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. General morphofunctional characteristic of large salivary glands and their classification.
29. Structural peculiarities of the parotid, submandibular and sublingual glands secretory portions (acini).
30. Main microscopic and ultrastructural signs of muco- and serocytes.
31. Large salivary glands excretory ducts.
32. Salivary glands excretory products and hormones.
33. Morphogenesis and regeneration of the salivary glands.
34. Salivary glands aging.
35. Description of the liver’s double blood supply.
36. Description of the complex structure of a hepatocyte and relation of its structure to its main functions.
37. Description of the classic hepatic lobule, the portal lobule, and the hepatic acinus (of Rappaport).
38. Principal components of a portal triad.
39. Characteristic of the major cell types that border the hepatic sinusoids and cells that border the space of Disse.
40. Description of the composition and production of bile.
41. Functions of the gall bladder.
42. Comparison of the wall of the gall bladders with that of the small intestine.
43. Description of the size, staining properties, and distribution of the islets of Langerhans in the pancreas.
44. Distinguishing of the A (alpha), B (beta), D (delta), and F (PP) cells of the islets of Langerhans in terms of the hormones they secrete, their location in the islets, their relative numbers, the appearance of their granules, and any special staining properties.
45. Role of the cells of the islets of Langerhans in regulating blood glucose levels.
46. Cytochemical pecularities of pancreas acinar cells.
47. Description of two types of pancreatic exocrine secretions in term of their composition, role in the digestion, cells primarily responsible for their secretion, and the enteroendocrinal hormone that stimulates their release.
48. Main enzymes secreted by the exocrine pancreas.
49. 29. Development and tissues compounds of the small intestine wall.
50. 30. Peculiarities of the small intestine relief. System villus-crypt.
51. 31. Morphofunctional characteristic of the simple columnar brushed epithelium of the villi and crypts.
52. 32. Ultrastructure and functions of different enterocytes.
53. 33. Small intestine submucosa. Duodenal glands structure and functions.
54. 34. Lymphoid follicles in small intestine, disposition and functions.
55. 35. Muscular and external tunics of the small intestine.
56. 36. Absorption histophysiology in small intestine.
57. 37. The sources of the digestive tube middle and posterior portion embryonic development.
58. 38. Large intestine anatomical portion and wall tunics structure.
59. 39. Appendix structure and functions.
60. 40. Rectum portions and functional peculiarities.
61. 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.\
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.
liver
Liver is the largest gland in the human body. It lies in the abdominal cavity, in contact with diaphragm. Its mass is divided into several lobes, the number and size of which vary among species. In most mammals, a greenish sac – the gallbladder – is seen attached to the liver and careful examination will reveal the common bile duct, which delivers bile from the liver and gallbladder into the duodenum. The image below is of a liver from a dog, and illustrates these concepts. The panel on the left shows that aspect of the liver that faces the contents of the abdominal cavity. The right panel shows the flatter face of the liver which is in contact with the diaphragm.
Understanding function and dysfunction of the liver, more than most other organs, depends on understanding its structure. The major aspects of hepatic structure that require detailed attention include:
- The hepatic vascular system, which has several unique characteristics relative to other organs
- The biliary tree, which is a system of ducts that transports bile out of the liver into the small intestine
- The three dimensional arrangements of the liver cells, or hepatocytes and their association with the vascular and biliary systems.
The Hepatic Vascular System
The circulatory system of the liver is unlike that seen in any other organ. Of great importance is the fact that a majority of the liver’s blood supply is venous blood. The pattern of blood flow in the liver can be summarized as follows:
- Roughly 75% of the blood entering the liver is venous blood from the portal vein. Importantly, all of the venous blood returning from the small intestine, stomach, pancreas and spleen converges into the portal vein. One consequence of this is that the liver gets “first pickings” of everything absorbed in the small intestine, which, as we will see, is where virtually all nutrients are absorbed.
- The remaining 25% of the blood supply to the liver is arterial blood from the hepatic artery.
- Terminal branches of the hepatic portal vein and hepatic artery empty together and mix as they enter sinusoids in the liver. Sinusoids are distensible vascular channels lined with highly fenestrated or “holey” endothelial cells and bounded circumferentially by hepatocytes. As blood flows through the sinusoids, a considerable amount of plasma is filtered into the space between endothelium and hepatocytes (the “space of Disse”), providing a major fraction of the body’s lymph.
- Blood flows through the sinusoids and empties into the central vein of each lobule. The speed of blood flow is very slow.
- Central veins coalesce into sublobular veins and then hepatic veins, which leave the liver and empty into the vena cava.
The Biliary System
The biliary system is a series of channels and ducts that conveys bile – a secretory and excretory product of hepatocytes – from the liver into the lumen of the small intestine. Hepatocytes are arranged in “plates”(cords) with their apical surfaces facing and surrounding the sinusoids. The basal faces of adjoining hepatocytes are welded together by junctional complexes to form canaliculi, the first channel in the biliary system. A bile canaliculus is not a duct, but rather, the dilated intercellular space between adjacent hepatocytes. This structure is adequate to tubular secretory portion of exocrine gland.
Hepatocytes secrete bile into the canaliculi, and those secretions flow parallel to the sinusoids, but in the opposite direction that blood flows. At the ends of the canaliculi, bile flows into the cholangiols – short ducts which connect secretory portions and aroundlobular bile ducts, which are true ducts lined with epithelial cells. Bile ducts thus begin in very close proximity to the terminal branches of the portal vein and hepatic artery, and this group of structures is an easily recognized and important landmark seen in histologic sections of liver – the grouping of bile duct, hepatic arteriole and portal venule is called a portal triad.
The gall bladder is another important structure in the biliary system of many species. This is a sac-like structure adhering to the liver which has a duct (cystic duct) that leads directly into the common bile duct. During periods of time when bile is not flowing into the intestine, it is diverted into the gall bladder, where it is dehydrated and stored until needed.
Architecture of Hepatic Tissue
The liver is covered with a connective tissue capsule that branches and extends throughout the substance of the liver as septae. This connective tissue tree provides a scaffolding of support and the highway which along which afferent blood vessels, lymphatic vessels and bile ducts traverse the liver. Additionally, the sheets of connective tissue divide the parenchyma of the liver into very small units called lobules.
The hepatic lobule is the structural unit of the liver. It consists of a roughly hexagonal arrangement of plates of hepatocytes radiating outward from a central vein in the center. At the vertices of the lobule are regularly distributed portal triads, containing a bile duct and a terminal branch of the hepatic artery and portal vein. Lobules are particularly easy to see in pig liver because in that species they are well deliniated by connective tissue septae that invaginate from the capsule.
Bile flows out of the liver through hepatic ducts, which join and extend as the common bile duct (also known simply as the bile duct) to traverse the wall of the duodenum and deliver bile into its lumen. In species with a gallbladder, the hepatic ducts join with the cystic duct, which conveys bile to and from the gall bladder.
Hepatocytes are the chief functional cells of the liver and perform an astonishing number of metabolic, endocrine and secretory functions. Roughly 80% of the mass of the liver is contributed by hepatocytes.
In three dimensions, hepatocytes are arranged in plates that anastomose with one another. The cells are polygonal in shape and their sides can be in contact either with sinusoids (sinusoidal face) or neighboring hepatocytes (lateral faces). A portion of the lateral faces of hepatocytes is modified to form bile canaliculi. Microvilli are present abundantly on the sinusoidal face and project sparsely into bile canaliculi.
Hepatocyte nuclei are distinctly round, with one or two prominent nucleoli. A majority of cells have a single nucleus, but binucleate cells are common. The micrographs below (H&E stain) demonstrate these features in sections of liver from a pig (left) and raccoon (right).
Hepatocytes are exceptionally active in synthesis of protein and lipids for export. As a consequence of these activities, ultrastructural examination of hepatocytes reveals bountiful quantities of both rough and smooth endoplasmic reticulum. In contrast to most glandular epithelial cells that contain a single Golgi organelle, hepatocytes typically contain many stacks of Golgi membranes. Golgi vesicles are particularly numerous in the vicinity of the bile canaliculi, reflecting transport of bile constituents into those channels.
Another important function of hepatocytes is to synthesize and secrete very low density lipoproteins. These complexes are seen in electron micrographs as electron-dense particles within smooth endoplasmic reticulum. Another type of particle observed in copious quantities in liver is glycogen. Glycogen is a polymer of glucose and the density of glycogen aggregates in hepatocytes varies dramatically depending on whether the liver is examined shortly after a meal (abundant glycogen) or following a prolonged fast (minimal quantities of glycogen). When viewed with an electron microscope, glycogen particles are typically arranged in chrysanthemum-like clusters of electron-dense particles, as seen in the following image:
In paraffin sections of liver stained with hematoxylin and eosin, accumulations of glycogen in hepatocytes do not stand out. However, when stained with using the periodic acid-Schiff (PAS) technique, glycogen stains bright pink in color. The images below represent PAS-stained sections of liver from two mice:
- Left panel: from a mouse that fasted overnight and thus had very low levels glycogen in liver.
- Right panel: from mouse that stuffed himself on mouse chow two hours prior to fixing the liver, and thus had high levels of hepatic glycogen. These accumulations are seen as pink areas of PAS-positive material throughout the section.
Sinusoids are low pressure vascular channels that receive blood from terminal branches of the hepatic artery and portal vein at the periphery of lobules and deliver it into central veins. Sinusoids are lined with endothelial cells and flanked by plates of hepatocytes.
The space between sinusoidal endothelium and hepatocytes is called the space of Disse. Sinusoidal endothelial cells are highly fenestrated, which allows virtually unimpeded flow of plasma from sinusoidal blood into the space of Disse. This arrangement has at least two important consequences:
Hepatocytes are bathed in plasma derived in large part from venous blood returning from the small intestine. Following meals, that plasma is nutrient-rich.
Plasma which collects in the space of Disse flows back toward the portal tracts, collecting in lymphatic vessels and forming a large fraction of the body’s lymph.
Another important feature of hepatic sinusoids is that they house an important part of the phagocytic system. Sinusoids are populated by numerous Kupffer cells, a type of fixed macrophage. Identifying Kupffer cells in conventionally-stained sections of liver is not easy. However, they stand out sharply when full of phagocytosed ink particles.
The images below are of a mouse liver fixed two hours after an intravenous injection of a small quantity of India ink, which provides a clear view of these tiny warriors. All of the black masses are ink-laden Kupffer cells lying in sinusoids.
As a final point of interest, examine the following micrograph of ovine liver (H&E stain). The sinusoids are packed with clusters of basophilic cells that have not been seen in previous micrographs of liver – what are they?
This section of liver was taken from a 100 day sheep fetus. The clusters of cells in sinusoids are various types of immature blood cells. In the fetus, the liver is a major site of hematopoiesis, or formation of blood cells. Hepatic hematopoiesis is not normally seen after birth, although it can occur under certain pathologic conditions.
The hepatic acinus is the other functional unit of the liver
The acinus is more difficult to visualize than the lobule, but represents a unit that is of more relevance to hepatic function because it is oriented around the afferent vascular system.
As seen overlayed onto lobules in the diagram below, the acinus consists of an irregular shaped, roughly ellipsoidal mass of hepatocytes aligned around the hepatic arterioles and portal venules just as they anastomose into sinusoids.
The acinus is roughly divided into zones that correspond to distance from the arterial blood supply – those hepatocytes closest to the arterioles (zone 1 below) are the best oxygenated, while those farthest from the arterioles have the poorest supply of oxygen. This arrangement also means that cells in the center of the acinus (again, zone 1) are the first to “see” and potentially absorb blood-borne toxins absorbed into portal blood from the small intestine.
The net result is that a variety of pathologic processes lead to lesions that reflect acinar structure; for example, necrosis of hepatocytes at the periphery of the acinus. In the image below, a depiction of the arterial and venous blood supply emanating from one triad has been painted onto a micrograph of porcine liver.
The liver is bounded by a connective tissue capsule which extends into its substance as highly branched septae. The afferent blood vessels and lymphatics follow this connective tissue highway throughout the liver. Efferent vessels traverse a route separate from connective tissue scaffolding.
In the section of equine liver below (Masson’s trichrome stain), the capsule and septae are stained blue , while hepatocytes are magenta . Notice how the capsule extends as a septum into the liver about one-third of the way from left, immediately below a large capsular blood vessel.
The connective tissue septae invaginating from the capsule delineate hepatic lobules, the structural unit of the liver. Relative to other common species, the connective tissue surrounding lobules is particularly abundant and easy to identify in pig livers, as shown below in an H&E-stained section
As you can observe above, a lobule is a roughly hexagonal arrangement of plates of hepatocytes radiating outward from a central vein (CV) in the center. Central veins are quite prominent and provide an easy means of orientation in sections of liver.
At the vertices of the lobule are regularly distributed portal triads (also known as portal tracts). Examination of a triad in cross section should reveal a bile duct and branches of the hepatic artery and hepatic portal vein. Due to plane of section, one can often observe more than one of each of these structures in a given portal tract or absence of one or more structures. Lymphatic vessels are also present, but are tough to see in standard paraffin sections, which is probably why it is not called a portal tetrad (portal tract which includes 4 compounds – interlobular artery. Interlobular vein, interlobular bile duct and lymphatic capillary).
Portal tract in equine liver (trichrome stain)
Portal tract in porcine liver (H&E stain) without visible bile duct
Lobules are almost impossible to miss in porcine liver, but one should also be able to recognize them in other species. Although the precise boundaries of lobules are sometimes difficult to discern, orienting on central veins and portal tracts allow “easy” identification. It goes without saying, of course, that the majority of lobules seen in a tissue section are not as “typical” as seen here and in other histology texts.
THE GALL BLADDER
The gallbladder is a distensible sac and, wheot distended, its mucosa is thrown into many folds. The lumen of the gallbladder is lined with a high columnar epithelium. The connective tissue wall contains abundant elastic fibers and layers of smooth muscle which predominantly run obliquely. The mucosa, connective tissue and muscularis of a canine gall bladder are seen below (H&E stain).
As seen below at a higher magnification, the epithelium of the gall bladder is quite uniform in appearance throughout the organ, and rests on a highly vascularized lamina propria. This is simple columnar brushed epithelium. Epithelial cells have microvilli over the apical surface and are devoted to absorption of inorganic salts and water, and provide the mechanism for the gallbladder’s ability to concentrate bile. Between the smooth muscle layers and serosa is a thick subserosal layer of connective tissue. One face of the gallbladder is attached to the liver, and in that area, the connective tissue of the two organs is shared.
PANCREAS
The pancreas is a elongated organ, light tan or pinkish in color, that lies in close proximity to the duodenum. It is covered with a very thin connective tissue capsule which extends inward as septa, partitioning the gland into lobules. The image below shows a canine pancreas in relation to the stomach and duodenum.
The bulk of the pancreas is composed of exocrine and endocrine portions. The pancreas is surrounded by a very thin connective tissue capsule that invaginates into the gland to form septae, which serve as scaffolding for large blood vessels. Further, these septae divide the pancreas into distinctive lobules, as can clearly be seen in the image of mouse pancreas below (H&E). The large spaces between lobules seen in this image are a commonly-observed artifact of fixation.
Serous acini are structural units of exocrine pancreas. Each acinus consists of 10-12 pancreatic exocrine cells and their intercalated ducts. Embedded within this exocrine tissue are roughly one million small clusters of cells called the Islets of Langerhans, which are the endocrine cells of the pancreas and secrete insulin, glucagon and several other hormones. In the histologic image of an equine pancreas seen below, a single islet is seen in the middle as a large, pale-staining cluster of cells. All of the surrounding tissue is exocrine.
The Acinus
The exocrine pancreas is classified as a compound tubuloacinous gland. The cells that synthesize and secrete digestive enzymes are arranged in grape-like clusters called acini, very similar to what is seen in salivary glands. In standard histologic sections, most acini are cut obliquely, making it difficult to discern their characteristic shape. In the image of equine pancreas below, one fairly-good cross section through an acinus is circled; note the wedge-shapped cells arranged around a small lumen:
Pancreatic Ducts
Digestive enzymes from acinar cells ultimately are delivered into the duodenum. Secretions from acini flow out of the pancreas through a tree-like series of ducts. Duct cells secrete a watery, bicarbonate-rich fluid which flush the enzymes through the ducts and play a pivotal role ieutralizing acid within the small intestine. Pancreatic ducts are classified into four types which are discussed here beginning with the terminal branches which extend into acini.
Intercalated ducts receive secretions from acini. They have flattened cuboidal epithelium that extends up into the lumen of the acinus to form what are called centroacinar cells.
Intralobular ducts have a classical cuboidal epithelium and, as the name implies, are seen within lobules. They receive secretions from intercalated ducts.
Interlobular ducts are found between lobules, within the connective tissue septae. They vary considerably in size. The smaller forms have a cuboidal epithelium, while a columnar epithelium lines the larger ducts. Intralobular ducts transmit secretions from intralobular ducts to the major pancreatic duct.
The main pancreatic duct received secretion from interlobular ducts and penetrates through the wall of the duodenum. In some species, including man, the pancreatic duct joins the bile duct prior to entering the intestine.
Histologic features of the pancreatic duct system are illustrated in the following images:
A longitudinal section through an intercalated duct (Cynomologous monkey pancreas, H&E stain). The duct is running from upper left to lower right. Note the low cuboidal, almost squamous epithelium.
Section of equine pancreas (H&E stain) showing a longitudinal section through an intercalated duct emptying into an intralobular duct. Note the cuboidal epithelium in the intralobular duct.
Small interlobular ducts (equine pancreas; H&E stain): note the columnar epithelium. A thin interlobular septum is seen running horizontally immediately above the duct.
A low magnification image of equine pancreas (H&E stain) showing a large interlobular duct in association with a pancreatic artery (A) and vein (V). An intralobular duct (D) is seen on the right side.
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Pancreatic exocrine cells are arranged in grape-like clusters called acini (a single one is an acinus). Basal portions of cells (homogenous zonez) are basophilic because of well developed rough endoplasmic reticulum producing enzymes. The exocrine cells themselves are packed with membrane-bound secretory granules which contain digestive enzymes that are exocytosed into the lumen of the acinus. These granules which contain a mixture of inactive enzymes lie in the apical part of cells (zymogenous zone). From there these secretions flow into larger and larger, intralobular ducts, which eventually coalesce into the main pancreatic duct which drains directly into the duodenum.
The lumen of an acinus communicates directly with intralobular ducts, which coalesce into interlobular ducts and then into the major pancreatic duct. Epithelial cells of the the intralobular ducts actually project “back” into the lumen of the acinus, where they are called centroacinar cells. The anatomy of the main pancreatic duct varies among species. In some animals, two ducts enter the duodenum rather than a single duct. In some species, the main pancreatic duct fuses with the common bile duct just before its entry into the duodenum.
If you examine a section of pancreas at low magnification, you will undoubtedly observe distinctive areas of pale staining embedded within the exocrine tissue of lobules. These are the Islets of Langerhans, the endocrine component of the pancreas. Islets contain several different endocrine cell types:A,B,D,D1 and PP cells. A cells, B cells and D cells are the major endocrine cells; D1 cells, and PP cells are minor endocrine cells. A cells usually are located along the periphery of islets. A cells have an irregularly shaped nucleus and secretory granules that contain glucagon. B cells comprise about 70% of the islet endocrine cell population and are centrally located in islets. B cells have large, round nuclei. D-cells located at the islet periphery, close to A cells.
The most abundant are beta cells (basophilic cells), which produce insulin, and alpha cells (acidophilic cells), which secrete glucagon. D – dendritic cells are producing somatomedin, D1 – vasoactive intestinal polipeptyde and PP cells – pancreatic polipeptyde. In sections stained with H&E, the different endocrine cell types cannot be differentiated from one another. Special stains, or better yet, immunostaining, is required to identify specific cell types. The endocrine cells within islets are arranged as irregular cords around abundant capillaries, which receive the secreted hormones for delivery into the systemic circulation. Examine the images of islets below and note their rich vascularity, evidenced by clusters and streams of red blood cells next to endothelial cells that can be identified by their flattened nuclei.
Student’s Practical Activities:
Students must know and illustrate such histologic specimens:
Specimen. 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. 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. 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. 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. 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. 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. 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. 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”?
Specimen . Liver
Stained with haematoxylin and eosin.
The principal cells of the liver, the hepatocytes, are arranged into lobules. The boundaries of the human liver lobules are not defined by distinct connective tissue. Portal tracts define the angles of the lobule margins and a central vein defines the centre of each lobule.
The plates of hepatocytes are usually only one cell. You can focuse on a portal triads between three adjacent liver lobules. Each portal triad contains vessels of three main types. Firstly, the largest diameter vessels are branches of the hepatic portal vein which have the typical, thin-walled structure and irregular outline of all veins. Secondly, the smaller diameter, thick-walled vessels, with the typical structure of arterioles and arteries, are branches of the hepatic artery which supplies oxygenated blood to the liver. Thirdly, ducts of variable size lined by simple cuboidal or columnar epithelium are the bile-collecting ducts which ultimately drain into the common bile duct. A fourth type of vessel, lymphatics, are also present in the portal tracts, but since their walls are delicate and often collapsed they are not readily seen.
Illustrate and indicate:
Specimen Pancreas.
Stained with haematoxylin and eosin.
The pancreas is a highly lobulated gland invested by a loose connective tissue capsule which extends as delicate septa between the lobules. The exocrine component of the pancreas consists of closely packed, secretory acini which drain into a highly branched duct system.
The endocrine tissues of the pancreas form islets of various sizes, the islets of Langerhans, which are scattered throughout the exocrine tissue.
At higher magnification, details of the pancreatic acini and duct system can be seen. Each acinus is made up of an irregular cluster of secretory cells which drain into a minute, central duct. These minute ducts then drain into the system of ducts of progressively increasing size. The small ducts are lined by simple cuboidal epithelium which becomes stratified cuboidal in the larger ducts. With increasing size, the ducts are supported by a progressively thicker layer of dense connective tissue and the wall of the main pancreatic duct contains smooth muscle.
The pancreatic acini are seen to consist of roughly pyramidal-shaped cells with their apices projecting towards the lumen of a minute duct. The acinar cells are typical protein-secreting (zymogenic) cells. The nuclei are basally located and surrounded by basophilic cytoplasm rich in rough endoplasmic reticulum; the apices of the cells are packed with eosinophylic, zymogen secretory granules. The smallest excretory ducts merge with the lumen of the acini, and duct lining cells are often seen in the centre of secretory acini; these duct lining cells are thus described as centroacinar cells and are recognized by their pale-stained nuclei and sparse, pale-stained cytoplasm. Cells of similar appearance can be seen between the acini. The cells lining the small ducts are responsible for elaborating the bicarbonate component of pancreatic secretion.
Illustrate and indicate:
Task No 3. Students should be able to indicate elements in the electron micrographs:
1. Hepatocytes in hepatic lobule.
2. Biliary poles of hepatocytes.
3. Binucleated hepatocyte.
4. Hemocapillary in liver.
5. Interlobular bile duct.
6. Pancreatocytes (apical portion).
7. Pancreatocytes (basal portion).
8. Insulocytes.
References:
1. Douglas F. Paulsen. Basic Histology. – Prentice – Hall International Inc. – 1990.
2. Paul R., Wheater H., George Burkitt, Victor G. Daniels. Functional Histology: a text and color atlas. – Churchill Livingstone Inc. – 1987.
3. L. Carlos Junqueira, Jose Carneiro, Robert O. Kelley. Basic Histology. – 7th ed. Appleton and Lange.
4. Inderbir Singh. Textbook of Human Histology with colour atlas. – 4th ed. Jaypee Brothers Medical Publishers (P) LTD, 2002, pp. 248-258.
5. Wheater P.R., Burkitt H.G., Daniels V.G. Functional Histology: a text and colour atlas. – 2nd ed. Longman Group UK Limited, 1987. – pp. 224-236.
6. Victor P. Eroschenko. Atlas of Histology with functional correlations. – 9th ed. Lippincott Williams and Wilkins, 2000. – pp. 221-237.
7. Ham, A. W.: Histology, eg.7.
8. Webster’s Medical Desk Dictionary. –
9. Tables:
http://intranet.tdmu.edu.ua/index.php?dir_name=kafedra&file_name=tl_34.php#n15
10. Disk:
http://intranet.tdmu.edu.ua/index.php?dir_name=cd&file_name=index.php#3
11. Волков К.С. – Ультраструктура клiтин i тканиню Навчальний посiбник-атлас. 1997 р., 95 с.
http://intranet.tdmu.edu.ua/data/books/Volkov(atlas).pdf
12. О.Д.Луцик і співавт. – Гiстологiя людини. – Київ: Книга плюс, 2003 р. –592 с.
http://intranet.tdmu.edu.ua/data/books/gistologia_lucyk.pdf
http://www.morphology.dp.ua/hist.php?lang=uk
http://en.wikipedia.org/wiki/Histology
http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.html
http://www.kumc.edu/instruction/medicine/anatomy/histoweb/
http://www.udel.edu/biology/Wags/histopage/histopage.htmhttp://intranet.tdmu.edu.ua/data/books/gistologia_lucyk.pdf
