This includes the cecum; the ascending, transverse, descending, and sigmoid colon; and the rectum. It converts undigested material received from the small intestine into feces by removing water and adding mucus. The colon is shorter than the small intestine and has a wider lumen. The colon's lining has no folds, except in the rectum. No villi are present. The epithelium is simple columnar with a great abundance of goblet cells. The mucosa has many deep crypts of Leiberkuhn, containing abundant goblet cells and few enteroendocrine cells.
Parts of Small Intestine
1. Duodenum. The major distinguishing feature of this C-shaped first part of the small intestine is the presence of duodenal (Brunner's) glands in the submucosa. The mucous cells of these glands produce an alkaline secretion. It protects the duodenal lining from the acidity of the chyme and raises the luminal pH to the optimum level for pancreatic enzyme activity. It is also the point of entry for the bile and pancreatic ducts, which penetrate the full thickness of the duodenal wall. It typically exhibits fingerlike or leaflike villi and relatively few goblet cells.
2. Jejunum. An intraperitoneal organ, the jejunum has long leaflike vilii, many plicae circulares, and an intermediate number of goblet cells. The key to its identification, however, is that although it has villi (and is thus part of the small intestine), it contains neither Brunner's glands nor Peyer's patches.
3. Ileum. This intraperitoneal organ has fewer villi, which are short and broad-tipped (clublike), and relatively abundant goblet cells.
Small Intestine Properties
1. Villi. The presence of these epithelium-covered fingerlike mucosal projections into the lumen is the most diagnostic feature of small intestine structure. The villi increase the mucosal surface area about 10-fold and thus enhance absorption; their shape and abundance differ according to the region where they are located.
2. Intestinal glands (crypts of Lieberkuhn), These simple tubular glands (often coiled) extend into the lamina propria below the bases of the villi. They are lined by absorptive, goblet, Paneth's, enteroendocrine, and undifferentiated cells. Their secretions enter the lumen via small openings between the villi. Similar glands are seen in the large intestine, where they contain many more goblet cells.
3. Enterocytes (absorptive cells). These are the predominant cell type covering the villi. They occur in small numbers in the crypts. The approximately 3000 microvilli per cell give the cell-lumen border a striped appearance, referred to as a striated border.
4. Goblet cells. These lie between the absorptive cells, with more in the surface epithelium than in the crypts. They gradually increase in number from the duodenum to the ileum.
5. Paneth's cells. Lying in the bases of the crypts, these cells synthesize a protein polysaccharide complex. In addition to RER and Golgi complexes, they have many large acidophilic secretory granules that contain lysozyme, an antibacterial enzyme that may help control the intestinal flora.
6. Enteroendocrine cells. Most known types of enteroendocrine cells are found in the crypts of the small intestine. Those that occur mainly in this area produce hormones and amines such as secretin, which increases pancreatic and biliary bicarbonate and water secre tion; cholecystokinin, which increases pancreatic enzyme secretion and gallbladder contraction; gastric inhibitory peptide, which decreases gastric acid production; and motilin, which increases gut motility.
7. Undifferentiated cells. Mucosal epithelial cells undergo continual turnover. Replacement occurs through the mitosis of undifferentiated (stem) cells located near the base of the crypts. Products of these divisions differentiate into all the cell types described above; by a mechanism that is still unclear, they move toward the crypt base or toward the tips of the villi, from which they are finally sloughed into the lumen.
SMALL INTESTINE
The small intestine, which includes the duodenum, jejunum, and ileum, receives chyme from the stomach, bile from the liver, and digestive enzymes from the pancreas. Here, nutrients are hydrolyzed into an absorbable form; they are absorbed and transferred to blood and lymphatic capillaries. Undigested material is moved to the large intestine by peristalsis. The word small refers to diameter, not length: the small intestine is longer and narrower than the large intestine.
A. General Structure: The walls of the small intestine have the same layers as do the rest of the tract. A series of permanent folds, the plicae circulares (valves of Kerckring), composed of both submucosa and mucosa, extend into the lumen and increase the surface area about 3-fold. The main distinguishing features of the small intestine (as viewed through the microscope) are in the composition and organization of the mucosa.
B. Mucosa of the Small Intestine: This consists of simple columnar epithelium with goblet cells, underlain by a lamina propria and separated from the submucosa by a muscularis mucosae.
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.
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
Subscribe to:
Posts (Atom)