General Overview and Components.
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.  




 Ingestion and fragmentation of food


Fragmentation of food


Fragmentation and swallowing

Salivary Glands

Fragmentation and moistening of food; swallowing


Passage of food from oral cavity to the 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


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.


  1. Filiform papillae are highly keratinized, sharply pointed and aid in mechanically breaking up food material.  They are numerous in ruminants and cats where they are  used in lapping milk.
  2. Fungiform papillae are smooth with a rounded surface.  They help manipulate the food and may also have taste buds on their lateral surfaces.
  3. Conical papillae are somewhat larger than fungiform papillae, are used in manipulating and breaking down ingested food.  They can be distinguished from fungiform papillae by their larger size, tendency to project above other papillae and they do not have taste buds.
  4. Foliate papillae, covered with non-keratinized stratified squamous epithelium, are leaf-shaped structures defined by an invagination of the mucous membrane on their sides.  Many taste buds on their lateral surfaces indicate their role in gustation.  They are absent in ruminants but well developed in the horse and dog.
  5. Circumvallate papillae are the largest (up to 1/8 in diameter) papillae, are surrounded by a deep indentation of the mucous membrane and are not numerous. They do not rise above the surface of the tongue. Many taste buds are located on their sides.  Serous von Ebner's glands empty into the "moat" around these papillae and help keep it free of food particles.













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.



Micrograph of a cat tongue.  Note the the skeletal muscle in the tongue runs in three planes.  (A) horizontal plane or x direction; (B) vertical plane or y direction and (C) plane in z direction -into and out of screen.
(Lab slide 41)

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.


Micrograph of Parotid Salivary Gland
(Lab slide 45). 

Micrograph of Parotid Salivary Gland.  Note small intercalated duct lined with low cuboidal epithelial cells.
(Lab slide 45). 

Micrograph of Parotid Salivary Gland.  Note the striated duct and nearby capillary.
(Lab slide 45). 



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.
(Lab slide 43). 

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). 



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).  

  • Tunica mucosa: This layer is composed of epithelium, connective tissue and muscle.  These tissues can usually be found in distinct layers as follows:
    • 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
  • Tunica submucosa: consists of loose connective tissue, nerves, blood vessels, and glands in some organs 
  • Tunica muscularis: consists of at leasttwo layers, an inner circular and an outer longitudinal with parasympathetic ganglia located between the layers
  • Tunica adventitia or tunica serosa: consists of loose connective tissue.  
    • If the organ is surrounded by other tissues, this layer is called a tunica adventitia and its connective tissueblends 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. 

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Micrograph of cat duodenum: 
illustrating basic plan of the digestive tube  
(Lab slide T).

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

  • Tunica mucosa:
  • 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
  • lamina propria: consists of loose connective tissue which often has scattered lymph nodules esp. in pigs and humans
  • 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) 


  • Tunica submucosa: consists of loose connective tissue that is very elastic allowing for expansion when food is present; ties the overlying epithelium to the underlying muscle layers; seromucous glands present in most species and numerous in the dog but absent in horses and cats.  Lymphoid nodules may be present as show in this micrograph of the pig esophagus. 
  • Tunica muscularis: consists of smooth and/or skeletal muscle; inner circular and outer longitudinal layers usually begin as skeletal muscle at the cervical end (voluntary control of swallowing) changing to smooth near the distal end close to the stomach; skeletal muscle throughout in ruminants and the dog.  
  • Tunica serosa/adventitia: consist of  typical loose connective tissue that blends into the connective tissue of surrounding tissues.

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Micrograph of section of pig esophagus. 
TM = Tunica mucosa; 
TS = Tunica submucosa;
TMus = Tunica muscularis 
( only the inner circular layer is visible in this micrograph) 
(Lab slide 253)


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.  


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:

  • cardia: contains many mucus-secreting glands
  • fundus: mostly glands secreting acid-peptic gastric juices; some mucus-secreting glands
  • pylorus: contains two different types of mucus-secreting glands; also contains endocrine cells secreting gastrin


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

  • Tunica mucosa: in the empty stomach, this layer is thrown into deep longitudinal folds called rugae that extend from lamina muscularis mucosae to the lumen; in full stomach the rugae are much reduced in size as a result of distension of the tunica mucosa to accomodate the presence of a large amount of food material
  • Tunica submucosa: typical loose connective tissue contains parasympathetic ganglia located in submucosal plexuses also known as Meissner's plexuses 
  • Tunica muscularis: typical smooth muscle consisting of at least two layers, an inner circular layer and an outer longitudinal layer; parasympathetic ganglia located between the two muscle layers in the myenteric or Auerbach's plexus
  • Tunica serosa: typical typical small amount of loose connective tissue with overlying simple squamous epithelium or mesothelium

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Wall of the fundic stomach 
(Lab slide 208)

Layers of the Tunica Mucosa of the Stomach - Fundic Region

  • Lamina epithelialis: consists of simple columnar epithelium that forms branched, tubular glands; organized into gastric pits that open onto the lumen and gastric glands that empty into the base of the gastric pits
  • Lamina propria: consists of loose areolar connective tissue that in the glandular stomach is minimal between gastric glands and difficult to see in sections; highly vascular containing many blood and lymphatic capillaries
  • Stratum compactum: consists of dense connective containing thick collagen fibers; located at between the lamina propria and the lamina muscularis mucosae; prominent in carnivores where it probably helps prevent the perforation of the wall of the stomach by sharp objects such as bones that might be present in the lumen
  • Lamina muscularis mucosae: consists of several layers of smooth muscle oriented both longitudinally and circularly; usually not very thick

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Tunica muscosa of the fundic stomach 
(Lab slide 208)


Four cell types in the gastric gland

  • Surface mucous cells: line the gastric pit and secrete mucous and bicarbonate ions to protect the epithelium from digestion by gastric juice (contains HCl and pepsin) present in the stomach lumen
    (not shown in image)
  • Neck mucous cells: found dispersed between the parietal cells;  secrets a mucus that is thinner than that secreted by the surface mucous cells; mucus protects other glandular cells from action of proteases and HCl
  • Parietal (oxyntic) cells:  found throughout the gastric gland; round cells that contain distinct eosinophilic (pink) cytoplasm and round, prominent nucleus;  Secrete HCl and intrinsic factor, needed for absorption of vitamin B12
  • Chief (peptic, zymogenic) cells: found mostly near the base of the gastric glands; very basophilic (purple) containing basally positioned nucleus and prominent basophilic apical cytoplasm filled with many ribosomes;  secrete pepsinogen, which is converted to pepsin in the acidic milieu of the stomach.
  • Neuroendocrine cells: difficult to distinguish by conventional light microscopy; Several types are present; some secrete serotonin, gastrin, glucagon, and somatostatin, among other hormones. (not labelled)
  • Stem cells: located primarily in the neck region; difficult to identify in routine H&E sections; undergo mitosis to form more cells then differentiate into the other cell types present in the gland

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Micrograph of gastric glands in the stomach




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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Micrograph of the tunica muscularis of the fundic stomach 
(Lab slide 208).   Note nucleus of a parasympathetic ganglion cell.

Compound stomach.  

The compound stomach as found in ruminants has four parts.

Chamber Histology Function
(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
(part of forestomach)
non-glandular; keratinized stratified squamous epithelium


(part of forestomach)
non-glandular; keratinized stratified squamous epithelium


Abomasum glandular; simple columnar glandular epithelium enzymatic digestion




  • Tunica mucosa: characterized by the presence of long (1.0-1.5 cm long) conical projections called papillae that extend in to the lumen
    • Lamina epithelialis mucosae - keratinized stratified squamous.

    • Lamina propria - typical; no glands

    • Lamina muscularis mucosae- absent; NOTE:  It is easy to confuse a thickened layer of connective tissue that extends into the papilla with a lamina muscularis mucosae but this tissue is connective tissue, not smooth muscle.

  • Tunica submucosa: merges with lamina
     propria; no glands or lymphoid aggregates.
  • Tunica muscularis: typical
  • Tunica serosa: typical


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Micrograph of the rumen of a cow. 
(Lab slide 55, Masson's trichrome stain)



  Similar to rumen, except as noted  below:

  • Tunica mucosa: When viewed from the lumen of the reticulum, the mucosa looks like a "honeycomb" or reticulum.  The basis of this honeycomb is a series of connected vertical primary folds that give rise to secondary and tertiary papillae which project into the lumen.  

  • Lamina muscularis mucosa:  a layer of smooth muscle extends from the tips of the papillae down to the position of the lamina muscularis mucosa although inthe reticulum this layer is not quite typical.  However, the smooth muscle in the reticulum is continuous with the smooth muscle of the lamina muscularis mucosa in the esophagus.  

  • Other tunics are typical

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Micrograph of reticulum of a cow. 
(Lab slide 56, Masson's trichrome)







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. 





















Micrograph of omasum of a cow showing the numerous primary folds or laminae of the tunica mucosa. 
(Lab slide 57)

Micrograph of omasum of a cow showing the small secondary projections of the lamina epithelialis mucosae.     
(Lab slide 57)

Micrograph of omasum of a cow showing the smooth muscle layers of the primary laminae.  
(Lab slide 57)




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. 
(Lab slide 58)

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.
(Lab slide 58)




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
  • Tunica mucosa: This layer protrudes out into the lumen as projections called villi and it dips down to the underlying lamina muscularis mucosae forming pockets called crypts.

    • Lamina epithelialis mucosae - simple columnar epithelium 

      - 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 - loose connective tissue rich in blood and lymphatic vessels present in the core of the villi and between crypts

    • Lamina muscularis mucosae- thin layer of smooth muscle located at the base of the crypts

  • Tunica submucosa: This layer blends with the lamina
     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: typical consisting of an inner circular layer and an outer longitudinal layer

  • Tunica serosa: typical









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.











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. 




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. 


  •  individual cells have numerous projections of their apical plasma membranes called microvilli

  • the lamina epithelialis and lamina propria together form folds that project out into the lumen called villi


  • the tunica mucosa and tunica submucosa together form large transverse folds into the lumen called plicae circulares 

  • the small intestine is extremely long (usually several meters)


Micrographs of cat small intestine (Lab slide T)

Regional variations in the small intestine:


  • presence of Brunner's glands in the submucosa 

    - serous in pig and horse

    - mucous in ruminant and dog

    -mixed in cat

  • presence of chyme in the small intestine induces cells of Brunner's glands to secrete alkaline mucus that neutralizes gastric acid and pepsin and further promotes digestion

  • no plicae circulares

  • longest villi of all three 

  • regions

  • highest number of goblet cells

Micrographs of cat small intestine (Lab slide T)




  • no glands in the submucosa

  • no lymphoid nodules


Micrographs of cat small intestine (Lab slide T)




  • permanent aggregated lymphoid nodules in the submucosa

  • shortest villi; least number of goblet cells


   Micrographs of cat small intestine (Lab slide T)





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.

  •  Tunica mucosa:  
    - 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
    lamina propria- loose connective tissue that contains numerous blood and lymphatic vessels, collagen, lymphocytes and plasma cells
    - lamina muscualris mucosae- present beneath the base of the crypts and prominent; undergoes rhythmic contractions

  • Tunica submucosa: typical 

  • Tunica muscularis:  inner circular and outer longitudinal layers; outer longitudinal layer is organized into three separate bands known as taenia coli; movement of more solid waste to the rectum

  • Tunica serosa is typical.


Commensal bacteria reside in the large intestine and play a role in the continued digestion of food.  

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Micrographs of pig colon (Lab slide 53)


Copyright 2002 Charlotte L. Ownby
Histology Part 2 Index