Lab 8 GI System Intro

Objectives:

Develop an understanding of the basic morphologic structure of the wall of digestive tube.
Correlate morphologic modifications in the basic pattern of G-I tract with specific functions.
Describe the gross anatomic relationships of the major organs comprising the digestive system in mammals.
Describe basic morphologic features and identify histologic sections of the following organs and/or structures:
1. buccal wall
2. Tongue
3. salivary glands
4. Esophagus
5. simple (monogastric) stomach
6. ruminant forestomachs (rumen, reticulum, and omasum)
7. small intestine (duodenum, jejunum, and ileum)
8. large intestine
9. anal glands

Slides:

#139 Lip & buccal wall of heifer

#143 Developing tooth

#144 Tongue from a dog

#145 Tongue from a cat

#147 Tongue from a cow

#148 Tongue from a rabbit

Unit 1

Buccal Wall - Glass slide #139

Skin:

This is a section of the lip and buccal wall from a calf. Skin on the outer surface is hairy, with modest stratum corneum (stained red).

Topic: Skin

Green arrows = components of hair shafts; Blue arrows = exterior surface epithelium.

Orbicularis muscle:

The middle layer of the buccal wall consists of fascicles of the striated orbicularis oris or buccinator muscle (depending on proximity to the lip) intermixed with connective tissue.

Buccal gland:

Many minor salivary glands in the buccal wall lubricate the oral cavity (along with the major salivary glands). The buccal glands are small, compound tubuloacinar (tubuloalveolar) glands that may be serous, mucous, or mixed; depending upon species and locale. Scan your section and try to identify among the connective tissue septa any ducts that drain these glands through the oral mucosa (in this image near the mucosal surface, middle, between papillae). These glands are often bounded by connective tissue amid fascicles of the buccinator muscle.

Topic: Buccal gland

Blue asterisks = buccal gland; Red asterisk = adipose tissue; Green asterisk = muscle.

Mucocutaneous Jct.:

The mucocutaneous junction demonstrates the profound difference in the thickness and structure of the surfaces. Whereas the hairy outer skin contains relatively delicate epithelium on a base of strong dermis, the oral mucosa consists of denser epithelium. Cornification and surface specializations vary depending on diet and species.

Topic: Mucocutaneous Jct.

blue arrow = mucocutaneous junction

Conical papillae:

Many species have papillae that help manage mastication. These conical papillae help prevent slippage of foods toward the lips.

Topic: Conical papillae

Blue arrows = conical papillae

Developing tooth - glass slide #143

Dental sac:

This section depicts the encapsulated dental sac. It is a delicate cellular membrane filled mostly with mesenchyme. As the dental papilla and the enamel organ develop, they displace the mesenchyme within the sac. It is typically less distinct near the tooth's root, where it helps form the cementum and stimulates formation of alveolar bone. Apically, the margin of the sac is incorporated into the outer enamel epithelium as those cells differentiate. Locate the vessels and nerves at the base of the dental papilla. In this example the lower papilla has cracked.

Topic: Dental sac

Blue asterisk = mesenchyme within the dental sac. Red asterisk = a crack in the dental papilla - artifact of preparation; Green line = general position of the margins of the dental sac, which are difficult to define near the base of this developing tooth.

Enamel organ:

At the edges of the dental sac find the enamel organ, which forms an inverted cup over the dental papilla. On the apical side the enamel organ is covered with mesenchyme, while near the root it is surrounded by developing bone.

Topic: Enamel organ

Green line = approximate position of the basement membrane separating dentin from enamel. Red arrows = outer edges of the enamel organ; Blue arrows = odontoblast layer

Dental papilla

The developing dental papilla consists of cells that are similar to mesenchyme outside the dental sac, though perhaps slightly more dense. These cells are programmed to mold the enamel organ into the desired shape and size.

Nerve:

the dental papilla contains nerves, arteries and veins to supply the developing tooth.

Topic: Nerve

Red arrow = nerves.

Alveolar bone:

Early alveolar bone takes the form of spicules in which the osteoid ground substance is not extensively mineralized. [With triple stain the non-mineralized ground substance is blue, mineralized is red.]

Topic: Alveolar bone

Blue arrows = spicules of alveolar bone.

Dental Sac:

At higher power, the margins of the dental sac are seen to be irregular connective tissue cells forming an incomplete separation between the mesenchyme above and the stellate reticulum, below. Near the apex of the tooth these cells collapse around the inner enamel epithelium, contributing to the enamel organ.

Topic: Dental Sac

Extent of the dental sac in this specimen.

Dental papilla:

The dental papilla forms the shape of the developing tooth and is covered by the double-layered enamel organ.

Odontoblasts:

Odontoblasts develop from the dental papilla (below and left in this example). Black arrows indicate typical odontoblasts. They produce a matrix that mineralizes to form dentin. In this image the dentin matrix nearest the cells stains blue, indicating the presence of collagen. This is sometimes referred to as 'pre-dentin'. Older, more mature dentin stains red as the matrix begins to mineralize.

Topic: Odontoblasts

Between the blue arrows are the cells that form the mineralized components of teeth; Green line = approximate location of the basement membrane dividing dentin from enamel. Black arrows in the zooming image = odontoblasts.

Dentin:

In this image the black arrows indicate the oldest dentin. Early in development it induces ameloblast differentiation by the inner enamel epithelium. Later, when enamel is present, the dentin contacts the enamel, separated only by remnants of a basement membrane. The space in this portion of the zooming image (ie. in which the black arrows are found) is an artifact of preparation.

Topic: Dentin

Ameloblasts

The ameloblast layer in this portion of the image shows their columnar shape, although they are somewhat separated from the deep red enamel that they elaborate. Ameloblasts differentiate from the inner enamel epithelium, present in earlier stages of tooth development.

Enamel

Black arrows indicate eamel. In this location the ameloblast layer is disrupted where it was pulled from the enamel.

Stellate reticulum:

The stellate reticulum lies between the inner and outer enamel epithelia. In this example the inner enamel epithelium had differentiated into mature ameloblasts.

Topic: Stellate reticulum

Components of the enamel organ: Green arrow = ameloblast layer; Blue arrow = stellate reticulum; Red arrow = outer enamel epithelium.

Outer enamel epith.:

Black arrows in the zooming image indicate the outer enamel epithelium. Only several cells thick, it forms a capsule around the enamel organ.

Topic: Outer enamel epith.

Green arrow = ameloblast layer; Blue arrow = stellate reticulum; Red arrow = outer enamel epithelium.

Tongue - Glass slide #144

Filiform papilla:

Note the epithelium of the ventral aspect of the tongue has a smooth non-cornified surface while the dorsum of the organ is covered with many epithelial projections called papillae. The papillae are variable in shape; most have a keratinized surface, and a central core comprised of connective tissue derived from the underlying lamina propria. In this example, most of the papillae are filiform. Realize that in a coronal section such as this, the curvature of the posteriorly-directed filiform papillae is not visible because the tips of the papillae will project above or below the section.

Topic: Filiform papilla

Green asterisk = fungiform papilla; Blue arrows = filiform papillae.

Fungiform papilla:

Rarely are gustatory glands found in this region of the tongue. In this example fungiform papillae are visible. Look for similar structures on your glass slide 144 and examine these papillae for taste buds (described later).

Topic: Fungiform papilla

Green asterisk = fungiform papilla; Blue arrows = filiform papillae.

Ventral surface:

The ventral surface of the tongue contains none of the specializations found on the dorsal side.

Tongue - Glass Slide #144

The greater portion of the tongue is filled with striated muscle fibers. In this example note that the fibers run in all three anatomical planes. The muscle is innervated by the myelinated hypoglossal nerve (Cranial nerve 12), branches of which can be observed scattered throughout the tongue. In some sections, non-myelinated nerves and associated autonomic ganglia can be observed. Numerous filiform papillae are present in cross section.

Cat tongue - glass slide #145

Filiform papillae:

Glass Slide 145 is a longitudinal section of the tongue from a cat. Here we observe filiform papillae cut in longitudinal section. These heavily cornified papillae project caudally above the surface of the tongue and serve a mechanical function in the movement of food within the oral cavity. Note the non-cornified secondary papillae located anterior to the primary papillae that serve to buttress the main structure. Compare the configuration of these longitudinal sections to the cross sections of filiform papillae observed on glass Slide 144.

Bovine tongue - glass slide #147

Moat:

This slide contains a portion of the vallate papilla from a calf. Vallate (circumvallate) papillae are located in a V configuration near the caudal aspect of the tongue. On glass Slide 147, note the circular furrow or moat that surrounds the papilla (in this example only the left-side furrow is visible).

Topic: Moat

Blue arrows = crevice of the moat.

Gustatory glands:

Serous glands and their associated ducts can be observed in the lamina propria directly below the vallate papillae. These gustatory glands (glands of von Ebner) are associated with papillae that possess taste buds. The secretion of these glands provides an aqueous medium by which the molecules that give rise to the sensation of taste are conveyed to the taste buds.

Topic: Gustatory glands

Green asterisks = gustatory glands; blue arrows = ducts

Taste bud:

An aggregate of taste buds is located on the lateral surface of the papilla. In this example only one full taste bud is sectioned.

Topic: Taste bud

Green outline = portion of image containing the papilla; Blue arrow = center of the papilla; Red arrow = taste bud.

Salivary glands:

These lingual glands are classified as minor salivary glands. Note that they may originate deeper than the gustatory glands.

Topic: Salivary glands

Green asterisks = salivary glands

Salivary ducts:

Numerous ducts are present - find and identify.

Topic: Salivary ducts

Blue arrows = salivary ducts

Plasma cells

What is the significance of the large number of plasma cells located in the connective tissue septa of the glandular units in this section?

Center of papilla:

In our class preparations, some papillae have a visible center, others do not.

Rabbit Tongue - Glass slide #148

Foliate papillae:

This is a section of rabbit tongue with leaf-like foliate papillae. These non-cornified folds of lingual mucosa are located on the caudo-lateral aspect of the tongue of some species. Find these structures on slide 148.

Topic: Foliate papillae

Foliate papillae outlined in blue.

Taste buds:

Pale staining sensory structures, called taste buds, are well represented here. Recall they are also present on the upper surface of fungiform papillae and lateral wall of vallate papillae (e.g., slides #144 and 147). Taste buds are ellipsoidal structures with spindle shaped cells at the periphery which extend from the basement membrane of the epithelium to an opening at the surface called the taste pore. Centrally located cells possess receptor-laden microvilli that project toward the taste pore.

Topic: Taste buds

Blue arrows = taste buds; red arrows = taste pores.

Gustatory glands:

As noted in the calf tongue, gustatory glands and ducts may be found deep to the sensory structures.

Topic: Gustatory glands

Green asterisks = gustatory glands; Red arrows = gustatory ducts; Blue arrows = taste buds.

Sensory nerve:

Numerous sensory nerves serving the taste buds are present in the connective tissue around gustatory glands.

Topic: Sensory nerve

Blue arrows = sensory nerves

Dog Parotid Gland - Glass slide #140

Parotid septa:

Glass slide #140 is a section of a canine parotid gland. A portion of the collagenous capsule of the gland is visible giving rise to connective tissue septa, which divide the gland into irregular lobules. Major vascular supply and exocrine ducts travel within the septa.

Topic: Parotid septa

Blue arrows = collagenous septa; Red asterisks = parotid lobules

Interlobular duct:

The connective tissue investments of vasculature and ducts may be delicate or may be heavy. These interlobular ducts are obviously within large C.T. septa. Note the duct epithelium is either simple columnar or stratified cuboidal/columnar in type.

Topic: Interlobular duct

Blue arrows = interlobular ducts.

Dog Parotid Gland - Glass slide #140

Serous acini:

The parotid gland is a serous salivary gland but a few mixed mucoserous acini can often also be observed. The acini are closely spaced, which makes it diffucult to delineate one from another.

Topic: Serous acini

Green line = approximate outline of one parotid acinus; Blue arrow = small central lumen of acinus.

Striated ducts:

Striated ducts consist of simple columnar epithelium that has prominent acidophilic stria or puncta in the basal cytoplasm. These are aggregates of mitochondria, which supply the needed energy for the secretory activity of these cells.

Topic: Striated ducts

Blue arrows = basal acidophilic striations.

Intercalated duct:

Intercalated ducts are typically short. They originate in the acini and give rise to the striated ducts.

Topic: Intercalated duct

Blue arrows = intercalated duct.

Interlobular duct:

Compare the duct epithelial lining of this familiar interlobular duct with a striated and intercalated duct.

Topic: Interlobular duct

blue arrows = interlobular duct

Mandibular gland - glass slide #141

Mucous acini:

The duct system, innervation, vasculature, and connective tissue stroma of the mandibular gland are similar to the parotid gland. However, the secretions of the mandibular gland are primarily mucus in the dog although many of the acini contain serous demilune cells. In these mixed acini, mucous secreting cells occupy the center while the serous demilune cells are found at the periphery. Also present are scattered serous acini, although many fewer serous than mixed acini.

Serous demilune cells:

Serous demilune cells can be observed at the periphery of many acini in the mandibular gland. Note the compressed nuclei of the mucous cells are typically displaced to the base of the cell. The apical portions of these cells contain a non-staining mucus. The cytoplasm of serous demilune cells stains more intensely than mucous-producing cells and the nuclei are more central with some euchromatin and nucleoli. The secretion of the serous demilune cells is conveyed to the lumen of the acinus via small intercellular canaliculi.

Topic: Serous demilune cells

Blue arrows = serous cells forming demilunes

Striated duct:

Striated ducts are very similar to those found in the parotid gland.

Topic: Mucous acini

Blue asterisks = mucolus acini; Red arrows = striated duct

Topic: Striated duct

Blue asterisks = mucolus acini; Red arrows = striated duct

Intercalated duct:

Intercalated ducts are the first channel that exits the acini. They are secretory, helping to dilute the secretory products of the glands. Note the simple cuboidal epithelial type.

Topic: Intercalated duct

Blue arrows = intercalated duct

Interlobular duct:

Mandibular gland stroma are less dense than the parotid. This interlobular duct has both simple columnar and stratified cuboidal epithelium, just as we observed in the parotid.

Detail of the mandibular gland - glass slide #141

Myoepithelial cell:

Myoepithelial cells resemble smooth muscle cells in their staining properties and nuclear morphology. These examples were generated by digitally combining three different focal planes at high power. The thin cytoplasmic processes of myoepithelial cells may be difficult to follow around the outside of the acini except in exceptional examples. Contrast the cytoplasmic shapes, staining, and nuclear morphology with that of serous demilune cells.

Prime examples of the intralobular ducts are also present here.

A myoepithelial cell - nucleus and cytoplasm resemble smooth muscle.

Black arrows -> myoepithelial cells. Red arrows -> serous demilune cells.

Striated duct:

Striated duct in mandibular gland of the dog. Black arrows -> myoepithelial cell cytoplasm.

Intercalated duct:

Intercalated duct in mandibular gland of the dog.

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