DIGESTIVE SYSTEM. ALIMENTARY TRACT
1. Digestive system general feature
2. Oral cavity (lips, cheek, gingiva, soft and hard palate)
3. Tongue
4. Teeth
5. Tonsils
6. Oesophagus
7. Stomach
8. Large and small intestine
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. he 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.
General structure of oral cavity includes next tunices:
І. Mucosa with 3 layers:
1. Еpithelium – stratified squamous nonkeratinized epithelium (180-600 mkm)
2. Lamina propria – loose connective tissue
3. Muscularis mucosa-smooth muscular tissue
ІІ. Submucosa – loose connective tissue
(glands)
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 keratinized 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.
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.
1. Hear-bearing skin :
а) epithelium stratified squamous keratinized (4 layers)
• b) lamina propria (loose connective tissue with hair follicles sweat and sebaceous glands)
2. Transitional (vermilion): external (smooth) and internal zone
3. Mucosal
а) epithelium stratified squamous nonkeratinized
b) lamina propria (papillae)
• c) submucosa
CHEEKS
Cheek has two surfaces:
1. External (lined with skin)
2. Internal (lined with mucosa)
Starting from outside cheek has next structures:
І. Skin
ІІ. Muscle
ІІІ. Mucosa
Mucosa of inner surface of cheek has 3 zones:
1. Upper (maxillary)
2. Lover (mandibulary)
3. Intermediate (has no salivary glands, parotid glands ducts lie here)
Mucosa includes:
• а) stratified squamous nonkeratinized epithelium (smear);
• б) lamina propria (dense connective tissue, elastic fibers).
Submucosa containa a lot of minor salivary glands.
Muscular tunic (skeletal muscle). Newborn have adipose body of Bisha. Dense nodule of adipose tissue which helps in suckling.
PALATE
Septum which separates oral and nasal cavity. It includes 2 components: hard and soft palate.
І. Hard palate has 3 zones: 1) adipose, 2) mucous and 3) marginal
At the middle line (epithelial bodies – “perls” may be observed iewborn)
ІІ. Soft palate. Oral surface is lined with stratified squamous nonkeratinized epithelium. Nasal surface is cowered by respiratory ciliated epithelium.
TONGUE
Tongue functions
1. Mechanical
2. Receptory (general and taste)
3. Articulation
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.
Tunices of the tongue are the next:
І. Mucosa
ІІ. Submucosa
ІІІ. Muscular
ІV. Aponeurosis
The body of the tongue 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.
Morphofunctional peculiarities of the lover surface of the tongue
1. Thin permeable epithelium
2. Well prominent blood supply
3. Salivary glands
TEETH
Each tooth has 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 mineralized enamel which covers it entirely. The bulk of the tooth is made up of dentine, a mineralized tissue which has a similar chemical composition to bone. The dentine has a central pulp cavity containing the dental pulp, which consists of specialized 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 jaw 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 gingival crevice. All of the tissues which surround and support the tooth are collectively known as the periodontum.
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 odontoblastic 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 Hertwig’s sheath and are known as epithelial rests of Malassez.
GINGIVA
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 molarsThe 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.
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.
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.
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.
7 tonsils : 2 tubular, 2 palatine, lingual, pharyngeal and laryngeal belong to lymph epithelial ring of Pirogov-Waldeier.
These are aggregations of lymph nodules disposed in mucosa. They perform next functions:
1. Protective
2. Immune
І. Mucosa over these nodules includes:
а) epithelium stratified squamous nonkeratinized infiltrated with leucocytes;
b) lamina propria – loose connective tissue with B lymphocytes.
ІІ. Submucosa – septae, capsule.
ІІІ. Muscular tunic has skeletal muscle.
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.
Digestive tube wall along alimentary tract has common general features: it has next tunices: I. Mucosa:
1. Epithelium
2. Lamina propria
3. Muscularis mucosae
II. Submucosa
III. Muscularis externa
IV. The outer tunica (adventitia or serosa)
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. Seretory (gastric juice, mucous)
2. Mechanical
3. Antianemic factor
4. Absorption of water
5. Excretory
6. Endocrine
7. Protective
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!
Wall of stomach has usual four tunices: mucosa with typical 3 layers, submucosa, muscularis externa and serosa. Relief of gastric mucosa showes folds, fields and pits. Fold is outgrowth of mucosa consisting of mucosa and submucosa. Longitudinally arranged folds of stomach are known as rugae. Pit is ingrowth of epithelium in underlying lamina propria. Usually pit contains opening of gastric gland (neck) in the bottom. Group of pits which are surrounded by connective tissue with vessels are turmed “field”. They are seen only at surface wievof mucosa. The stomach inner surface is cowered by the simple columnar glandular epithelium. Mucosa surface has small gastric pits. Enough thick layer of lamina propria connective tissue underlies the epithelium. A lot of tubular glands are disposed there. These are the proper gastric glands in fundus and body, cardial glands in cardia and pyloric glands in pyloric part of stomach.
Predominantly they are long sected and lie very closely one to each other in fundus and body. Lumen of the glands is worse seen and is boarded by cells with light cytoplasm – mucocytes (mucous producing cells), basophilic chief cells (pepsinogen producing cells), large oxyphilic parietal (or oxyntric) cells which are producing antianemic (gastric intrinsic) factor. There are few endocrine cells here too, they perform local regulatory function. In the neck of glands small basophilic cells are placed which have cambial functions. These mitotically dividing cells promote bidirectional regeneration of gastric epitheluim: both superficial glandular and cells inside glands.
Then muscularis mucosa has 3 layers. 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 (peritoneum) is the outer tunic of the stomach wall which consists of mesothelium and underlying connective tissue.
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: duodenum, a receiving area for chemicals and partiallydigested food from stomach, the jejunum, where most nutrients are absorbed into the blood and the ileum, where the remaining nutrients are absorbed before moving into the large intestine. The intestines process about 2.5 gallons of food, liquids and bodily waste every day.
Relief of small intestine includes C-like folds, villi and crypts. In order for enough nutrients to be absorbed into the body, it must come in contact with large numbers of intestinal cells which are folded like gathered skirts. Each of these cells contain thousands of tiny finger-like projections called “villi,” and each villus contains microscopic “microvilli”. In one square inch of small intestine, there are about 20,000 villi and ten billion microvilli. Each villus brings in fresh, oxygenated blood and sends out nutrient-enriched blood. The villi sway constantly to stir up liquefied food and remove the nutrients which can be absorbed and then passed through the membranes of the villi into the blood and lymph vessels. The fatty nutrients go to the lymph vessels, and glucose and amino acids go to the blood and on to the liver.
Villi are lined with intestinal epithelium – simple columnar brushed (absorptive). Enterocytes over villi include 3 groups of cells: most numerous ones brashed with microvilly, Goblet cells (mucocytes) and enteroendocrine cells.
Crypt or intestinal (Liberkun) gland is tubular invagination of epithelium into underlying lamina propria. 5 types of cells lie in crypt. They are 3 types previously described in villy and two more types – Panneth cells and nonbrushed (cambial) cells. The latter ones lie in the bottom of crypt and promote unidirectional reheneration of intestinal epithelium. Turnower of epitheliocytes is about 3 days.
The muscles which encircle small intestine constrict about seven to twelve times a minute to move the food back and forth, to churn it, knead it, and to mix it with gastric juices. The small intestine also makes waves which move the food forward, but these are usually weak and infrequent to allow the food to stay in one place until the nutrients can be absorbed. If a toxic substance enters the small intestine, these movements may be strong and rapid to expel the poisons quickly.
Duodenum contains special duodenal glands in submucosa secretion of which has to converce acidic reaction of stomach content into slight alkaline in intestine. These part of intestine contains especially a lot of endocrine cells which are necessary for local regulation of nearest organs whose functions are tightly connected with intestine (stomach liver, gall bladder and pancreas).
LARGE INTESTINE
Digestion and absorption in large intestine are less active, so it has no villy, but C-like folds and crypts never the less are present in relief here.
Digestion takes place almost continuously in a watery, slushy environment. The large intestine absorbs water and salts from its inner contents and stores the rest until it is convenient to dispose of it. Thanks to E.Coli which “lives” here large intestine is the only part of intestine which is involved in cellulosa digestion. Also it synthesise some vitamins like (K, A).
Attached to the first portion of the large intestine is a troublesome pouch called the (veriform) appendix. The appendix has no digestive function in modern humans, however it is believed to have been part of the digestive system in our primitive ancestors and plays and impotant role in immune protection. Presence of numerous lymph nodules in the wall of this organ allowes to name it “abdominal tonsil”. Intestinal epithelium over lymphatic agregations contains special M-cells. These are microfold cells (macrophages) have which antigenpresented function.
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.
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. Special feature of colon is discontinuous outer (longitudinal) layer of muscularis externa. Myocytes in this layer are arranged i 3 tenia colli, haustrae are seen between them. 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.
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. Rectum has pelvic and anal portions which have their own peculiarities. Simple columnar brushed epithelium in pelvic part is replaced by simple cuboidal, then in anal part by stratified nonkeratinized and stratified keratinized at the exit. Submucosa of anal portion contains remnants of anal glands and well developed mesh of hemorroidal veins.
