Stomach

Stomach:
The stomach connects the esophagus to the intestines and in the most species serves not only to continue the breakdown of foodstuffs via the use of digestive enzymes and acid but it also as a storage depot for food. Usually food remains in the stomach a few hours during which it is converted into a liquid material, called chyme.
Regional variation in the glands of the tunica mucosa of the stomach
Not all regions of the stomach mucosa have the same histological structure. They vary as follows:
•cardia: contains many mucus-secreting glands
• fundus: mostly glands secreting acid-peptic gastric juices; some
mucus-secreting glands
. pylorus: contains different types of mucus-secreting glands.
Wall of the Glandular Stomach
mucosa: in the empty stomach, this layer is thrown into deep longitudinal folds called rugae that extend from the lamina muscularis mucosae to the lumen; in the full stomach the rugae are much reduced in size as a result of distension of the tunica mucosa to accomodate the presence of a large amount of food material
• submucosa: typical loose connective tissue contains parasympathetic ganglia located in submucosal plexuses also known as Meissner’s plexuses
musculris externa: typical smooth muscule consisting of at least two layers, an inner circular layer and an outer longitudinal layer; parasympathetic ganglia located between the two muscle layers in
Layers of the Mucosa of the Stomach - Fundic Region . *Epithelia : consists of simple columnar epithelium that forms branched tubular gland, organized into gastric pits that open onto the lumen and gastric glands that empty into the base of the gastric pits.
*Lamina propria: consists of loose areolar connective tissue that in the glandular stomach is minimal between gastric glands and difficult to see in sections; highly vascular containing many blood and lymphatic capillaries
*Muscularis mucosac: consists of several, layers of smooth muscle oriented both longitudinally and circularly usually not very thick.
cell types in the gastric gland:
* Surface mucous cells: line the gastric pit and secrete mucous and bicarbonate ions to protect the epithelium from digestion by gastric juice ( contains HCL and pepsin ) present in the stomach lumen . *Neek mucous cell : found dispersed between the parietal cells ;secret a mucous that is thinner than secreted by the surf ace mucous cell ; mucus protects other glandular cell from action of proteases and HCL .
Parietal cells : found throughout the gastric gland round cells that contain distinct eosinophilie(pink) cytoplasm and round prominent nucleus ; secrete HCL and intrinsic factor needed for absorption of vitamin B12 .
Chief (peptic , zymogenic ) cell : found mostly near the base of the gastric gland ; very basophilie (purple) containing basally positioned nucleus and prominent basophilie apical cytoplasm filled with many ribosomes; secrete pepsinogen which is converted to pepsin in the acidic milieu of the stomach.
Enteroendocrine cells : difficult to distinguish by conventionl light microscopy , several types are present some secrete serotonin , gastrin , glacagon somatostatin among other hormones .
Pylorus region:
Simple columnar mucous epithelium lines the surface of the stomach extends into and lines the gastric pits ,as in the cardia region only one type of cell is normally found in these glands this is tall columnar
Cell , with slightly granular cytoplasm1ight1y stained because of mucien content and flattened or oval nucleus at the base.
The remaining structures in this region are similar to those in the fundus region.
Small Intestine;The small intestine is a typical tubular organ in that it has all of the typical layers. However, the mucosa is especially modified to fulfill the function of absorption. Also the three regions of the small intestine, the; duodenum, the jejunum and the ileum each have special modifications to the wall to enable each region to better perform its particular function. In the small intestine digestion occurs in the lumen as well at the surface of the lining epithelial cells . Pancreatic enzymes such as trypsin, chymotrypsin , elastase, carboxypeptidases, peptide hydrolases, amylase and lipases are adsorbed onto the membrane surface of the epithelial cells where they mix with the chyme present in the lumen catalyzing thebreakdown of proteins carbohydrates and lipids. The smaller breakdown products are then absorbed by the lining epithelial
cells that are called enteroc ytes.
Layers of the Small Intestine
*mucosa : This layer protrudes out into the lumen as projections called villi and it dips down to the underlying muscula ris mucosa forming pockets called crypts .
-epithelia - simple columnar epithelium.
-villus – avillus contains enterocytes (absorption) ,goblet cell (protective mucus) in its upper region and neuroendocrine cell (local hormones).
-crypt: a crypt (crypt of Lieberkhn) contains goblet cell, paneth cell(defensive),enteroendocrin cell, stem cell,intraepithelial lymphocytes (defensive).
Lamina propria - loose connective tissue rich in blood and lymphatic vessels present in the core of the villi and between crypts.
Muscularis mucosa- thin layer of smooth muscle located at the base of the crypts
• Submucosa : This layer blends with the lamina propria and is typical In the duodenum it has coiled branched glands known as Brunners glands, the ducts of which seen into the base of the crypts.
Muscularis externa: typical consisting of an inner circular layer and an outer longitudinal layer.
Serosa : typical.
Enteroendocrine cell: These cells secrete hormones such as secretin, somatostatin,enteroglucagon and serotonin ;one hormone per type of cell .
Paneth cell : These remarkable cell contain large granules that contain defensins (antimicrobial peptides ) as well as lysozymes and phospholipase A. These chemicals represent the first –line of defense against microbes that enter through the digestive tract. Compared to the other cells present in the epithelial lining, Paneth celLs are long -lived, weeks versus a few days for the other cells.
Specializations to enhance absorption ability
The small intestine has al1 of the layers of typical tubular organ but the tunica mucosa is highly specialized to perform the function of absorption .To fulfill this function it uses several strategies to increase the surface area of the plasma membrane of the absorptive epithelial cell.
• individual cells have numerous projections of their apical plasma
membranes called microvilli
• the lamina epithelialis and lamina propria together form folds that project out into the lumen called villi
• the tunica mucosa and submucosa together form large transverse folds
into the lumen called plica circulares
• the small intestine is extremely long (usually several meters).
Regional variations in the small intestine
Duodenum
* presence of Brunners glands in the submucosa
* no plica circulares
*longest villi of all three regions least number of goblet cells
Jejunum
*no glands in the submucosa
*.presence of plica circulares
lleum
• permanent aggregated lymphoid nodules in the lamina propria
• shortest vill i; highest number of goblet cells.
Large intestinet
Unlike the small intestine, there are no plicae circulares or villi in the
large intestine so the surface of the tunica mucosa is more uniform and
flatter than that at the small intestine.
• mucosa:
o epitbelia -simple columnar epithelium that forms straight tubular
glands lined with absorptive columnar cells ( recovering water and salt)
and numerous gobt cells (producing mucus to facilitate passage of dry
waste material); stern cells and lymphocytes are also present .
o lamina propria – loos connective tissue that contains numeroues blood and lymphatic vessls,collagen,lymphocytes and plasma cell.
o muscularis mucosae- present beneath the base of the crypts and
prominent.
• submucosa: typical
• muscularis externa: inner circular and outer longitudinal layers; outer longitudinal layer is organized into three separate hands known as taenia coli; movement of rnore solid waste to the rectum
Serosa - is typical
Commensal bacteria reside in the large intestine and play a role in the continued, digestion of food .