All regions of the gastrointestinal tract have a mucosa, submucosa, muscularis, and serosa, but these layers display different specializations in different regions of the tract. By studying the histology of the liver and pancreas, the functions of these organs are better understood.

The tubular digestive tract, including the esophagus, stomach, small intestine, and large intestine, consists of four major layers (fig. 10.2). From the innermost layer outward, they are as follows:
1. The mucosa, or mucous membrane, consists of an inner epithelium supported by a thin layer of connective tissue, the lamina propria, which is bordered by a ribbon of smooth muscle, the muscularis mucosa. The epithelium is stratified squamous in the esophagus and anal canal and simple columnar in the stomach, small intestine, and large intestine.
2. The submucosa is connective tissue and therefore has abundant extracellular space for blood vessels, nerves, and mucus-secreting glands.
Parasympathetic fibers and ganglia can be seen as the submucosal (Meissner's) plexus in the submucosa.
3. The muscularis externa consists of smooth muscle, which in most of the digestive tract is arranged in
an inner circular and outer longitudinal layer. Parasympathetic fibers and ganglia can be seen as the myenteric (Auerbach's) plexus in this layer.
4. The serosa consists of a simple squamous epithelium and connective tissue, and is the outermost covering of the digestive tract.

A. ESOPHAGUS AND STOMACH
The esophagus is lined with a stratified squamous epithe- lium. The muscles of the first third of the esophagus, like those of the pharynx and mouth, are striated to provide voluntary control of swallowing. The middle third con- tains a mixture of striated and smooth muscle, and the last third of the esophagus contains only involuntary smooth muscle (fig. 10.3).
The submucosa of the stomach is thrown into large folds, or rugae, which can be seen with the unaided eye. Microscopic examination of the mucosa shows that it, too, is folded. The openings of these folds into the stomach lumen are called gastric pits. The cells that line the folds of mucosa are secretary and form the gastric glands (figs. 10.4 and 10.5).
The gastric glands include (1) goblet cells, which se- crete mucus; (2) parietal cells, which secrete hydrochloric acid (HCI); (3) chief cells, which secrete pepsinogen (the inactive precursor of pepsin, a protein-digesting enzyme); (4) argentaffin cells, which secrete serotonin and hista- mine; and (5) G cells, which secrete the hormone gastrin into the blood. The gastric mucosa also secretes a poly- peptide called intrinsic factor, which aids in the absorp- tion of vitamin B12 in the intestine.

Procedure
1 -Observe a cross section of the esophagus under 100x (using the lOx objective lens) and note the four major layers.
2. Hold a slide of a stomach section up to a light source and observe a fold, or ruga. Now, place the slide on a microscope and, under 100x, observe the gastric pits and glands in the mucosa, the submucosa, and the muscularis externa.
3. Using the high-dry objective lens (45x),
observe the gastric glands in the mucosa under a total magnification of 450x. Identify goblet cells near the surface of the gastric pits. These mucus-secreting cells are numerous and clear in appearance. Near the base of the glands, identify parietal cells (with red-staining cytoplasm) and chief cells (with blue-staining cytoplasm). Argentaffin and G cells cannot be easily identified without specially stained slides

B. SMALL INTESTINE AND LARGE INTESTINE
The small intestine is approximately 21 feet long (in a cadaver) and divided into three regions. The first region, approximately 12 inches long, is called the duodenum. The next region, the jejunum, is about 8 feet long and constitutes two-fifths of the entire length of the intestine. The ileum, about 12 feet long (constituting three-fifths of the intestine), is the terminal region.
The mucosa and submucosa of the small intestine form large folds called the plicae circulates. The surface area of the mucosa is further increased by microscopic folds that form fingerlike projections called villi (fig. 10.6). Each villus is covered with a simple columnar ep- ithelium around a core of connective tissue (the lamina propria). The apical surface (facing the lumen) of each epithelial cell has a slightly blurred, "brush border," ap- pearance because of numerous projections of its cell membrane in the form of microvilli. Microvilli can be clearly seen only with an electron microscope. The mi- crovilli, villi, and plicae circulates increase the surface area of the small intestine tremendously, thus maxi- mizing the rate at which the products of digestion can be absorbed by transport through the epithelium into the blood. Various digestive enzymes-called brush border- enzymes-are fixed to the cell membranes of the micro- villi and act together with enzymes in pancreatic juice to catalyze hydrolysis reactions of food molecules.
The epithelium at the base of the villi invaginates to form pouches called crypts of Lieberkiihn. Although these somewhat resemble the gastric glands of the stomach, the crypts do not secrete enzymes. Instead, it appears that the cells within the crypts undergo rnitotic division and push upward to replace those cells that are continuously lost from the tips of the villi. Within the submucosa of the duodenum are mucus-secreting Brunner's glands (fig. 10.6).
Waste products from the small intestine pass into the colon of the large intestine where water, sodium, and po- tassium are absorbed. The mucosa of the large intestine contains crypts of Lieberkiihn but not villi, so its surface has a flat appearance. As with the small intestine, nu- merous lymphocytes can be seen in the lamina propria, and large lymphatic nodules appear at the junction of the mucosa and submucosa. Lymphatic nodules are clearly evident in a section of the appendix (fig. 10.7), which is a short outpouching from the cecum.

Procedure
1 .Use the lowest power available on the microscope to observe the layers of a section of small intestine. Identify the viiii, submucosa,
and muscularis externa.
2. Observe the viiii using the 45x objective lens. Identify the goblet cells in the epithelium and
the numerous lymphocytes (small, blue-staining cells) in the lamina propria within each villus. Also within the lamina propria, note the central lacteal-a lymphatic vessel that transports absorbed fat from the intestine.
3. Observe a slide of the large intestine using the
1 Ox objective lens. Note the absence of viiii
and the appearance of the lymphatic nodules.
C. LRVER
The liver aids digestion by producing and secreting bile, which emulsifies fat. Bile leaves the liver in the common hepatic duct, which branches to form the cystic duct and common bile duct. The cystic duct channels bile to the gallbladder where it is stored and concentrated. The common bile duct, together with the pancreatic duct, emp- ties into the duodenum.
The liver also serves to modify the composition of the blood that drains from the intestine into the hepatic portal vein. Before this venous blood can return to the heart, it must pass through sinusoids in the liver tissue (figs. 10.8 and 10.9). These sinusoids function as liver capillaries, although they differ from other types of capillaries in that they are wider and are lined with phagocytic cells (called Kupffer cells). Blood from the sinusoids is drained by

small central veins that ultimately merge to form the hepatic vein (fig. 10.9), which carries blood away from the liver. The sinusoids also receive arterial blood from branches of the hepatic artery. Arterial blood mixes with blood from the portal vein as it passes through the sinu- soids to the central vein.
Bile is produced and secreted by the liver cells (he- patocytes), but it does not mix with blood because it is secreted into bile canaliculi that are located between ad- jacent hepatocytes, and so it does not enter the sinusoids (fig. 10.10). Bile is drained from the canaliculi into bile ducts, located near the openings of the portal vein and hepatic artery. The grouping of the portal vein, hepatic artery, and bile duct that one sees in a microscopic view of the liver is called a portal triad.
Procedure
1 .Examine a slide of the liver under low magnification and locate a central vein and a portal triad. The central vein appears as a large "hole" into which a number of sinusoids empty. The portal vein may also appear as a large
hole, but it can easily be distinguished from the central vein because the portal vein, hepatic artery, and bile duct are in close proximity.
2. Observe a portal triad under high magnification and distinguish between the portal vein, hepatic artery, and bile duct. The portal vein is the largest of the three. The hepatic artery can be identified by its layer of smooth muscle and lining of simple squamous endothelium; by contrast, the bile duct lacks smooth muscle and is lined with simple columnar epithelium.

D. PANCREAS
The pancreas is both an exocrine and an endocrine gland. Most of the pancreas is composed of exocrine tissue, with secretary cells arranged in clusters, or acini (fig. IO. II). The pancreatic acini secrete pancreatic juice into open- ings of the pancreatic duct, which carries these secretions to the duodenum. Pancreatic juice contains water, bicar- bonate, and a variety of digestive enzymes, including
trypsin, lipase, and amylase (for the digestion of protein, fat, and carbohydrates, respectively).
Scattered among the exocrine acini are islands of en- docrine cells. These are the islets of Langerhans (fig. 10.11). The islets contain alpha cells, which secrete the hormone glucagon, and beta cells, which secrete the hormone insulin. These hormones are secreted into sur- rounding blood capillaries rather than into the pancreatic duct.