odontobiastic deposition of dentine matrix and its subsequent calcification: unlike ameloblasts, each odontoblast leaves behind a slender cytoplasmic extension, the odontoblastic process, within a fine dentlal tubule. When dentine formation is complete, the dentine is thus pervaded by parallel odontoblastic processes radiating from the odontoblast layer on the dentinal surface of the reduced dental papilla which now constitutes the dental pulp. After tooth formation is complete, a small amount of less organised secondary dentine continues to be laid down resulting in the progressive obliteration of the pulp cavity with advancing age. Parallel dentine tubules containing odontoblastic processes extend through a narrow pale-stained zone of uncalcified dentine matrix called predentine into the mature dentine. Underlying the odontoblastic layer, a relatively acellular layer, called the cell free zone of Weil ,gives way to the highly cellular dental pulp.
Enamel hardest material of the body, it is composed almost entirely of the mineral hydroxyapatite (Ca10(PO4)6(OH)2), which is arranged in highly packed hexagonal enamel rods or prisms about 4 mm in diameter, although some may measure up to 8 mm. each enamel rod extends through the full thickness of the enamel. The small interstices between adjacent rods are occupied by hydroxyapatite crystals. A small amount of organic matrix (protein and polysaccharide) represents the remnants of the matrix synthesized and excreted by the enamel-producing cell, the ameloblasts, and prior to mineralization of the enamel. Enamel covers the dentine only in the region of the exposed crown; in the root the dentine is covered by cementum.
Dental pulp. The dental pulp consits of a delicate supporting connective tissue resembling primitive mesenchyme. lt contains numerous stellate fibroblasts, reticuline fibres, flne poorly organised collagen fibres and much ground substance. The pulp contains a rich network of thin-walled capillaries supplied by arterioles which enter the pulp canal from the periodontal membrane, usually via one foramen at each root apex. The pulp is also richly innervated by a plexus of myelinated nerve fibres from which fine non-myellnated branches extend into the odontoblastic layer. Despite the acute sensitivity of dentine, nerve fibres are rarely demonstrable in dentine and the mechanism of sensory reception is unknown: it is suggested that the odontoblastic processes may act as sensory receptors.
CEMENTUM. The dentine, comprising the root, is covered by a thin layer of cementum which is elaborated by cells called cementocytes lying on the surface of the cementum. The cementum is an amorphous calcified tissue into which the fibers of the periodontal membrane are anchored. Fragments of alveolar bone have remained attached to the roots after extraction of these specimens.
Cementum consists of a dense, calcified organic material similar to the matrix of bone and is generally acellular. Towards the root apex the cementum layer becomes progressively thicker and irregular and cementocytes are often entrapped in lacunae within the cementum (cellular cementum). Cementocytes resemble osteocytes and remain viable throughout the life, being nourished through canaliculi which link the lacunae. They can become activated to produce new cementum when required.
In addition to cementocytes, which are scattered throughout the cellular cementum, there is a layer of cells called cementoblasts, which are similar to the actively synthetic osteoblasts of bone. Cementoblasts lie against the surface of the periodontal ligament and probably produce most new cementum by appositional deposition.
THE PERIODONTAL MEMBRANE forms a thin fibrous attachment between the tooth root and the alveolar bone; it consists of dense collagenous tissue. The collagen fibres, known as Sharpeys fibres, run obliquely downwards from their attachment in the alveolar bone to their anchorage in the cemehtum at a more apical position on the root surface. The periodontal membrane thus acts as a sling for the tooth within its socket, permitting slight movements which cushion the impact of masticatory forces. The points of attachment of the collagen fibres in both cementum and bone are in a contrast state of reorganisation to accomodate changing functional stresses upon the teeth. Osteoclastic resorption is often seen at one aspect of a tooth socket and complementary osteoblastic deposition of the tooth through the bone: this is the mechanism which permits tooth movement during orthodontic treatment. The periodontal membrane is richly supplied by blood vessels and nerves from the surrounding alveolar bone, the apical region and the gingiva. Small clumps of epithelial cells are often found scattered throughout the periodontal membrane: these cells are remnants of Hertwigs sheath and are known as epithelial rests of Malassez.
During tissue preparation the enamel has been completely dissolved from the surface of the crown, but the extent of the outer surface of the enamel, can be visualised by shreds of regaining organic debris which had been adherent to the tooth surface.
THE GINGIVA (GUM) may be divided into the attached gingiva, which provides a protective covering to the upper alveolar bone, and the free gingiva, which forms a cuff around the enamel at the neck of the tooth. Between the enamel and the free gingiva is a potential space, the ginglval crevice, which extends from the tip of the free gingiva to the cemento-enamel junction.
The thick stratified squamous epithelium, which constitutes the oral aspect of the gingiva, undergoes abrupt transition at the tip of the free gingiva to form a thin layer of epithelial cells tapering to only two or three cells thick at the base of the gingival crevice. This crevicular epithelium is easily breached by pathogenic organisms and the underlying supporting tissue is thus frequently infiltrated by lymphold cells. Collagen fibres of the periodontal membrane radiate from the cementum near the cemento-enamel junction into the dense supporting tissue of the free gingiva, these fibres, together with circular fibres surrounding the neck of the tooth, maintain the role of the gingiva as a protective cuff.
TOOTH DEVELOPMENT. The tissues of the teeth are derived from two embryological sources. The enamel is of epithelial (ectodermal) origin while the dentine, cementum, pulp and periodontal ligament are of mesenchymal (mesodermal) origin. The first evidence of tooth development in human occurs at 6 weeks of fetal life with the proliferation of a horseshoe-shape epithelial ridge from the basal layer of the primitive oral epithelium into the underlying mesoderm. In the position of the future jaws: this is known as the dental lamina. In each quadrant of the mouth, the lamina then develops four globular swellings which will become the enamel organs of the future deciduous central and lateral incisors, canines and first molar teeth.
Subsequently, the dental lamina proliferates backwards in each rich successively giving rise to the enamel organs of the future second deciduous molar and the three permanent molars. The permanent successors of the deciduous teeth will later develop from enamel organs which bud off from the inner aspect of the enamel organs of their deciduous predecessors. The primitive mesenchyme immediately subjacent to the developing enamel organ proliferates to form a cellular mass whilst, at the same time, the enamel organ becomes progressively cap-shaped, enveloping the mesenchymal mass which becomes known as the dental papilla.
During the cap stage, the cells lining the concave face ef the enamel organ in contract with the dental papilla begin to differentiate into tall columnar cells, the future ameloblasts, which will be responsible for the production of enamel. This, in turn, induces the differentiation of a layer of columnar odontoblasts, the future dentine-producing cells, in the apical region of the dental papilla. The interface between the differentiating ameloblast and odontoblast layers marks the position and shape of the future junction between enamel and dentine. As the enamel organ develops further,it assumes a characteristic bell shape, the free edge of the “bell” proliferating so as to determine the eventual shape of the tooth crown.
Meanwhile, the cells forming the main bulk of the enamel organ become large and star-shaped forming the stellate reticulum, the extracellular matrix of which is rich in glycosaminoglycans. Between the stellate reticulum and ameloblast layer two or three layers of flattened cells form the stratum intermedium whilst the outer surface of the enamel organ consists of a simple cuboidal epithelium called the external enamel epithelium. By the cap stage of development, the dental lamina connecting the enamel organ with the oral mucosa has become fragmented and, around the whole developing bud, a condensation of mesenchyme forms the dental follicle which will eventually become the periodontal ligament.
As ameloblasts and odontoblasts differentiate at the tip of the crown, a layer of dentine matrix is progressively laid down between the ameloblast and odontoblast layers. As the odontoblasts retreat, each leaves a long cytoplasmic extension, the odontoblastic process, embedded within the dentine matrix thereby forming the dentine tubules. Dentine matrix has a similar biochemical composition to that of bone and undergoes calcification in a similar fashion. Deposition of dentine induces the production of enamel by the adjacent aneloblasts. Each retreating ameloblast lays down a column of enamel matrix which then undergoes mineralisation resultingthe formation of a dense prismatic structure as described below.
With the deposition of dentine and enamel, the overlying stellate reticulum atrophies and the enamel organ is much reduced in thickness. A thin layer of dentine has been laid down by the underlying odontoblastic layer of the highly cellular dental papilla. The ameloblastic layer is about to lay down enamel in the area of the artefactual space: note that in this area the stellate reticulum has disappered. Note also the surrounding dental follicle and early formation of cancellous bone, by the time that dentine and enamel formation is well under way at the inclsual edge or tips of the cusps (as the case may be), the enamel organ will have fully outlined the shape of the whole tooth crown. A thin, densely-stained layer of poorly mineralised enamel can be seen covered at its external surface by the much thinner enamel orgaow. The unstained space between this and the underlying dentine represents fully mineralised enamel laid down earlier but dissolved away during tissue preparation.
Although enamel production is confined to the srown, the rim of the “bell” of the enamel orgaevertheless continues to proliferate, inducing dentine formation and thereby determining the shape of the tooth root. This part of the enamel organ, known as the epithelial sheath of Hertwig, disintegrates once the outline of the root is completed. The cementum which later is formed on the root surface is derived from the dental follicle which, as previosly stated, is of mesenchymal origin. As the dentine of the crown and root are progressively laid down, the dental papilla shrinks and eventually becomes the dental pulp contained within the pulp chamber and root canals. Growth of the tooth root is one of the principal mechanisms of tooth eruption and root formation is not completed until some time after the crown has fully erupted into the oral cavity.
Active ameloblasts are tall columnar epithelial cells which form a single layer apposed to the forming surface of the enamel. Each ameloblast elaborates a column of organic enamel matrix which undergoes progressive mineralisation by the deposition of calcium phosphate mainly in the form of hydroxliapatite crystals. Fully formed enamel contains less than 3% organic material and Ii the hardest and most dense tissue in the body.
The structure of mature enamel is not fully understood, but it appears that the process of mineralisation of the enamel matrix is not uniform and, as a result, mature enamel consists of highly calcified prisms separated by interprismatic material which may differ only in the orientation of its crystals. Each prism extends from the dentino-enamel junction to the enamel surface and may represent the enamel laid down by a single ameloblast. Underlying the ameloblast layer are several layers of cells, also of epithelial origin, which constitute the remainder of the enamel organ. As enamel formation progresses, the enamel organ becomes much reduced in thickness compared with earlier stages of its development. At tooth eruption, the enamel organ, including the ameloblasts, degenerates leaving the enamel exposed to the hostile oral environment, completely incapable of regeneration.
TONGUE. The tongue is a muscular organ covered by oral mucosa which is specialised for manipulating food, general sensory reception and the special sensory function of taste. A V-shaped groove, the sulcus terminalls, demarcates the anterior two-thirds of the upper tongue surface from the posterior one-third. The mucosa of the anterior two-thirds is formed into papillae of three types. The most numerous, the filiform papillae, appear as short “bristles” macroscopically. Among them are scattered the small red globular fungiform papillae. Twelve to twenty large circumvallate papillae form a row immediately anterior to the sulcus terminalis.
The body of the tonque consists of a mass interlacing bundles of skeletal muscle fibres which permit an extensive range of tonque movements. The mucous membrane covering the tonque is firmly bound to the underlying muscle by a dense, collagenous lamina propria which is continuous with the epimysium of the tonque muscle. Numerous small serous and mucous accessory salivary glands are scattered throughout the muscle and lamina propria of the tongue.
Filiform and fungiform papillae. Filiform papillae are the most numerous type and consist of a dense supporting tissue core and a heavily keratintsed surface projection. They are found all over the dorsum of the anterior two-thirds of the tongue. They are tall, narrow and pointed and keratinized, particularly at their tips. These papillae contaio identifiable taste buds. Fungiform papillae have a thion-keratinised epithelium and a richly vascularised supporting tissue core giving them a red appearance macroscopically amongst the much more numerous whitish filiform papillae.
Circumvallate papillae are the largest and least common type of papillae on the tongue. They are set into the tongue surface and encircled by a deep cleft. These papillae appear as flattened domes, the bases of which are depressed below the dorsal surface. Each circumvallate papilla is surrounded by a narrow, moat-like channel, in the epithelium of which are numerous taste buds. These taste buds are thought to detect bitter taste. Aggregations of serous glands, called von Ebners glands, open into the base of the circumvallate clefts, secreting a watery fluid which dissolves food constituents, thus facilitating taste reception.
The posterior third of the tongue is characterized by the presence of lymphoid tissue. Low-smooth dome-shaped elevations of the covering epithelium of the part of the tongue are due to lymphoid tissue (the lingual tonsillar tissue) in the submucosa. This lymphoid tissue is part of the mucosa-associated lymphoid tissue system (MALT –see further) protecting the oral portal of entry (with the palatine tonsils and the pharyngeal adenoids). Lymphocytes are numerous within the overlying non-keratinizing stratified squamous epithelium, which extends down into the lymphoid tissue as narrow clefts, becoming more prominent and numerous near the line of circumvallate papillae.
Skeletal muscle in the tongue is arranged in many directions. The fibers run in bands longitudinally, vertically, transversely and obliquely, with a variable amount of adipose tissue in between. In the bulkier, less mobile posterior third of the tongue the adipose tissue is more abundant.
This arrangement gives the tongue great mobility to manipulate food around the mouth for efficient fragmentation, and for moving fragmented food backward prior to swallowing; it also provides the fine control of tongue movement that is essential for speech.
SALIVARY GLANDS.
The mouth also contains the salivary glands which are accessory digestive glands that produce a fluid secretion called saliva. Saliva functions as a solvent in cleansing the teeth and dissolving food particles so that they may be tasted. Saliva also contains starch-digesting enzymes and mucus, which lubricates the pharynx to facilitate swallowing. There are three major pairs of salivary glands. The largest of which is the parotid gland and is located anteriorly and inferiorly to the ear between the skin and the muscle of chewing, the masseter. The parotid duct carries its contents and drains into the mouth. It is the parotid gland that becomes swollen and infected with the mumps or parotitis. The submandibular gland is located inferiorly to the mandible or jawbone midway along the inner side of the jaw. It has a muscular covering and empties its contents by way of the submandibular duct into the floor of the mouth on both sides. The sublingual gland, as its name implies, lies under the floor of the mouth and on the side of the tongue. Each sublingual gland possesses several small sublingual ducts that empty into the floor of the mouth in an area posterior to the submandibular duct.
Ducts: A branched system of different but connected ducts is only present in major salivary glands starting from the acini these ducts are: Intercalated (lined by simple cuboidal epithelium) Striated (lined by simple columnar epithelium)
Excretory (lined by pseudo-stratified columnar epithelium with goblet cells) Terminal excretory = main (lined by stratified squamous epithelium)
Myoepithelial cells Oncocytes
Stroma:
Interstitial Connective Tissue
Neural and vascular elements
Lymphocytic aggregates
Fine structural features (at the electron microscopic level)
-Functions of Saliva:
Protection (Buffering action, Mineral homeostasis in enamel)
Digestion
Defense
Taste
-Characteristics of major and minor salivary glands
–Classification:
Several criteria are used to classify salivary glands such as size, location, type of secretion (most commonly used), mode of secretion etc Some of these classifications are listed below:
1. Size:
a) Major glands are the large extra-orally located pairs of glands, namely parotid, submandibular and sublingual.
b) Minor glands are small aggregates of salivary glands associated with oral mucosa and distributed in several regions of the oral cavity.
2. Type of secretion:
a) Serous acini produce a watery secretion containing most of the salivary proteins.
b) Mucous acini produce viscous secretion rich in carbohydrates.
c) Mixed acini produce both types of secretion.
Further, aN ational Board part I Examination question addresses other classifications. For answering such question you need to remember that salivary glands are classified as exocrine, their secretion is released into ducts, merocrine, that is the acinar cells remain intact after their secretion is released, and compound, meaning that their duct system is branched.
Development of Salivary Glands:
Major salivary gland primordia appear in the embryo during the 6th -8th weeks. The primordium of the parotid gland appears at the beginning of the 6th week, that of the submandibular appears at the end of the 6th week and the sublingual primordium at the 7th-8th weeks. Primordia of the minor salivary glands appear after the third month in utero. Acini and ductal cells differentiate during the last 2 months of gestation and in general the glands continue to grow after birth up to 2 years.
All these primordia are derived from the ectodermal lining of the stomodeum and these epithelial strands develop into the parenchyma of the glands i.e. the acini and the ducts. The surrounding ectomesenchymal cells form the stroma, that is the connective tissue septa that divide the major glands into lobes and lobules. Also in the parotid and submandibular glands a connective tissue capsule surrounds the gland.
During the development of salivary glands, epithelial/mesenchymal interactions playa significant role in guiding the organization and the differentiation of different components of the gland. Fibroblast growth factors (FGFs) and transforming growth factor J3 (TGFJ3) are the major factors involved in sequential development of major salivary glands. Evidence indicates that ectomesenchyme is the determining factor in formation of the different parts of the epithelial parenchyma and deciding which will differentiate into acinar cells or various ductal cells. This role is similar to the one played by the dental papilla which determines the type of the developing tooth and whether it will be an incisor, canine, premolar or a molar.
Histological Features (light microscopic level):
I. Epithelial parenchyma: 1. Acini:
a) Serous acinar ceUs: These are pyramidal shaped cells with round nuclei located in the basal third of the cell, that is the part adjacent to the connective tissue surrounding the acinar cells but separated from them by a basement membrane. The cytoplasm appears basophilic in H&E sections reflecting the presence of abundant granular endoplasmic reticulum cisternae. The apical cytoplasm contains numerous secretory granules.
b) Mucous acinar cells: These pyramidal shaped cells have flattened angular nuclei which are located at the basal cell membrane. The cytoplasm appears lightly stained in H&E stained sections.
c) Mixed acini: consist of mucous acinar cells that line the lumen. At their outer periphery i. e. at their basal ends crescent-shaped caps of serous acinar cells are present which are called serous demilunes. Intercellular canaliculi delivers the serous secretion of these demilunes to the lumen of the mixed acinus since the canaliculi are actually extensions of the lumen.
2. Ducts: Saliva from minor glands are transported to the oral cavity via short ducts but the major glands have a branched duct system which starting at the acinar lumina includes:
a) Intercalated ducts: These are located adjacent to and continuous with the acinar lumina. This is the first order of ducts through which saliva passes and they are lined by simple cuboidal cells. b) Striated ducts: These ducts are larger and longer than intercalated ones and they are lined by simple columnar cells. The term striated is derived from the observation of striations in the basal cytoplasm. c) Excretory ducts: These are lined by pseudo stratified columnar epithelium with few goblet cells. These ducts are mainly located in the connective tissue septa between lobules. d) Main ducts: These are the ducts that open into the oral cavity and they are lined by stratified squamous epithelium.
ll. Stroma:
These are the connective tissue elements which surround and support the acini and ducts. In addition, the major salivary glands have connective tissue septa surrounding their lobes and lobules. Also the parotid and submandibular glands have connective tissue capsule around their outer margins. The stroma also contains the vascular, neural and lymphatic supply to the glands.
–Myoepithelial cells:
These are cells of epithelial origin that have contractile filaments in their cytoplasm and share some of the functional characteristics of smooth muscle cells. Myoepithelial cells have many processes that extend on the outer surface of acinar cells and intercalated ductal cells but they are located on the epithelial side of the basement membrane.
–Oncocytes:
These are cuboidal cells with acidophilic cytoplasm and round nuclei that are present between ductal cells. They seem to increase with age. Their significance is unknown.
Nerve and vascular supply:
Salivary glands are innervated by the autonomic nervous system, sympathetic and parasympathetic divisions. Neural elements form a network adjacent to ducts and acini and axons may form endings within this network. Also naked axons penetrate the basal lamina and end close to the acinar and ductal cell membranes. These axonal/epithelial relationships are referred to as subepithelial (extra- parenchymal) or intraepithelial (intra-parenchymal), respectively. A rich network ofblood vessels within the stroma supplies the avascular epithelial acini and ducts. The main arteries branch into arteioles which further form capillaries which are located in close proximity to acini and ducts, particularly striated ducts.
Fine Structural features (electron microscopic level):
Serous acinar cells: These are polarized cells with contents present in specific locations. The basal cytoplasm has around nucleus surrounded by numerous parallel cisternae of granular endoplasmic reticulum and a prominent Golgi complex as well as numerous mitochondria. The apical cytoplasm extending from the maturing (trans) face of the Golgi complex to the luminal membrane is occupied by diverse shaped secretory granules. These features characterize these cells as highly active in protein synthesis. A number of intercellular junctions are present between these acinar cells including tight junctions that separate acinar lumina from intercellular spaces. These junctions have selective permeability which regulates passages of ions, water and other molecules to the lumen.
Mucous acinar cells: These are also polarized cells similar to serous cells except that their nuclei are angular and are more basally located. Also their contents are quantitatively different from serous cells as they contain lesser number of granular endoplasmic reticulum cisternae and a more prominent Golgi complex which reflects their synthetic role in formation of carbohydrate rich components of saliva such as mucins. The apical cytoplasm is full of mucous secretory granules which often fuse with each other and seem to compress the organelles in the basal cytoplasm.
Striated ducts: Their main function is reabsorption and secretion of electrolytes and their fine structure reflects this. The striations seen by light microscopy are actually deep invaginations of the basal membranes of the cells that enclose radially arranged mitochondria. This specialization characterizes these cells as involved in active transport as the invaginations increase the surface area available for ion exchange and the mitochondria provide the required energy. The apical cytoplasm contains dense granules, possibly secretory ones and clear vesicles that may be involved in endocytosis.
Excretory ducts: The pseudo stratified epithelial cells contain diverse dense granules and electron lucent vesicles which reflect their role in modifying the composition of saliva. They also contain cells with long branching processes that seem to extend between the ductal epithelium. These cells may be dendritic cells i.e. antigen presenting cells and thus are part of the immune defense system.
Connective tissue stroma:
Loose connective tissue stroma forming the septa between lobes and lobules also extend as thin strands supporting individual acini and ducts. This connective tissue contains the following cells: fibroblasts, macrophages, dendritic cells, mast cells, plasma cells as well as fat cells. The extra- cellular compartment contains collgen and elastic fibers as well as gIycoproteins and proteogIycans. The stroma adjacent to excretory ducts or ducts of minor salivary glands often contains aggregates of lymphocytes. Plasma cells located adjacent to the acini and intralobular ducts produce secretory immunoglobulin A (sIgA) which is a dimer (2 19A molecules connected by a J chain). Salivary epithelial cells have receptors for this dimer at their basolateral membranes and they endocytose receptor-bound sIgA and release it with part of the receptor called the secretory piece. Small quantities of IgG and IgM are also present in saliva.
Secretion of Saliva: Newly formed saliva is released into the lumina from acinar cells when they receive a secretion stimulus e.g. sympathetic neurotransmitter epinephrine. This release is a well regulated active process called exocytosis. The secretory granule membrane fuse with the acinar cell luminal membrane and the site of fusion open into the lumen. Thus the contents within the secretory granule are released while the integrity of the acinar cell membrane is preserved.
The composition of primary saliva collected from the intercalated ducts is modified while it passes through the striated and excretory ducts, mainly by reabsorption and secretion of electrolytes and possibly by addition of other constituents. The primary saliva is isotonic, with sodium and chloride concentrations higher than potassium while the saliva reaching the oral cavity is hypotonic with low sodium and cloride but high potassium concentrations.
Functions of Saliva:
The composition of saliva is complex and varies according to the type of stimulus and the rate of flow. However several important functions are carried out by different components, the essential function is saliva coats the teeth and the mucosal lining of the oral cavity and as such it provides protection as well as it facilitates a number of physiological activities. Humans who suffer from a decreased salivary secretion due to diseases affecting acini or as a consequence of other systemic diseases or as side effect of certain medications have difficulty in eating, swallowing or speaking. These persons are susceptible to mucosal infections and high incidence of dental caries.
Specifically saliva performs the following functions:
1. Protection: The fluid nature and the components of saliva protects the oral cavity in several ways. Saliva provides a washing action that clears non-adherent potentially harmful substances in the oral cavity. Viscous components e.g. mucins lubricate oral tissues and form a barrier against microbial products. Bicarbonate, phosphate ions as well as basic proteins in saliva maintain the near neutral pH in the oral cavity which prevents demineralization of enamel that would otherwise occur due to acids produced by sugar metabolizing bacteria. Salivary proteins form a thin coating on tooth surfaces, the salivary pellicle which contributes to protecting these surfaces. Saliva is supersaturated with calcium and phosphate ions and this state is maintained by certain calcium binding proteins, notably acidic proline-rich proteins and statherin. This leads to posteruptive maturation of enamel surfaces which increases their hardness and resistance to demineralization. Further, such environment favors remineralization of beginning carious lesions (white spots) provided that cavitation did not occur.
2. Digestion: Saliva contains two digestive enzymes, namely amylase which breaks down complex carbohydrates such as starch into glucose and maltose and lipase (a product of lingual glands )which hydrolyzes triglycerides into mono- and diglycerides.
3. Defense: Saliva plays a major bacteriostatic role in the oral cavity. It interferes with microbial colonization and mucins form a physical anti-microbial barrier. Salivary IgA is an important factor in oral immune defense, together with salivary agglutinins (glycoproteins) sIgA causes clumping of certain microorganisms thus preventing them from adhering to oral and dental surfaces. Other components namely histatins, lysozyme, lactoferrin and peroxidase inhibit bacterial growth.
4. Taste: The solubilization of ingested material by saliva is essential for the functioning of taste buds which act as chemoreceptors when they are in contact with the dissolved material and initiate taste sensation.
Characteristics of major and minor salivary glands: Parotid gland is purely serous in the adult but it contains few mucous acini in the newborn. It has long and branched intercalated ducts, numerous well developed striated and excretory ducts. Its main duct, Stensen‘ s duct opens in the mouth opposite the second maxillary molar. Its C. T. stroma consists of well developed capsule and septa with numerous fat cells specially in old persons.
Submandibular gland is mixed but predominantly serous as it contains mixed acini but the serous acini are more numerous than muCcous ones. The intercalated ducts are shorter than those in the parotid but the striated ducts are longer. Excretory ducts are well developed and the-main duct, Wharton’s opens in the floor of the mouth. Prominent septa and connective tissue capsule are present.
Sublingual gland is mixed but predominantly mucous as pure serous acini are rare or absent but serous acinar cells are present as demilunes in mixed acini. The intercalated and striated ducts are poorly developed. Several small terminal ducts as well as the main duct, Bartholin’s open into the floor of the mouth. Prominent C. T. septa are present but only a poorly developed capsule exists.
Minor glands are located in the submucosa or within the muscular tissue around the oral cavity. The distinct types of ducts seen in major glands do not exist in minor ones as a rapid transition occurs in ducts connecting the acini and the oral cavity. The mucous minor salivary glands include labial, buccal, glosso-palatine, palatine and posterior lingual glands. The anterior lingual glands are mixed while the Von Ebner’s glands associated with lingual circumvallate papillae are serous.
Saliva is produces by three pairs of major salivary glands, the parotid, submandibular and sublingual glands, and numerous minor accessory glands scattered throughout the oral mucosa. The minor salivary glands secrete continuously and are in general under local control whereas the major glands mainly secrete in response to parasympathetic activity which is induced by physical, chemical and psychological stimuli.
Endocrine function of the salivary glands includes the secretion of such hormones as: parotine, nerve and epithelium growth factors, insulin like protein.
Saliva is a hypotonic watery (99 %) secretion containing variable amounts of mucus, enzymes (amylase, maltase, nuclease, pepsin and tripsin like proteins and the antibacterlal enzyme lysozyme), antibodies and inorganic ions (Na, K, Ca, I , Fe). Different cells may be observed in the saliva – epithelial cells, neutrophils and lymphocytes.
Two types of secretory cells are found in the salivary glands: serous cells and mucous cells.
The parotid glands consist almost exclusively of serous cells and produce a thin watery secretion rich in enzymes and antibodies. The subingual glands have predominantly mucous secretory cells and produce a viscid secretion. The submandibular glands contain both serous and mucous secretory cells and produce a secretion of intermediate consistensy. The overall composition of saliva varies according to the degree of activity of each of the major gland types.
Traditionally, the role of salivary amylase has been considered to be initiation of starch digestion in the oral cavlty: its primary role, however, is more likely, to be as a cleansing agent for starch debris retained around the teeth.
The glands are divided into numerous lobules each containing many secretory units (acini). Supporting tissue septa radiate between the lobules from an outer capsule and convey blood vessels, nerves and large excretory ducts.
The salivary secretory unit consists of a terminal branched acinar structure composed exclusively of either serous or mucous secretory cells or a mixture of both types.
Serous secretory cells are organized into alveolar secretory unit.
Mucous secretory cells are organized into tubular secretory unit
In mixed secretory units where mucous cells predominate, serous cells often form semilunar caps called serous semilunes surrounding the terminal part of the mucous acini. Myoepithelial cells embrace the secretory units, their contraction helping to expel the secretory product. The terminal secretory units merge to form larger intercalated ducts which are also lined by secretory cells. The intercalated ducts drain into larger ducts called striated ducts which are named for their striated appearance in light microscopy. The striations result from the presence of numerous deep infoldings of the basal plasma membranes of the large cuboidal cells which line these ducts. In addition to the salivary proteins, the serous cells are involved in active secretion of a watery fluid containing a variety of inorganic ions. This secretion is isotonic with plasma. In striated ducts, the ionic content of the secretion is modified by active reabsorption and further secretion of ions to produce saliva which is hypotonic with respect to plasma. The ionic composition of saliva varies at different flow rates but the concentrations of sodium and chloride ions are below that of plasma, and the concentrations of potassium and bicarbonate ions are above that of plasma. Transport processes in the striated ducts are facilitated by the large surface area offered by the basal infoldlngs of the lining cells and are fulled by their numerous associated rnitochondria.
Both serous and mucous acini are embraced by the processes of contractile cells called myoepithelial cells which, on contraction, force secretion from the acinar lumen into the duct system. Myoepithelial cells are located between the basal plasma membranes of secretory cells and the basement membrane. These cells are flattened and have long processes which extend around the secretory acinus but in secretion they can only be recognised by their large flattened nuclei lying within the basement membrane surrounding the acinus.
The striated ducts are lined by large cuboidal cells with large nuclei located between the centre and luminal surface of the cell, the basal cytoplasm appears striated, reflecting the presence of deep basal infoldings of the plasma membrane and associated columns of mitochondrla: the apical cytoplasm is uniformly stained and not striated. In predominantly serous salivary glands, the striated ducts are larger than mucous ones, a feature associated with the role of the striated duct in modifying isotonic basic saliva to produce hypotonic saliva.
Parotid salivary glands are the complex branched alveolar glands which secrete a serous product. They are situated below and in front of the pinna on each side of the face. They are flat and well encapsulated and the facial nerve runs through them, dividing them into superficial and deep portions. Their long ducts open into the oral cavity opposite the second upper molar tooth on each side.
The parotids are composed entirely of the serous acini rich in zymogen granules, with a variable amount of adipose tissue in the interstitium between parotid lobules. Thus these glands have a well prominent lobular structure.
Submandibular glands are the complex branched alveolar-tubular glands with mixed type of secretion. These roughly ovoid in shape glands are disposed on either side of the neck just below the mandible. Their ducts open into the floor of the mouth near the frenulum.
Sublingual glands are the complex branched tubular glands with mucous type of secretion. They are located in the floor of the mouth, one on either side of the frenulum of the tongue. Their short ducts open into the mouth near to, or with, the submandibular ducts. These glands are composed predominantly of mucous cells.
PHARYNX. The pharynx, a transitional space between the oral cavity and the respiratory and digestive systems, forms an area of communication between the nasal region and the larynx. It is divided into oropharynx (the opening of the mouth into the pharynx) and nasopharynx (nasal opening). The Eustachian tube from the middle ear opens into the pharynx on each side. The oropharynx and pharynx proper are lined by largely stratified squamous epithelium of the mucous type, except in those regions of the respiratory portions that are not subject to abrasion. These latter areas have a ciliated pseudostratified epithelium with goblet cells.
The mucosa of the pharynx has many small mucous glands in its dense connective tissue layer. The submucosa is well endowed with lymphoid tissue (tonsils). The constrictor and longitudinal muscles of the pharynx are located outside this layer.
Tonsils are organs composed of aggregates of incompletely encapsulated lymphoid tissues that lie beneath, and in contact with, the epithelium of the initial portion of the digestive tract. They surround the entrance to the proper digestive tube being organized into pharyngeal lymphoepithelial rink of Pirogov-Waldeier Depending on their location, tonsils in the mouth and pharynx are called palatine (2), tube (2) and unpaired pharyngeal, laryngeal and lingual.
Palatine tonsils. The two palatine tonsils are located in the lateral walls of the oral part of the pharynx in the gap between the glossopalatine and pharynopalatine arches on each side. Under the squamous stratified epithelium, the dense lymphoid tissue in these tonsils forms a band that contains lymphoid nodules, generally with germinal centers. Each tonsil has 10-20 epithelial invaginations that penetrate the parenchyma deeply, forming crypts, whose lumens contain desquamated epithelial cells, live and dead lymphocytes, and bacteria. Crypts may appear as purulent spots in tonsils. Separating the lymphoid tissue from subjacent structures is a band of dense connective tissue, the capsule of the tonsil. This capsule usually acts as a barrier against spreading tonsillar infections.
Inflammatory process of palatine tonsils is known as angina and very often such chronicle process is accompanied with rheumatism.
Pharyngeal tonsil is a single tonsil situated in the superior-posterior portion of the pharynx. It is covered by ciliated pseudostratified columnar epithelium typical of the respiratory tract, and areas of stratified epithelium can also be observed.
The pharyngeal tonsil is composed of pleats of mucosa and contains diffuse lymphoid tissue and nodules. It has no crypts, and its capsule is much thinner than those of the palatine tonsils.
Hypertrophy of the pharyngeal tonsil resulting from chronic inflammation is called adenoid.
Lingual tonsil are smaller then the previous ones. It is situated at the base of the tongue and is covered by stratified squamous epithelium. Lingual tonsil has a single crypt.
Epiglottis
The epiglottis is the flap of cartilage lying behind the tongue and in front of the entrance to the larynx (voice box). At rest, the epiglottis is upright and allows air to pass through the larynx and into the rest of the respiratory system. During swallowing, it folds back to cover the entrance to the larynx, preventing food and drink from entering the windpipe. The throat contains both an air passage (the wind pipe) and a food passage (the esophagus). If these passages were both open when a person swallowed, air could enter the stomach and food could enter the lungs. Part of the safety hatch that seals off the windpipe is the “epiglottis,” a little valvelike cartilage, which works with the larynx to act as a lid every time we swallow. The larynx draws upward and forward to close the windpipe. This keeps solid food and liquid out of the respiratory tract. At the end of each swallow, the epiglottis moves up again, the larynx returns to rest, and the flow of air into the windpipe continues. The uvula (Latin for “little grape”) is a fleshy piece of muscle, tissue and mucous membrane that hangs down from the palate. It is the part that moves upward when we say, “Ah!” It flips up and helps close off the nasal passages when we swallow. Contrary to the depictions seen in cartoons, the uvula does not vibrate during singing and shouting and, in fact, has nothing to do with the voice.
Esophagus
The esophagus is a muscular tube which carries food and liquids from the throat to the stomach for digestion after it has been chewed and chemically softened in the mouth. Food is forced downward to the stomach (or upwards, if one is standing on his head) by powerful waves of muscle contractions passing through the walls of the esophagus. Because these contractions are so strong in the throat and the esophagus, we can swallow in any position — even upside-down! If the food is bad, poison, or more than we can “stomach,” it may travel back by the same force to be thrown out through the mouth, which is called vomiting. The esophagus has a ring of muscle at the top and at the bottom. These rings close or contract after the food passes through and enters the stomach, where there is an abundance of churning acid waiting to digest the food. If the bottom muscle weakens, stomach contents, along with the stomach acid, may return to the esophagus and cause an uncomfortable, burning sensation known as “heartburn”, although it is not connected with the heart at all, but be careful next time you are forced to swallow your pride.
Stomach
A hollow, sac-like organ connected to the esophagus and the duodenum (the first part of the small intestine), the stomach consists of layers of muscle and nerves that continue the breakdown of food which begins in the mouth. It is also a storage compartment, which enables us to eat only two or three meals a day. If this weren’t possible, we would have to eat about every twenty minutes. The average adult stomach stretches to hold from two to three pints and produces approximately the same amount of gastric juices every twenty-four hours. The stomach has several functions: (1) as a storage bin, holding a meal in the upper portion and releasing it a little at a time into the lower portion for processing; (2) as a food mixer, the strong muscles contract and mash the food into a sticky, slushy mass; (3) as a sterilizing system, where the cells in the stomach produce an acid which kills germs in “bad” food; (4) as a digestive tub, the stomach produces digestive fluid which splits and cracks the chemicals in food to be distributed as fuel for the body. The process of digestion is triggered by the sight, smell or taste of food, so that the stomach is prepared when the food arrives. Every time you pass a bakery shop or smell your mother’s good cooking, the body begins a digestive process. If the stomach is not filled, these gastric juices begin eroding the stomach lining itself, so fill ‘er up!
Small Intestine
If the small intestine were not looped back and forth upon itself, it could not fit into the abdominal space it occupies. It is held in place by tissues which are attached to the abdominal wall and measures eighteen to twenty-three feet in the average adult, which makes it about four times longer than the person is tall. It is a three-part tube of about one and one-half to two inches in diameter and is divided into three sections: (1) the duodenum, a receiving area for chemicals and partially digested food from the stomach; (2) the jejunum, where most of the nutrients are absorbed into the blood and (3) the ileum, where the remaining nutrients are absorbed before moving into the large intestine. The intestines process about
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Appendix
Digestion takes place almost continuously in a watery, slushy environment. The large intestine absorbs water from its inner contents and stores the rest until it is convenient to dispose of it. Attached to the first portion of the large intestine is a troublesome pouch called the (veriform) appendix. The appendix has no function in modern humans, however it is believed to have been part of the digestive system in our primitive ancestors
Large Intestine
The large intestine, or colon, consists of ascending, transverse, descending, and sigmoid portions. The ascending portion extends from the cecum superiorly along the right abdominal wall to the inferior surface of the liver and bends sharply at a right angle to the left at a curve called the hepatic flexure. From there, it crosses the abdominal cavity as the transverse colon to the left abdominal wall at the splenic flexure and begins the descending colon which traverses inferiorly along the left abdominal wall to the pelvic region. The colon then forms an angle medially from the pelvis to form an s-shaped curve called the sigmoid colon. The last few inches of the colon is the rectum which is a storage site for solid waste which leaves the body by way of an external opening called the anus, controlled by muscles called sphincters. Substances which have not been absorbed in the small intestine enter the large intestine in the form of liquid and fiber. The large intestine or “bowel” is sometimes called the “garbage dump” of the body, because the materials that reach it are of very small use to the body and are sent on to be disposed of. The first half of the colon absorbs fluids and recycles them into the blood stream. The second half compacts the wastes into feces, secretes mucus which binds the substances, and lubricates it to protect the colon and ease its passage. Of the two to two and one-half gallons of food and liquids taken in by the average adult, only about twelve ounces of waste enters the large intestine. Feces are comprised of about three quarters water. The remainder is protein, fat, undigested food roughage, dried digestive juices, cells shed by the intestine, and dead bacteria.
A common disorder of the large intestine is inflammation of the appendix, or appendicitis. Waste that accumulates in the appendix cannot be moved easily by peristalsis since the appendix has only one opening. The symptoms of appendicitis include muscular rigidity, localized pain in the right lower quarter of the abdomen, and vomiting. The chief danger of appendicitis is that is may rupture and empty its contents of fecal matter and waste into the abdominal cavity producing an extremely serious condition called peritonitis.
Rectum
The rectum is a short, muscular tube that forms the lowest portion of the large intestine and connects it to the anus. Feces collects here until pressure on the rectal walls cause nerve impulses to pass to the brain, which then sends messages to the voluntary muscles in the anus to relax, permitting expulsion.
Specimen 1. Human tongue. Filiform and fungiform papillae.
Stained with haematoxylin and eosin
At a low magnification watch the mucous membrane of the tongue, this has special structures -papillae on the upper and lateral surfaces. All the papillae are cowered by the stratified squamous epithelium, which rests on the mucosa lamina propria. Filiform papillae are the most numerous and despite of others are cowered by the stratified squamous keratinized epithelium. Fungifirm papillae are cowered by the stratified squamous nonkeratinized epithelium. Loose connective tissue of the papillae border with muscles of the tongue. Bundles of striated muscular fibers are disposed in three different turns and are separated by the loose connective tissue.
Illustrate and indicate: 1. Filiform papilla. 2. Fungiform papilla: a) stratified squamous epithelium; b) lamina propria. 3. Tongue muscles. 4. Glands of the tongue.
What are the differences between filiform and fungiform papillae of the tongue?
What muscular tissue does tongue consists of?
What peculiarities salivary glands may be recognized and assigned by in the specimen?
Specimen 2. Human tongue. Foliate papilla.
Stained with haematoxylin and eosin
At a low magnification of microscope find the foliate papillae on the lateral surface of the tongue. At a high magnification light oval-shaped structures – taste buds (receptors of taste) – may be observed in the epithelium, which covers the lateral surface of the foliate papillae.
Illustrate and indicate: 1. Foliate papilla :a) stratified sqamous epithelium; b) lamina propria. 2.Taste buds.
Name, please, the papillae, which contain the taste buds.
Indicate the disposition of the taste buds on the foliate papillae.
Specimen 3. Tooth development early stage.
Stained with haematoxylin and eosin
At a low magnification of microscope watch the structure, which looks like a glass with two walls. This is tooth bud (enamel organ), which is connected with tooth lamina by the cord of cells – the neck. Mesenchime introducing into the enamel organ is called dental papilla, that one which surrounds it is dental sac. Epithelial cells of the enamel organ boarding with dental sac are the outer cells of enamel organ. Prismatic shaped cells, which touch in contact with dental papilla are the inner epithelial cells of the enamel organ. Pulp is disposed inside in the tooth bud.
Illustrate and indicate 1. Enamel organ: a) external epithelium; b) pulp; c) internal epithelium. 3. Dental papilla. 4. Dental sac. 5. Neck of the enamel organ.
Name the tooth structures, which give the origin to the soft and dark tissues of the tooth.
Development of the deciduous teeth is continued in postembryonic period. What portion of the tooth appears at that time?
Three types of cells are seen in the specimen of enamel organ: inner, outer and intermediate. What are the producers of the enamel? What are their names?
Specimen 4. Tooth development later stage.
Stained with haematoxylin and eosin
At a low magnification of microscope find the enamel organ at the later stage of development. Spindle-like cells are differentiated at the top of dental papilla. These are the odontoblasts. Dentin is disposed up to them. It consists of two layers – lighter – predentin and darker (pink) layer rich with lime salt – dentin. Enough thick layer of enamel is seen above the dentin. Alveolar bone formation occurs in the surrounding connective tissue.
Illustrate and indicate: 1. Internal epithelium of the enamel organ (enameloblasts).2. Enamelum. 3. Mature dentin. 4. Predentin. 5. Odontoblastic layer.
Specimen 5. Parotid gland
Stained with haematoxylin and eosin
At a low magnification there seen a lobular structure of the gland. There are vessels, interlobular excretory ducts and nerves in the connective tissue layers. At a high magnification thin intercalated ducts consisting of dark small cells are well observed in the lobules. Striated ducts are larger and consist of tall epithelial cells with basal striations (cell membrane folds). They continue into interlobular ones, whose wall consists of bilayer epithelium and common duct – of stratified epithelium. Secretory portions are round-shaped and consist of serocyte and myoepithelial (basket) cells, which surround the first ones.
Illustrate and indicate: 1. Lobules. 2. Interlobular connective tissue septa. 3. Vessels. 4. Excretory ducts: a) interlobular; b) striated; c) intercalative. 5. Secretory portion: a) serocytes; b) myoepithelial cells.
What are the typical secretory portions of the parotid gland?
Which excretory ducts epitheliocytes are darker and why?
Specimen 6. Submundibular gland
Stained with haematoxylin and eosin
At a high magnification two types of secretory portions are seen: serous and mixed. The serous ones are round-shaped and consist of 12-18 serocytes inside and peripherally disposed myoepithelial cells and then – basement membrane. Mixed acini are larger and have two types of cells: serocytes and mucocytes. Mucous cells are conical with wide basal part and light cytoplasm. Serocytes lie near the basal portion of mucocytes thus producing so called serous semilunes or caps of Djianutsi. Serocytes have basophilic cytoplasm and centrally disposed nuclei. Outside they are surrounded with myoepithelial cells layer. Excretory ducts are similar to that ones in the parotid glands.
Illustrate and indicate: 1. Mucous acini. 2. Mixed acini: a) mucocytes; b) serocytes; c) myoepithelial cells.
What kinds of cells do you see in the mixed acini?
What kind of cells serous semilunes are composed of?
Specimen 6. Cross section of the upper third of oesophagus.
Stained with haematoxylin and eosin.
At a low magnification 4 tunics of the oesophageal wall are seen: mucosa, submucosa, muscular and adventitia. Mucosa consists of three layers: stratified squamous nonkeratinized epithelium under which lamina propria with cardiac glands is disposed. Then muscularis mucosa is seen. Submucosa is well developed and consists of loose connective tissue with oesophageal glands secretory portions. Muscular tunica has inner circular and outer longitudinal layers of skeletal muscles. Adventitia- loose connective tissue with blood vessels. Watch the specimen at a high magnification.
Illustrate and indicate:I. Tunica mucosa: a) stratified squamous nonkeratinized epithelium; b) lamina propria; c) muscularis mucosa. II. Tunica submucosa: d) connective tissue; e) proper oesophageal glands. III. Tunica muscularis: f) internal circular layer; g) external longitudinal layer. IV. Adventitia: h) vessels.
What are the differences between cardiac and proper glands of oesophagus?
What structures may be observed in the relief of oesophagus?
Specimen 7. Palatine tonsil.
Stained with haematoxylin and eosin.
At a low magnification of microscope it is seen that basically palatine tonsil is formed by the mucosa folds in lamina propria containing numerous lymph follicles. Epithelium invaginations between these folds form the crypts. Special attention should be paid to the stratified squamous nonkeratinized epithelium and lamina propria infiltration with lymphocytes and neutrophils. Outside tonsil is cowered by the connective tissue capsule.
Illustrate and indicate: 1. Crypt. 2. Stratified squamous epithelium: a) noninfiltrated; b) infiltrated with leucocytes. 3. Lamina propria: c) lymph nodules. 4. Tonsil capsule.
What are the peculiarities of tonsil cowering epithelium?
What cells besides epithelial are observed in the stratified squamous epithelium of tonsils?
Specimen 8. Oesophageo-gastric junction.
Stained with haematoxylin and eosin
At a low magnification watch the specimen, special attention should be paid to the mucosa, in which transition of the oesophageal stratified squamous epithelium into simple columnar gastric epithelium is well seen. In gastric mucosa gastric pits are observed. There are groups of light cells in the gastric lamina propria – cardial glands secretory portions. Muscularis mucosa has two layers (like in the oesophagus). Submucosa consists of loose connective tissue. Muscular tunic contain three layers of smooth muscles. Serosa is the outer tunic.
Illustrate and indicate:
Name, please the characteristic features, which differ the relief of oesophagus from stomach.
How to distinct the mucosal epithelium of stomach and oesophagus?
What are the differences between muscular tunics of above-mentioned organs?
Specimen 9. Fundus of the stomach.
Stained with haematoxylin and eosin
Watch the specimen at a low magnification; find the stomach inner surface, which is cowered by the simple columnar epithelium. Mucosa surface has small gastric pits. Enough thick layer of lamina propria connective underlies the epithelium. A lot of tubular glands are disposed there. These are the proper gastric glands. Predominantly they are long sected and lie very closely one to each other. Then muscularis mucosa is seen. Submucosa lies outside to mucosa, and then there is well developed muscular tunic, which consists of three layers with nerve plexus disposed between them. Serosa is the outer tunic of the stomach wall. At a high magnification watch the structure of the glands, which are disposed in the mucosal lamina propria. Special attention should be paid to the worse seen lumen of the glands, which is boarded by the cells with light cytoplasm – mucocytes. There are some round-shaped parietal cells with pink cytoplasm outside to these parietal cells.
Illustrate and indicate:
What is the best-developed tunic of the gastric fundus?
What structures are disposed in this tunic?
Specimen 10. Pyloric stomach.
Stained with haematoxylin and eosin
Watch the specimen at a low magnification, special attention should be paid to the presence of the deep gastric pits, less amount of gastric glands in the lamina propria and their structure: their secretory portions are branched very much and predominantly consist of one type of cells in opposite to the fundal glands. In this portion of stomach muscular tunic is developed most of all.the other tunics of pyloric stomach are similar tocardial and fundul ones.
Illustrate and indicate: 1. Tunica mucosa: a) gastric pits. 2. Simple columnar glandular epithelium. 3. Lamina propria: a) pyloric glands. 4. Muscularis mucosa. ІІ. Tunica submucosa. ІІІ. Tunica muscularis. IV. Serosa.
What is the best-developed tunic of the pyloric stomach?
What portion of gastric mucosa do gastric pits occupy?
Specimen 11. Duodenum.
Stained with haematoxylin and eosin
At a low magnification watch the specimen and find four tunics in the intestinal wall: mucosa, submucosa, muscularis and serosa. Pay attention on its surface – there are villi with disposed crypts in between them. At a high magnification it is seen that both are cowered by simple columnar brushed epithelium. Loose connective tissue of the lamina propria underlies this epithelium. Then muscularis mucosa is disposed. Submucosa consists of loose connective tissue with secretory portions of duodenal glands and nerve plexus of Meissner. Muscular tunic contains two layers of smooth muscles and myenteric plexus (Awerbach’s). serosa has typical structure.
Illustrate and indicate:
. What structure do you see in the duodenal submucosa?
What are the peculiarities of epithelial cells, which allow naming this organ “abdominal hypophysis”?
Specimen 12. Jejunum
Stained with haematoxylin and eosin
Principal difference of jejunum wall structure compare to the duodenum is thicker and taller villi and deeper crypts. Submucosa has no glands. All these peculiarities are well seen at a low magnification. All the others tunics and layers are similar to those in duodenum.
Illustrate and indicate: I Tunica mucosa. 1. Villi.2. Crypts. 3. Simple columnar brushed epithelium. ІІ. Tunica submucosa. 1. Connective tissue. ІІІ. Tunica muscularis. IV. Tunica serosa.
What is the difference between relief of duodenum and jejunum?
Specimen 13. Large intestine. Cross section of the colon.
Stained with haematoxylin and eosin
Watch the specimen at a low magnification. Special attention should be paid on the differences in the mucosa structure. It has no villi but contains a lot of deep crypts large aggregations of lymphocytes are disposed in the mucosal lamina propria and submucosa. Outer layer of muscular tunic is discontinuous. Serosa has typical structure.
Illustrate and indicate:
What are the relief compounds in the large intestine?
What is the predominant type of cells in large intestine mucosa epithelium?
Specimen 14. Appendix.
Stained with haematoxylin and eosin
Lumen of the appendix looks like a wide fissure. Mucosal crypts are small, they are cowered by the simple columnar epithelium with a few endocrine cells. Mucosa is continued into submucosa, which contains a lot of lymph follicles (nodules) with light centers. Muscular tunic and serosa have typical structure.
Illustrate and indicate:
Name the peculiarities of appendix mucosa and submucosa.
Why does appendix is named “abdominal tonsil”?
References:
1. Practical classes materials from www.tdmu.edu.te.ua
2. Lecture presentations from www.tdmu.edu.te.ua
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby, 2000. – P. 177-214
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J., Stevens A., Heath J.]. – Elsevier Limited, 2006. – P. 307-334
5. Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. – [fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002. – P.217-247
6. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. – Lippincott Williams and Wilkins, 2011. – P.526-627
b) additional
1. Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P. [tenth edition]. – Lippincott Williams and Wilkins, 2008. – P.151-217
2. Junqueira L. Basic Histology / L. Junqueira, J. Carneiro, R. Kelley. – [seventh edition]. –
3. Charts:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#inf3
4. Disk:
http://intranet.tdmu.edu.ua/data/teacher/video/hist/
5. Volkov K. S. Ultrastructure of cells and tissues / K. S. Volkov, N. V. Pasechko. – Ternopil : Ukrmedknyha, 1997. – P.
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.udel.edu/biology/Wags/histopage/histopage.htm
