Digestive System Study Guide

Samantha Blum

The Lip: Red region• The redness in this region comes from the presence of capillaries within the dermal papillae, which are

unusually deep (i.e. close to the surface) here. • Note the keratinized stratified squamous epithelium (4X, 10X). • Identify pronounced dermal papillae (4X, 10X). • Note the absence of sebaceous glands and salivary glands. How does the lip stay moist?

– We unconsciously lick our lips!

Dermal papillae

The Lip: Oral region• Note the non-keratinized stratified squamous epithelium (10X). • Identify lamina propria (10X) of the mucosa. • Identify the labial glands (4X, 20X) in the submucosa; these are minor salivary glands. • Compare the thickness of the oral epithelium with that of the other two regions.

– The oral epithelium is thicker than the red region

The Lip: Skeletal muscle core• Observe that the three lip surfaces surround a core of muscle (4X, 10X, orbicularis oris) and

elastic fibroconnective tissue (10X).

The Tongue: Mucosa• The mucosa of the upper surface (anterior two-thirds) of the tongue has

many lingual papillae, which are projections of mucosal epithelium with a lamina propria core. Some types of papillae contain taste buds within their epithelium. The lower surface of the tongue lacks papillae. The posterior third of the upper tongue surface has an irregular, nodulated surface (lingual tonsils).

The Tongue: Mucosal epithelium• Note the stratified squamous epithelium, keratinized in some regions, present at the papillated upper

(dorsal) surface (4X). • Note that the lower (ventral) surface (4X) is a non-keratinized stratified squamous epithelium. • Identify filiform (4X, 10X), fungiform (4X, 10X), foliate (4X, 4X, 10X), and circumvallate papillae (4X).

Note the taste buds down the side of the foliate papillae

Foliate papillae with taste buds

The Tongue: Lamina Propria• Note that the lamina propria is just deep to the mucosal epithelium. • Notice that it extends into papillae (5X, 20X). • Look for diffuse lymphatic tissue (5X, 20X, 20X). • Identify blood vessels (10X, 10X) and nerves (40X). • Compare the lamina propria of dorsal and ventral surfaces; which is thinner?

The Tongue: MuscularisNerves. Identify nerves (10X, 20X, 40X) within the body of the tongue.Blood Vessels. Notice the great supply of blood vessels (10X, 10X) to this area.

Salivary Glands:Parotid Gland

• Examine the general structure of a parotid gland, and identify the fibrous capsule and septa (4X). • Note that the gland is divided into lobes and lobules (4X) by septa. • Identify secretory alveoli (50X). Note that they are comprised entirely of serous-secreting cells. • Note that the secretory cells (100X) of the acini have centrally-located nuclei, and granules in their

cytoplasm.

Secretory alveolus circled

Salivary Glands:Submandibular Gland

• Note that this gland contains both serous and mucous acini (20X), but that serous acini predominate. • Identify serous demilunes (50X) within some mucous acini. Would the secretory product of such an acinus

be mucous or serous? • Compare (50X) the nuclear positions and staining characteristics of mucous and serous cells. • Note that mucous acini generally have a clearly visible lumen, while serous acini do not. • Identify myoepithelial cells (100X). • Be careful not to confuse mucous cells with adipocytes (50X), which are also present here.

Salivary Glands:Sublingual Gland

• Note that this is also a mixed sero-mucous salivary gland, but that mucous acini predominate (20X). • Identify purely serous acini, purely mucous acini (40X), and mixed acini (40X, mucous with serous

demilunes). • Identify the basement membranes (100X) of these serous and mucous secreting epithelial cells.

Teeth: Enamel• Ameloblasts are the cells that produce the acellular material called enamel. Enamel is produced only

before the tooth erupts, at which point the ameloblasts degenerate. Therefore, you should use slide O-068 (developing tooth) to study ameloblasts.

• Identify the cells that comprise the enamel organ (4X, 10X). These epithelial cells give rise to the ameloblasts.

• Identify ameloblasts (40X) at the periphery of the crown. These are the tall columnar cells with apical projections (Tomes' processes, 100X) that border on the enamel matrix (40X, 100X).

Developing tooth

Teeth: Dentin• Dentin is produced throughout life, so you can use both slides (developing and adult tooth) to study the

cells that produce it, the odontoblasts. These cells line the pulp cavity (10X), and give rise to the dentin (4X, 10X, 40X).

• Identify dentinal tubules (40X). These structures surround the odontoblast processes, which extend from the cell apex to the dentin-enamel junction. Look for the tubules in both cross-sections and longitudinal sections within the dentin matrix.

Teeth: Pulp cavity• Note the type of connective tissue that fills the pulp cavity (4X, 10X, 20X). • Identify the root canal (10X; 10X). • Identify blood vessels (20X). • Identify odontoblasts (10X, 40X), which line the cavity. • Where are the ameloblasts and odontoblasts located, with respect to the dentin of the tooth?

Soft Palate: Mucosa• This structure has both a nasal (respiratory) surface and an oral surface, with a core of skeletal muscle

posteriorly (soft palate) or bone anteriorly (hard palate). Mucosa. • Note that, in humans, the mucosa of the oral surface is covered by a non-keratinized or para-keratinized

stratified squamous epithelium (4X, 20X, 50X). • Identify the lamina propria (4X, 20X). Large numbers of un-named salivary glands are located here (1X, 2X,

5X), as they are throughout the oral mucosa. • Note that the epithelium of the respiratory surface (4X, 20X, 100X) is pseudostratified ciliated columnar.

Epiglottis• This structure has both a nasal (respiratory) surface and an oral surface, with a core of elastic cartilage. Mucosa. • Identify the thick stratified squamous epithelium that covers the oral side of the epiglottis (20X). • Identify the pseudostratified columnar epithelium that covers the respiratory side (20X); this side faces the

trachea, which the epiglottis covers during swallowing.

Oral side Respiratory side

Pharynx• This structure connects the oral and nasal cavities with the esophagus and larynx. It is divided into three

regions: nasopharynx, oropharynx, and laryngopharynx. Mucosa. • Note that the epithelium (10X, 40X) of this section of pharynx is non-keratinized stratified squamous. • Note the pronounced fibroelastic lamina propria (10X, 40X).

Lingual Tonsils• The lingual tonsils (4X, 4X, 40X, 4X, 40X) are located behind the sulcus terminalis,

on the dorsal surface of the tongue.

Palatine Tonsils• The palatine tonsils (4X, 4X, 10X, 40X) are located between the palatoglossal and

palatopharyngeal folds. • Note the type of surface epithelium (10X) that covers this tonsil.

Pharyngeal Tonsils• Also known as adenoids, the pharyngeal tonsils are located within the

nasopharynx (4X, 20X, 40X) . • Note that the surface epithelium here (20X, 40X) is pseudostratified columnar,

because it is located within the nasal cavity.

Esophagus:Mucosa

Epithelium. • Note the presence of non-keratinized stratified squamous epithelium (4X, 10X).Lamina propria. • Note the presence of areolar C.T. (20X) • Identify lymphocytes in lamina propria, both diffuse (10X) and in small aggregations (4X). • Identify mucous glands, also known as cardiac glands, 4X, 10X) in the upper 1/3 and in the lower 1/3 of

the esophagus.

Esophagus:Submucosa

• Note the presence of dense irregular C.T. (20X) • Identify esophageal mucous glands (4X, 40X) in the submucosa; these glands are most abundant in the

lower esophagus. • Try to find ducts of esophageal glands coursing through more superficial layers to the esophageal lumen. • Note that some mucous glands are also present in the lamina propria of the lower esophagus.

Esophagus:Muscularis Externa

• The type of muscle present in this layer varies with the region of esophagus. Skeletal muscle predominates in the upper esophagus, and is gradually replaced with smooth muscle caudally. Why?

Upper 1/3 of Esophagus• Note that, at this level, the entire external muscular layer is composed of skeletal muscle (this specimen

is from the rat).• Identify fascicles of the thin muscularis mucosa, which is composed of smooth muscle (20X).• Identify the two layers of skeletal muscle in the external muscularis, and note that the fibers lie at right-

angles to one another (40X). This structural arrangement is found throughout the digestive tract, and permits the production of peristaltic waves of contraction that force the food through the alimentary tube.

Esophagus

Serosal membrane

Adventitia

LUMEN

Stomach:Submucosa

• Note that a typical loose C.T. submucosa is present in the stomach (4X, 10X). • Identify blood vessels and lymphatics here. • A Meissner's plexus is present within the submucosa, but its component ganglia are small and infrequent (

5X, 20X); they are best seen in the pyloric region of slide O-027.

Stomach:Serosa

• This mesothelial layer invests the outermost loose C.T. layer of stomach wall (1X, 20X, 40X, 100X). • Identify blood vessels, lymphatics and nerves (10X, 20X).

Transition From Stomach to Duodenum

Small Intestine: Mucosa• The mucosa of the small intestine is thrown into large folds that are grossly visible, the plicae circulares (

4X). At the microscopic level, the surface epithelium is evaginated into villi (4X, 10X, 10X, 10X).

Small Intestine: Lamina Propria• As in the stomach, this lamina propria contains

areolar C.T. elements and mucosal digestive glands. The crypts of Lieberkühn (10X), or intestinal glands, begin at the bases of the villi and extend through the lamina propria toward the muscularis mucosa. The glands (20X) are lined by the same epithelium (100X) that covers the villi. Identify Paneth cells (20X, 100X, 100X) in the bases of the glands. These cells release lysozyme, an enzyme that digests the cell walls of some bacteria; they are, therefore, thought to have an antibacterial function.

• Identify solitary lymphoid nodules (4X, 10X) and Peyer's patches (4X, 10X) in the lamina propria. The latter are often especially pronounced in ileum.

Small Intestine: Muscularis mucosae

• Identify inner circular smooth muscle and outer longitudinal smooth muscle (50X).

Small Intestine: Submucosa• Note that Brunner's glands (4X, 10X, 20X) are abundant in the duodenal submucosa. • Try to find ducts of Brunner's glands (50X) penetrating the muscularis mucosa and emptying into an

intestinal crypt or gland. • Identify submucosal (Meissner's) plexus (20X, 50X).

Small Intestine: Serosa• Identify this outermost layer (40X, 50X), that covers the small intestine.

Small Intestine: Appendix• The vermiform ("worm-like") appendix is a blind-ended tube, 2-4 inches in length, that evaginates from the

caecum (dissection; dissection). Its structure is similar to that of the rest of the digestive tube, although its glands are shorter, it contains abundant large lymphoid nodules, and the outer muscular wall does not have taenia coli (1X, 5X).

• Long considered vestigial, the appendix may function as a "safe house" for the beneficial bacteria living in the human gut. According to this hypothesis, the beneficial bacteria in the appendix that aid digestion can ride out a bout of diarrhea that completely evacuates the intestines and emerge afterwards to repopulate the gut.

Large Intestine: Mucosa

• The large intestine lacks villi. The surface epithelium invaginates into the intestinal glands.

Large Intestine: Epithelium• Note the increase in abundance of goblet cells (10X, 10X, 40X, 40X, 40X), relative to the small intestine. • Compare the absorbing cells (enterocytes) of large intestine with those of small intestine (100X, 100X).

Note that the striate border (100X) is lower (thinner). • Paneth cells are occasionally found in the large intestine, but not in the large numbers that characterize

the small intestine. • Compare the surface epithelia of different regions (10X, 40X) of large intestine. • Classify the epithelium (20X) that is present at the anal valves.

Large Intestine: Muscularis mucosae

• Identify inner circular and outer longitudinal layers (50X).

Large Intestine: Submucosa• Note that this is similar to the submucosa of small intestine. • In the anal mucosa, identify the venous plexus (4X); enlargements of these vessels are called hemorrhoids.

Large Intestine: Muscularis externa• Note that the colon has a complete inner circular muscle layer, but that the outer longitudinal muscular

layer is incomplete. In a cross-section of the colon, look for three bands of longitudinal muscle, the taenia coli (photo, 1X, 4X).

• Note that, in the anal canal, the inner circular layer is complete, and enlarges to form the internal anal sphincter (schematic). The complete outer longitudinal layer attaches to connective tissue. Unlike the internal sphincter, the levator ani muscle and external anal sphincter contain skeletal muscle.

• Identify myenteric (Auerbach's) plexus (4X, 4X) between smooth muscle layers of the muscularis externa.

Large Intestine: Serosa & Adventitia

• Colon, appendix and part of the rectum are covered by peritoneum (20X). The anal canal and retroperitoneal portions of rectum lack a serosal covering and have only a loose C.T. adventitia (4X) instead. Verify this on your slides.

Colon, appendix, part of rectum Anal canal & retroperitoneal part of rectum

Liver: Hepatic lobules• These are the microscopic anatomical units of liver, and are roughly in the shape of hexagonal prisms (4X).

Identify the central vein (10X, 50X) at the center of the lobule. • Find portal triads (20X, 20X) at the periphery of lobules. Identify portal vein, hepatic artery, and bile duct. • Identify plates of hepatocytes (50X, 50X) in the stroma of the lobule. • While the liver lobule is easier to visualize, the liver acinus more accurately represents the basic unit of

liver function; centered around a portal tract, it is defined by several zones that differ in their susceptibility to ischemic injury.

Liver: Parenchyma• Hepatic cells (hepatocytes) are arranged in anastomosing plates (100X) that appear as cords in cross

section. Between plates are the liver sinusoids (100X). • Note that the plates are usually one cell thick (100X), such that cells are exposed on two sides to blood. • Note that the hepatocytes are polygonal, with spherical or ovoid nuclei (100X) that may vary considerably

in size from one cell to another. Hepatocytes may be binucleate (100X).

Liver: Blood channels• Sinusoidal spaces surround the plates of hepatocytes. • Note that sinusoidal spaces are much larger than capillaries. Look for erythrocytes (100X, 100X, EM) in

the sinusoids. • Study the two cell types lining the sinusoidal spaces: endothelial-type cells (100X, 100X) and Kupffer cells

(100X). Endothelial-type cells form an incomplete sinusoidal lining, with fenestrae much larger than those of type II capillary endothelium. Kupffer (stellate) cells are lower in abundance, and are phagocytic; note their extensive cytoplasm and large pale nucleus.

• The liver sinusoid is unusual in that it has no basement membrane. Instead, it is supported by a network of reticular fibers; these reinforce the vessel, but also allow it to be extremely permeable (schematic).

• Identify the perisinusoidal space of Disse (100X, 100X) between hepatocytes and endothelial lining cells. • Verify the fact that sinusoids are continuous with, and empty into, central veins (50X, 50X, 50X). • Look for larger sublobular veins into which central veins empty.

Kuppfer Cell

Liver: Bile channels

• These channels collect bile, the secretory product of liver, from hepatocytes. Note that bile flows in a direction opposite to blood flow in the lobule. Bile canaliculi are the smallest bile channels. Canaliculi have no cellular lining. Rather, they are spaces between adjacent hepatocytes. Canaliculi form a hexagonal network of channels within the plane of each parenchymal plate.

Liver: Extrahepatic bile ducts

• Bile drains from the common bile duct, entering the gall bladder via the cystic duct. On contraction of the gall bladder during a meal, the (concentrated) bile is injected into the duodenum via the common bile duct and the sphincter of Oddi.

Gallbladder: Epithelium• Note a tall, simple columnar epithelium (4X, 50X). • Identify the striate border (50X).

Gallbladder: Lamina propria• Note the high vascularity of this loose C.T.

Gallbladder: Muscularis mucosa

• None is present.

Gallbladder: Submucosa

• None is present.

Gallbladder: Muscularis externa• Note the presence of collagenous, reticular, and elastic fibers (4X, 20X, 50X) in this layer of smooth muscle.

Gallbladder: Adventitia & Serosa• The region of gallbladder that attaches to the liver is surrounded by an adventitia; the portion not attached

to the liver shows a typical serosa (4X, 4X) instead.

Pancreas: General structure• Note that the pancreas is divided into indistinct lobules (4X). • Note that the exocrine pancreas is present as serous acini and associated ducts, while the endocrine

pancreas is present as paler staining islets of Langerhans.

Pancreas: Exocrine acini• Identify acidophilic zymogen granules (100X) in the cells of serous acini. • Identify centroacinar cells (100X).

Centroacinar Cell

Exocrine Pancreas:Pancreatic ducts

• The exocrine duct system actually begins within the acinus, with centroacinar cells (compare with serous alveoli of salivary glands). Centroacinar cells. (100X) These squamous cells lie within the lumen of an acinus.

• Intercalated (intralobular) ducts. (50X, 100X) These ducts have small lumens lined with cuboidal epithelial cells.

• Interlobular (excretory) ducts. These ducts are located in the C.T. septa (20X).

Intercalated/Intralobular duct Interlobular/Excretory duct

Endocrine Pancreas:(Islets of Langerhans)

• Compare the pale-staining cytoplasm (40X) of these clumps of cells (40X) to the highly staining exocrine acini.

• Be aware that islets contain more than one cell type: alpha, beta and delta cells. These different cell types cannot be readily distinguished in H&E preparations.

• Identify capillaries (40X) within islets of Langerhans. Why is the capillary network so rich here? – The islets of Langerhans communicate with cells and also deliver the hormones produced through the blood stream

Clinical Correlation• 1. Barrett's esophagus (HD085)

In chronic indigestion, the esophagus is exposed to acidic stomach contents that percolate, or reflux, upward from the stomach. In response to this change in environment, the mucosal lining of the esophagus may change from its normal stratified squamous structure to an abnormal, more glandular, gastric type of structure (1X, 5X).

2. Squamous cell carcinoma of the esophagus (HD086)

Left untreated, cases of prolonged esophageal reflux may proceed from Barrett's esophagus to a malignant, squamous cell carcinoma. Moving along the section (1X), identify 1) normal SSE (5X), 2) small islands of malignant squamous cells within the lamina propria (5X) that retain their characteristic keratinocyte appearance (40X), 3) larger islands of transformed cells (5X), and 4) complete breakdown of the epithelial surface, and large numbers of squamous cell islands (5X). 4) complete breakdown of the epithelial surface,

and large numbers of squamous cell islands

Clinical Correlation• 3. Acute appendicitis (HD005)

This is a life-threatening inflammation of the appendix (1X), treated by surgical removal of the inflamed organ. Inflammatory cells, mostly neutrophils and lymphocytes, must be present within the muscular wall (5X) to permit the diagnosis of acute appendicitis. The mucosal lining is often eroded and infiltrated with immune cells (5X).

4. Carcinoma of the colon (HD094)

Carcinomas derived from glandular epithelia are called adenocarcinomas; this differentiates them from tumors derived from squamous cells, which are called squamous cell carcinomas. Colonic epithelium is glandular rather than squamous, so this specimen is an adenocarcinoma. Note that only a small region of normal mucosa remains (low mag, 1X). Examine this tumor (1X); is it microinvasive, or is it a carcinoma in situ?