Histology Of STOMACH


This dilated portion of the digestive tract temporarily holds ingested food, adding mucus, acid, and the digestive enzyme pepsin, Muscular contractions of the stomach blend these components into a viscous mixture called chyme, The chyme is then divided into parcels for further digestion and absorption by the intestines.
A. General Structure: The stomach wall has the same layers as the rest of the tract. The complex mucosa contains numerous gastric glands, a 2-3-layer muscularis mucosae that helps empty the glands, and an intervening lamina propria. When the stomach is empty and contracted, the mucosa and underlying submucosa are thrown into irregular, temporary folds called rugae, that flatten when it is full. The smooth muscle of the muscularis externa is arranged in 3 layers: outer longitudinal, middle circular, and inner oblique. The stomach has 4 major regions: cardia, fundus, body, and pylorus.
B. Gastric Mucosa: The stomach lining of simple columnar epithelium is perforated by numerous small holes called foveolae gastricae, The foveolae are the openings of epithelial invaginations, the gastric pits, which penetrate the lamina propria to various depths. The pits serve as ducts for the branched tubular gastric glands, Each gland has 3 regions: an isthmus at the bottom of the pit, a straight neck that penetrates deeper into the lamina propria (perpendicular to the surface), and a coiled base that penetrates deeper still and ends blindly just above the muscularis mucosae. The mucosa is characterized by the following epithelial cell types.
1. Surface mucous cells. They secrete a neutral mucus that protects the stomach's surface from the acidity of the gastric fluid.
2. Undifferentiated cells. Low columnar cells with basal ovoid nuclei are found scattered in the neck of the gastric glands.
3. Mucous neck cells occur singly or in clusters between the parietal cells in the neck of the gland. They differ from the surface mucous cells by secreting acidic mucus.
4. Parietal (oxyntic) cells secrete HCI and intrinsic factor. Parietal cell secretion is stimulated by cholinergic nerve endings. Acid production is greatly enhanced by histamine and gastrin produced by enteroendocrine cells in gastric glands (and elsewhere).
5. Chief (zymogenic) cells secrete pepsinogen and some lipase. These cells are smaller than parietal cells. They are basophilic owing to the ribosomes associated with their RER. They also contain membrane-limited pepsinogen-filled zymogen granules. Pepsinogen is an inactive proenzyme or zymogen that is converted to the active protease pepsin when exposed to the acidic environment of the stomach lumen. Gastric lipase has only weak lipolytic activity. 6. Enteroendocrine cells. In the stomach, these cells occur mainly in the base of gastric glands. They produce various endocrine and paracrine amines leg, histamine, serotonin) and peptide hormones leg, gastrin).

C. Regional Differences:
1. Cardia. the cardia surrounds the point of entry of the esophagus. Here, the lamina propria contains simple or branched tubular cardiac glands like those in the terminal part of the esophagus. The basal portions of these glands are often coiled, with wide lumens.
2. Fundus and body. The glands in these regions are similar in structure and function. The body is the stomach's largest region, extending from the cardia to the pylorus. The fundus is a smaller, roughly hemispheric region that extends above the cardia. Gastric glands--termed fundic glands in both regions--are characterized by shallow pits and long glands. The pits extend about a third of the distance from the mucosal surface to the base of the glands. Fundic glands contain abundant parietal and chief cells. Parietal cells are concentrated in the neck and upper part of the base, while chief cells predominate in the lower portion. Serotonin (5-hydroxytryptamine)-secreting cells are typically found at the bases of these glands.
3. Pylorus. This makes up the distal 4-5 cm of the stomach, leading to the small intestine. Pyloric glands are characterized by deep pits and short glands. Chief cells are especially scarce in this region. Gastrin-secreting cells (G cells) are typical of the bases of these glands. At the pylorus-small intestine junction, a thickened band of the middle circular layer of the muscularis externa, the pyloric sphincter, controls the passage of chyme.

PHARYNX

A short, broad, muscular tube that lies behind the tongue and soft palate, the pharynx is shared by the respiratory and digestive tracts. Its superior portion, the respiratory pharynx, lies above the soft palate; it communicates with the nasal cavity and is lined by respiratory epithelium. The inferior portion, the oral pharynx (oropharynx), lies below the level of the soft palate.
It communicates with the oral cavity and is lined by nonkeratinized stratified squamous epithelium. The pharynx also communicates with both the esophagus and the larynx. During swallowing, the back of the tongue helps close the epiglottis to direct food away from the larynx and into the esophagus.

ORAL CAVITY

The upper end of the digestive tract is bounded anteriorly by the teeth and lips, posteriorly by the oral pharynx, laterally by the teeth and cheeks, superiorly by the hard and soft palate, and inferiorly by the tongue and floor of the mouth.

A. Wall Structure: The mucosa includes the lining epithelium and the underlying lamina propria. Nonkeratinized stratified squamous epithelium (mucous membrane) covers all internal surfaces of the oral cavity and pharynx except the teeth. The lamina propria is a vascular connective tissue with papillae like those of the dermis. The papillae contain capillaries that nourish the epithelium. The oral cavity has no muscularis mucosae. The submucosa is a more fibrous connective tissue than the lamina propria; it contains many blood vessels and small salivary glands. The oral cavity lacks a standard muscularis externa. Skeletal muscle underlies the submucosa in the lips, cheeks, tongue, floor of the mouth, oral pharynx, soft palate, and its downward extension, the uvula. Bone underlies the thin submucosa of the hard palate and gums (gingiva).

B. Lips: Here, there is a transition from nonkeratinized mucous membrane to the keratinized stratified squamous epithelium of the skin. The thin keratinized layer covering the lips' ver million border allows the reddish color of blood in vessels of the lamina propria to show through. Hair follicles, keratin, and additional pigment help distinguish the outer lip surface from the inner in tissue sections.

C. Tongue: This is a mass of skeletal muscle covered by a mucosa. The mucosa is bound tightly to the muscle by the lamina propria, which penetrates between the bundles of muscle fibers. There is little or no submucosa. The muscle is arranged in bundles of many sizes; these are separated by connective tissue and cross each other in 3 planes. This gives the tongue the flexibility required for speech, positioning food, chewing, and swallowing. The mucosa differs on the dorsal (upper) and ventral (lower) surfaces. The ventral surface has a thin nonkeratinized stratified squamous epithelium underlain by a lamina propria. The epithelium covering the dorsal surface is partly keratinized. The anterior two-thirds of the dorsal surface is separated from the posterior third by a V-shaped groove. Behind this, the epithelium invaginates to form the crypts of the lingual tonsils. Cryptless patches of lymphoid tissue in the lamina propria cause surface bulges in this region. The anterior two-thirds of the dorsal surface has many papillae-projections of the mucosal surface. There are 4 types of papillae.

1. Filiform papillae are the most numerous. They are sharp, often partly keratinized, conical projections that lack taste buds.
2. Fungiform papillae resemble mushrooms. Each has taste buds on its expanded upper surface but not on its narrow stalk. Fungiform papillae occur singly and are scattered among the filiform papillae.
3. Foliate papillae are poorly developed in humans. They occur in rows separated by furrows into which serous glands in the lamina propria drain. The furrow walls (sides of the papillae) harbor many taste buds.
4. Circumvallate papillae are the largest and least numerous, with only 7-12 occurring near the V-shaped groove at the back of the tongue. Each is surrounded by a ringlike ridge of mucosa from which it is separated by a circular furrow, whose walls contain taste buds on both sides. As with the foliates, ducts from serous (von Ebner's) glands empty into the furrow and periodically wash the chemical stimuli from the taste buds, allowing new tastes to be sensed

Functions of the digestive tract


C. General Functional Features: The main functions of the digestive tract are the absorption of nutrients and water and the excretion of wastes and toxins.

1. Digestion. Enzymatic degradation of foods is a prerequisite for absorption; enzymes act mainly at food surfaces. Chewing exposes more surface area. Lip, cheek, and tongue muscles help position food between the teeth. Saliva dissolves water-soluble particles and contains enzymes that attack carbohydrates. Taste buds check for contaminants toxins, and nutrients. The tongue moves chewed food back into the oral pharynx and closes the epiglottis to protect the airway. The esophagus adds mucus to reduce friction, but mainly moves material to the stomach. Glands in the stomach wall add acid (HCI), a protease (pepsin), and mucus to the mixture (now called chyme). Smooth muscles in the stomach wall mix and pulverize the chyme and move it to the small intestine (duodenum), where pancreatic enzymes and bile are added. The enzymes hydrolyze nutrients to an absorbable form. The detergent action of bile disperses water-insoluble lipid into tiny droplets, increasing the surface area available to pancreatic lipases. The lining epithelial cells (enterocytes) of the small intestine have additional enzymes on their luminal surfaces to complete the hydrolysis of certain nutrients.

2. Absorption. This primary function of the digestive tract occurs mainly in the intestines: the small intestines absorb nutrients, and the large intestines absorb water. To maximize the absorptive surface, the small intestine's lining has multiple permanent folds including plicae circulares and villi. Intestines are lined by absorptive cells (enterocytes) whose apical microvilli further increase the surface area. These cells absorb and transfer amino acids and sugars to capillaries in the lamina propria, whose blood carries them to the liver for further process ing. Enterocytes assemble chylomicrons from absorbed lipids and transfer them to lymphatic capillaries (lacteals) in the lamina propria. From here, lipids reach the blood through the lymphatic vascular system.

3. Excretion. Metabolic wastes are excreted by the liver as bile and emptied into the duodenal lumen by the bile duct. Smooth muscles in the walls of the small intestine move undigested material and waste products to the large intestine (colon). Here, more mucus is added and most of the water is extracted. This concentrates and solidifies the intestinal contents, forming feces. This material is further dehydrated and stored in the rectum and finally expelled through the anal canal.


4. Endocrine function. Individual cells with characteristics of the diffuse neuroendocrine system are scattered among the epithelial cells lining the tract's mucosal glands and crypts. These enteroendocrine cells were formerly called argentaffin, ar gyrophilic, and enterochromaffin cells because of their affinity for stains containing silver and chromium. They secrete hormones and amines leg, serotonin, secretin, gastrin, somatostatin, cholecystokinin, glucagon) that regulate such local gastrointestinal functions as gut motility and the secretion of acid, enzymes, and hormones by other cell types. 5. Innervation. Distributed along and in the walls of the tract are the myenteric (Auerbach's) and submucosal (Meissner's) autonomic nerve plexuses. These include postsynaptic sympathetic fibers, pre- and postsynaptic parasympathetic fibers, parasympathetic ganglion cell bodies, and some visceral sensory fibers. After voluntary swallowing, these autonomic plexuses coordinate peristaisis-wavelike contractions of the muscularis externa that propel ingested material through the tract. They also control the independent activity of the muscularis mucosa, which maintains contact between the mucosa and the contents of the tract and help empty mucosal glands. These plexuses also modulate the secretory activity of certain DNES-like cells. In general, sympathetic action inhibits gut motility and parasympathetic action has the opposite effect. 6. Blood supply. Mesenteric branches of the abdominal aorta branch further in the mesenteries to form a series of arcades. Small arteries penetrate the tract walls to feed capillaries of the lamina propria. Amino acids, sugars, small fatty acids, and any toxins absorbed in the intestine thus travel directly to the liver to be metabolized, stored, or detoxified before reaching the general circulation. 7. Protection. The extensive absorptive surface of the digestive tract increases the risk of infection. The risk is reduced by immunoreactive cells--including IgA-secreting plasma cells--in the lamina propria and submucosa. Other defenses include lysozyme secreted by Paneth's cells, digestive enzymes in the lumen, the layer of mucus covering the epithelium, and the tight junctions between absorptive cells. Toxic substances that do reach the blood are carried directly to the liver for detoxification in the SER of the hepatocytes.

GENERAL FEATURES OF THE DIGESTIVE TRACT

-A. Components: The digestive tract is a series of organs forming a long muscular tube whose continuous lumen opens to the exterior at both ends. The organs include the oral cavity, oral pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (cecum and appendix; ascending, transverse, and descending colon), rectum, and anal canal.
-B. General Structural Features: The walls of each organ consist of 4 concentric layers: the mucosa, submucosa, muscularis externa, and serosa or adventitia
-1. Mucosa, This layer borders the lumen and has 3 parts. The epithelium (mucous membrane) derives from endoderm. It is stratified squamous in the oral cavity, oral pharynx, esophagus, and anal canal; it is simple columnar in the stomach, intestines, and rectum. The lamina propria is a layer of loose connective tissue beneath the endothelium; it contains small blood and lymphatic vessels. The muscularis mucosae is a thin layer of smooth muscle bordering the submucosa.
2. Submucosa, This dense, irregular connective tissue layer contains blood and lymphatic vessels and the submucosal (Meissner's) plexus of nerves. Some organs are characterized by glands and lymphoid nodules in this layer.
3. Muscularis externa, This consists of 2 layers of smooth muscle--an inner circular and an outer longitudinal-through most of the tract. Between them lies the myenteric (Auerbach's) plexus. The muscle around the oral cavity is skeletal; where it is absent leg, hard palate, gingiva) the submucosa binds tightly to bone. In the upper esophagus, this layer contains mainly skeletal muscle, which is replaced by smooth muscle in the lower portion. The stomach's muscularis externa has 3 layers: outer longitudinal, middle circular, and inner oblique. The colon's outer longitudinal layer is gathered into 3 bands, the taeniae coli. The smooth and skeletal muscles encircling the anal canal form involuntary and voluntary sphincters, respectively.
4. Serosa and adventitia. The tract's outer covering differs by location. The esophagus and rectum are surrounded and held in place by a connective tissue adventitia like that around blood vessels. Intraperitoneal organs (stomach, jejunum, ileum, transverse and sigmoid colon) are suspended by mesenteries and covered by a serosa composed of a thin layer of loose connective tissue covered by simple squamous epithelium (mesothelium). Retro peritoneal organs (duodenum, ascending and descending colon) are bound to the posterior abdominal wall by adventitia and covered on their free (anterior) surfaces by serosa.