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Material Type: Notes; Class: VET HISTOLOGY; Subject: Veterinary Medicine ; University: Oklahoma State University - Stillwater; Term: Unknown 1989;
Typology: Study notes
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The digestive system includes the gastrointestinal tract as well as associated organs like the pancreas and liver. Digestive System I will cover the oral cavity (lips, tongue, major salivary glands) and the gastrointestinal tract, i.e. esophagus, stomach, small and large intestines. The digestive system consists throughout most of its length of a series of tubular organs lined with specific types of epithelium to fulfill specific functions related to the digestion and absorption of nutrients from a food source and the elimination of waste products.
Organ FuFunctionn ction
Lips Ingestion and fragmentation of food
Teeth Fragmentation of food
Tongue Fragmentation and swallowing
Salivary Glands Fragmentation and moistening of food; swallowing
Esophagus Passage of food from oral cavity to the stomach
Stomach
Completion of fragmentation and beginning of digestion
Small Intestine - duodenum
Digestion; emulsificaton of fats by enzymes from the pancreas and bile from the liver
Small Intestine - jejunum & ileum
Completion of digestion and absorption
Large Intestine- cecum Absorption of water from liquid residue
Large Intestine - colon Absorption of water from liquid residue
Large Intestine - rectum Storage of feces prior to defecation
Anus Route for defecation of feces outside the body
Organs that make up the oral cavity include the lips, teeth, tongue and major salivary glands. These organs function to obtain and ingest food, fragment it into smaller particles, moisten and swallow it. Teeth will not be covered in this course.
Lips
hypotonic solution containing various enzymes (esp. amylase and lysozyme) and other proteins such as antibodies, glycoproteins as well as electrolytes. Saliva in the buccal cavity is the combined secretion of the numerous salivary glands, both major and minor. The secretions of salivary cells can be either of a serous type, i.e., watery and rich in enzymes and antibodies or mucous, i.e., viscid containing more glycoproteins. Individual salivary glands may contain mostly cells of the serous type, of the mucous type or a mixture of both types. The final composition of saliva at any given time depends on the proportion contributed by specific salivary glands and is determined in the major glands by the parasympathetic nervous system resulting from physical, chemical and psychological stimuli.
Salivary Gland Type of Secretory Cells
Parotid Serous
Sublingual Mucous
Submandibular Mixed
From the esophagus to the anus, the digestive system is basically a tube very similar to other tubular organs in the body. All such tubular organs are composed of several tissue layers arranged around a lumen. In a "generic" tubular organ, these layers are as follows (from the lumen to the ablumenal layer).
lamina epithelialis mucosae: consists only of epithelium lamina propria mucosae: consists of either loose areolar or reticular connective tissue lamina muscularis mucosae: consists of smooth muscle
If the organ is surrounded by other tissues, this layer is called a tunica adventitia and its connective tissue blends with that of the surrounding tissues.
If the organ is suspended in the body cavity, this layer is called a tunica serosa and it is covered by a simple squamous epithelium that is called mesothelium.
The esophagus connects the oral cavity with the stomach allowing and aiding in the movement of food particles to the stomach. It is a muscular tube having the layers described above for the typical tubular organ. In the esophagus the layers are specialized for the function of further fragmenting food particles.
Layers of the Esophagus
o lamina epithelialis: consists of stratified squamous epithelium that can be highly folded in an empty organ; may be highly keratinized in animals that ingest hard, dry materials such as herbivores o lamina propria: consists of loose connective tissue which often has scattered lymph nodules esp. in pigs and humans o lamina muscularis mucosae: consists of smooth muscle; distribution and continuity is highly species variable as follows: (1) continuous in human (2) separate muscle bundles that fuse in horses, ruminants and cats, (3) absent in cervical part in dogs, (4) absent in pigs in cervical region but complete near the stomach
The stomach connects the esophagus to the intestines and in most species serves not only to continue the breakdown of foodstuffs via the use of digestive enzymes and acid but it also as a storage depot for food. Usually food remains in the stomach a few hours during which it is converted into a liquid material called chyme.
Four cell types in the gastric gland
Parasympathetic Ganglia
Aggregations of parasympathetic ganglion cells are found throughout the digestive tube in two locations. Some are located in the submucosa and are usually called Meissner's plexus; others are located between the inner circular and outer longitundinal layers of smooth muscle in the tunica muscularis. The latter ones are usually called myenteric or Auerbach's plexus. Postganglionic fibers from Meissner's plexus innervate the lamina muscularis mucosae whereas postganglionic fibers from the myenteric plexus innervate the smooth muscle of the tunica muscularis. The two layers of smooth muscle in the tunica muscularis inherently contract in a wave of peristalsis that helps move stomach contents toward the small intestine. However, contractions of the smooth muscle are regulated by the autonomic nervous system as well as other factors such as hormones released into the stomach. An increase in peristalsis results from an increase in parasympathetic stimulation; a decrease in peristalsis results from an increase in sympathetic stimulation.
Meissner's plexus and the myenteric plexus both consist of the cell bodies of parasympathetic ganglion cells that are easily identified by their large size in comparison with other cells in the area and also by the large, round nucleus that contains a prominent nucleolus. These cell bodies are found in the midst of unmyelinated nerve fibers and near areas of myelinated axons.
Chamber Histology Function
Rumen
(part of forestomach)
non-glandular; keratinized stratified squamous epithelium
mechanical and chemical breakdown of food; breakdown of food by microbes; production of volatile fatty acids; absorption of volatile fatty acids, lactic acid, ammonia, inorganic ions and water
Reticulum
(part of forestomach)
non-glandular; keratinized stratified squamous epithelium
Omasum
(part of forestomach)
non-glandular; keratinized stratified squamous epithelium
Abomasum glandular; simple columnar glandular epithelium enzymatic digestion
Rumen
Reticulum
Similar to rumen, except as noted below:
o Lamina propria - loose connective tissue rich in blood and lymphatic vessels present in the core of the villi and between crypts o Lamina muscularis mucosae- thin layer of smooth muscle located at the base of the crypts
Enteroendocrine cells: These cells secrete hormones such as secretin, somatostatin, enteroglucagon and serotonin; one hormone per type of cell.
Paneth cells: These remarkable cells contain large granules that contain defensins (antimicrobial peptides) as well as lysozymes and phospholipase A. These chemicals represent the "first-line" of defense against microbes that enter through the digestive tract. Compared to the other cells present in the epithelial lining, Paneth cells are long-lived, i.e., weeks versus a few days for the other cells.
Specializations to enhance absorption ability
The small intestine has all of the "layers" of a typical tubular organ but the tunica mucosa is highly specialized to perform the function of absorption. To fulfill this function it uses several strategies to increase the surface area of the plasma membrane of the absorptive epithelial cells.
Regional variations in the small intestine
Duodenum
Jejunum
Ileum
Unlike the small intestine, there are no plicae circulares or villi in the large intestine so the surface of the tunica mucosa is more uniform and flatter than that of the small intestine.
o lamina epithelialis -simple columnar epithelium that forms straight tubular glands lined with absorptive columnar cells (recovering water and salt) and numerous goblet cells (producing mucus to facilitate passage of dry waste material); stem cells and lymphocytes are also present
o lamina propria - loose connective tissue that contains numerous blood and lymphatic vessels, collagen, lymphocytes and plasma cells
o lamina muscularis mucosae - present beneath the base of the crypts and prominent; undergoes rhythmic contractions mixed in cat
hepatic acinus, might better represent the functional unit of the liver. Both the hepatic lobule and the hepatic acinus will be described but first the basic histology of the liver will be described.
At low magnification the liver looks relatively homogeneous and on first examination little organization can be discerned. A closer look reveals the presence of "lobules" or groups of hepatocytes arranged around a blood vessel, the central vein, and defined by loose connective tissue in which the portal canals are found. This type of organization is most easily seen in the pig liver.
Hepatocytes are one of the primary functional cells of the liver. They are located in flat irregular plates that are arranged radially like the spokes of a wheel around a branch of the hepatic vein, called the central vein or central venule since it really has the structure of a venule.
Portal canal: Three structures are found gouped together in the loose connective tissue surrounding the plates of hepatocytes. These include branches of the hepatic artery, the hepatic portal vein (venule) and the intralobular bile ductule. This group of three structures has been called a portal triad but now is called a portal canal.
Portal canal:
Hepatocytes are arranged in rows that radiate out from the central vein. These rows are one cell wide and are surrounded by sinusoidal capillaries or sinusoids. This arrangement ensures that each hepatocyte is in very close contact with blood flowing through the sinusoids, i.e. bathed in blood.
The endothelial cells lining sinusoids are fenestrated and in most species lack a basal lamina. Gaps are also present between the endothelial cells. Taken together these two properties make the sinusoids extremely leaky and allow for the extremely close contact between the blood and the surface of hepatocytes. Many materials in the blood, except for whole blood cells, can pass between the spaces in the sinusoidal lining.
Although sinusoidal endothelial cells lie very close to hepatocytes, they do not actually make contact. A narrow space is present between the surface of the hepatocyte and the surface of the endothelial cell. This is called the space of Disse ; it is filled with numerous microvilli from the hepatocytes. As in other areas of the body, these structures serve to increase the surface area of the cell membrane that comes in contact with the blood facilitating exchange of molecules between hepatocytes and the blood.
What is the basic functional unit of the liver?
The hepatic acinus has three zones.
o Hepatocytes in Zone 1 are the first to receive blood and it is high in oxygen.
o Hepatocytes in Zone 2 are the second cells to receive blood and it is lower in oxygen.
o Hepatocytes in Zone 3 are the last to receive blood from a branch of the hepatic artery and it is lowest in oxygen.
o Thus, the cells with the highest metabolic potential are found in Zone 1 and those with the least are found in Zone 3. Importantly, the cells in Zone 3 are the most susceptible to ischemic conditions due to the already low level of oxygen that reaches them through the blood.
Secretion of bile in the liver
Bile is produced and secreted by hepatocytes into a special "duct" called a bile canaliculus. This "duct" is actually just a space formed between two hepatocytes that is separated from the connective tissue space around the hepatocytes by the presence of tight junctions. The bile canaliculi empty into branches of the bile ductules which eventually empty into the hepatic duct that carries the bile out of the liver to the gall bladder for concentration and storage. In the duct system, bile flows in the direction opposite to the flow of blood in the sinusoids.
The gall bladder receives bile from the liver. Bile is composed of bile salts that emulsify fats forming water-soluble complexes with lipids (micelles) to facilitate the absorption of fat. Bile salts in the small intestine also activate lipases in the intestine.
Organization of the pancreas
The bulk of the pancreas by volume consists of exocrine cells that secrete an alkaline solution of digestive enzymes. This secretion moves through a duct system that eventually leads to the pancreatic duct. Only about 5% of the volume of the pancreas consists of endocrine cells. These cells secrete peptide hormones that play a role in controlling carbohydrate metabolism. The endocrine cells are closely associated with large numbers of blood capillaries into which they secrete the peptide hormones.
The exocrine pancreas...
The exocrine portion of the pancreas is a compound acinar gland. It has many small lobules, each of which is surrounded by connective tissue septa through which run blood vessels, nerves, lymphatics, and interlobular ducts.
Exocrine secretion by the pancreas is controlled by hormones and nerves.
A compound acinar gland
the secretory cells. These granules are most abundant during fasting or between meals and least abundant after a meal has been ingested.
o Intercalated duct. The first part of the duct system is called the intercalated duct or intralobular duct. It is lined with low cuboidal epithelial cells that secrete bicarbonate ion into the secretory product. This duct actually extends into the acinar lumen, where its walls consist of the pale staining centroacinar cells. Intercalated ducts empty into the larger interlobular ducts.
o Interlobular ducts. These ducts are lined with a low columnar epithelium that may contain goblet cells. Interlobular ducts empty into the main pancreatic ducts that exit the pancreas.
The endocrine pancreas... The cells of the endocrine portion of the pancreas are arranged either in round-to-oval shaped areas rich in blood vessels known as the islets of Langerhans or they may be scattered throughout the exocrine portions of the pancreas near the acini or ducts. There are several different types of cells in the islet or other regions, each secreting a different peptide hormone. It is not possible to distinguish among these cells with routine hematoxylin and eosin stain used for histological preparations. Immunocytochemistry is necessary to identify which cells are secreting a particular peptide. This is done by staining with an antibody made to the specific peptide that is combined with a label that can be visualized at the light microscopic level such as immunoperoxidase.
Examples of peptide hormones secreted by the endocrine pancreas:
The monophyletic theory of hematopoiesis states that pluripotent stem cells multiply to produce more pluripotent stem cells, thus ensuring the steady and lasting supply of stem cells. Some of the pluripotent stem cells differentiate into precursor cells that are at least partially committed to become one type of mature blood cell.
Pluripotent stem cells multiply slowly into one of five possible unipotential stem cells , which then multiply rapidly into the precursor of the specific mature blood cell for which they are destined.
Although the pluripotent stem cells and the unipotential stem cells cannot be distinguished from one another histologically, the precursor cells can be distinguished with a trained and practiced eye.
helpful in distinguishing and identifying the different cells in a bone marrow smear or in an intact bone marrow preparation. Basically an immature, precursor cell goes from a cell that is making lots of protein to a cell that is making much less protein.
Since structure is (always) related to function, the structure of the precursor cell changes as it goes from making more protein to making less protein. Thus, a cell that is making a lot of protein will have a nucleus containing dispersed or active chromatin, i.e. that is being transcribed actively. When this cell is making less protein, the chromatin is condensed or clumped because it is not being transcribed. Likewise, a cell that is making a lot of protein will have many large nucleoli, the site of ribosomal RNA synthesis and assembly; as protein secretion decreases there are smaller and fewer nucleoli. Cells with high protein synthetic activity have more ribosomes in their cytoplasm and consequently the cytoplasm stains more basophilic (hematoxylin staining of the RNA in ribosomes). Cells with lower protein synthetic activity have fewer ribosomes, thus less basophilic staining with hematoxylin leaving the cytoplasm appearing more acidophilic due to the eosin staining of cytoplasmic proteins. In cells with high protein synthetic activity, the Golgi apparatus is highly developed, occupies much of the cytoplasm thus pushing the nucleus off to one side (acentric nucleus). Cells with low protein synthetic activity have a smaller Golgi and the nucleus tends to be more centrally located.
The chart below summarized these features.
Cell Organelle Making lots of Protein Making less Protein
Nucleus chromatin is dispersed chromatin is clumped
Nucleoli more fewer
Cytoplasm more ribosomes; basophilic fewer ribosomes; acidophilic
Golgi apparatus *large; nucleus off center *smaller; nucleus more centered
As the cells are maturing in the erythrocytic series, the cells are usually getting smaller, the nucleus is becoming smaller and more condensed and is eventually lost, and the cytoplasm is becoming pinker rather than blue.
The cells in the developing erythrocyte series are as follows:
