|
Organ |
FuFunctionnction |
| Ingestion and fragmentation of food | |
|
Teeth |
Fragmentation of food |
| Fragmentation and swallowing | |
| Fragmentation and moistening of food; swallowing | |
| Passage of food from oral cavity to the stomach | |
| Completion of fragmentation and beginning of digestion | |
| Digestion; emulsificaton of fats by enzymes from the pancreas and bile from the liver | |
| Completion of digestion and absorption | |
|
Large Intestine- cecum |
Absorption of water from liquid residue |
| Absorption of water from liquid residue | |
|
Large Intestine - rectum |
Storage of feces prior to defecation |
|
Anus |
Route for defecation of feces outside the body |
Oral Cavity. 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.
| The lips aid in obtaining food and placing it in the mouth so that the teeth and tongue can manipulate it and begin fragmenting it. Lips are covered by a stratified squamous epithelium that is usually keratinized on the outer surface and contains many hairs whereas the epithelium on the inner surface is more moist and non-keratinized. |  |
Tongue. The tongue is a highly muscular organ used to manipulate food in the mouth and for the sense of taste. One diagnostic feature of the tongue is the presence of skeletal muscle that is arranged in three planes or directions. It is covered with stratified squamous epithelium that in the anterior part forms specialized structures known as papillae useful in the manipulation of food as well as in the sense of taste. The types, numbers and distribution of papillae in the tongue varies greatly among species. In domestic animals there are usually five different types of papillae.
Skeletal muscle in the tongue: A diagnostic feature of the tongue is that the skeletal muscle is oriented in three different planes. In any section of the tongue skeletal muscle fibers can be seen that orient along planes in the X, Y and Z directions as seen in the adjacent micrograph. |
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Salivary glands. The salivary glands all empty their secretions into the buccal cavity. They vary as to their distance from the buccal cavity, their size and the nature of their secretory products. They can also be divided into major and minor gands. We will consider only the major salivary glands of which there are three: parotid, sublingual and submandibular. These glands all have the tubuloalveolar glandular structure and all are compound, i.e., composed of numerous secretory endpieces connected by an elaborate system of branching ducts. In general saliva is a dilute, 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 |
|
Parotid Salivary Gland
This salivary gland is composed primarily of serous secretory units. These units are organized into lobules that are separated and defined by loose connective septa containing nerves, blood vessels and the larger secretory ducts.
Serous cells secrete their watery product directly into intercalated ducts lined with low cuboidal epithelium that is continuous with the cells of the secretory units, sometimes called acini or alveoli.
From the intercalated duct the watery secretion passes through the larger striated ducts that are lined with a simple columnar epithelium. These epithelial cells are packed with basal infoldings between which lie numerous mitochondria. This peculiar feature gives rise to the striated appearance of these lining cells, hence the name striated duct. Structures such as this support the epithelial cell's active role in water and ion transport; the same type of fine structure is observed in kidney tubule cells. The large amount of cell membrane surface for transport of ions is coupled with the large numbers of mitochondria close by to provide the needed energy for such transport.
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Micrograph of Parotid Salivary
Gland
Micrograph of Parotid Salivary
Gland. Note small intercalated duct lined with low cuboidal epithelial
cells.
Micrograph of Parotid
Salivary Gland. Note the striated duct and nearby capillary.
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Submandibular Salivary Gland
The submandibular salivary gland is a mixed gland, containing secretory cells that are either serous or mucous. The overall organization of the gland is the same as the parotid, i.e., lobules of secretory units surrounded and separated from each other by loose connective septae containing nerves, blood vessels and the secretory ducts.
As in other glands of the mixed type, the secretory units or endpieces are usually arranged so that the mucous-secreting cells form the main lining of the unit and the serous-secreting cells lie on the periphery of the mucous cells. The few serous-secreting cells grouped together are called a "serous demilune".
Each acinus or secretory units is surrounded by a network of cells called myoepithelial cells because although they are epithelial in nature, they are also contractile. These small, flat cells lie between the serous and mucous glandular cells and their underlying basement membrane. They send out long processes to surround the acinus and upon stimulation, contract and squeeze the contents of the lumen of the acinus into the duct system.
|
Micrographs of Submandibular
Salivary Gland, a mixed gland containing both serous and mucous secretory
units.
Micrographs of Submandibular Salivary Gland. Note the arrangement of the serous demilunes peripheral to the mucous-secreting cells of the mixed secretory unit or acinus. (Lab slide 43).
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A tubular organ. From the esophagus to the anus, the digestive 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).
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Esophagus 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
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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.
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Stomachs are either simple or compound, i.e., consisting of one chamber or many chambers. Simple stomachs are composed primarily of glands, that is the tunica mucosa is filled with glands.
Ruminant stomachs are compound stomachs containing both non-glandular and glandular regions. The non-glandular regions include the reticulum, rumen and the omasum. The glandular region is the abomasum which has its own regions similar to those found in a simple stomach.
Regional variation in the glands of the tunica mucosa of the stomach Not all regions stomach mucosa have the same histological structure. They vary as follows:
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Diagram
of Simple Stomach
Diagram
of Compound Stomach: Reticulum, Rumen and Omasum are non-glandular
regions.
Diagram
of Compound Stomach: Abomasum |
Wall of the Glandular Stomach
|
Wall of the fundic
stomach |
Layers of the Tunica Mucosa of the Stomach - Fundic Region
|
Tunica muscosa of the
fundic stomach |
Four cell types in the gastric gland
|
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Parasympathetic Ganglia Aggregations of parasympathetic ganglion cells are found in 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 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 (see adjacent image).
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The compound stomach as found in ruminants has four parts.
| 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:
|
|
| Omasum
This part of the non-glandular region of the compound stomach is notable for the complexity of the foldings of its tunica mucosa.
These folds or laminae are covered with a highly keratinized stratified squamous epithelium.
Underlying this epithelium is the sparse loose connective tissue of the lamina propria.
The laminae muscularis mucosae extends into the primary laminae usually in two layers. In between these two layers of the laminae muscularis mucosae there is a layer of smooth muscle belonging to the tunica muscularis. These three layers of smooth muscle intertwine as they extend toward the tip of the laminae and eventually fuse to form one large mass of muscle at the tip.
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Micrograph of
omasum of a cow showing the numerous primary folds or laminae of the
tunica mucosa.
Micrograph of
omasum of a cow showing the small secondary projections of the lamina
epithelialis mucosae.
Micrograph of
omasum of a cow showing the smooth muscle layers of the primary laminae.
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|
Abomasum The abomasum is the glandular part of the compound stomach and histologically it is essentially the same as a simple stomach.
Micrograph of abomasum of a
cow.
Micrographs
of the tunica mucosa of the abomasum of a
cow. Left micrograph shows the apical region of the mucosal epithelium
and the right micrograph shows the basal region. Note presence of
lymphocytes.
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The small intestine is a typical tubular organ in that it has all of the typical tunics and layers. However, the tunica mucosa is especially modified to fulfill the function of absorption. Also, the three regions of the small intestine, the duodenum, the jejunum, and the ileum, each have special modifications to the wall to enable each region to better perform its particular function. In the small intestine digestion occurs in the lumen as well as at the surface of the lining epithelial cells. Pancreatic enzymes such as trypsin, chymotrypsin, elastase, carboxypeptidases, peptide hydrolases, amylase and lipases are adsorbed onto the membrane surface of the epithelial cells where they mix with the chyme present in the lumen catalyzing the breakdown of proteins, carbohydrates and lipids. The smaller breakdown products are then absorbed by the lining epithelial cells called enterocytes.
Layers
of the Small Intestine
- villus - a villus contains enterocytes (absorption), goblet cells (protective mucus) in its upper region and neuroendocrine cells (local hormones) - crypt - a crypt (crypt of Lieberkühn) contains goblet cells, paneth cells (defensive), neuroendocrine cells, stem cells, intraepithelial lymphocytes (defensive) Lamina propria - Lamina muscularis mucosae- Tunica submucosa: propria and is typical. In the duodenum it has coiled branched glands known as Brunner's glands, the ducts of which open into the base of the crypts. Tunica muscularis: Tunica serosa:
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 bodies
"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.
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Micrograph of cat small intestine (Lab slide T) showing goblet cells and enterocytes in a villus.
Micrograph of cat small intestine (Lab slide T) showing enteroendocrine cells in the crypts.
Micrograph small intestine showing Paneth cells. Note the prominent pink (eosinophilic) granules.
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| 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.
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Micrographs of cat small intestine (Lab slide T) |
Regional variations in the small intestine:
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Micrographs of cat small intestine (Lab slide T)
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Micrographs of cat small intestine (Lab slide T)
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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.
Commensal bacteria reside in the large intestine and play a role in the continued digestion of food. |
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Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index
Micrographs of cat small
intestine (Lab slide T)
