Digestion Tract: GIT
Large Intestine contains four histologic layers of the GI tract:
- Mucosa, which is smooth (no villi). It contains numerous crypts of lieberkuhn, which are straight tubular glands that extend through the full thickness of the mucosa. It is made of simple columnar epithelium, similar to those of the small intestine. They are absorptive cells, absorbing water and electrolytes. These absorptive cells outnumber goblet cells 4:1, with the ratio decreasing to 1:1 near the rectum.
- Lamina propria contains GALT, which is continuous with that of the terminal ileum (although more extensively developed). It also has large lymphatic nodules, with distorted regular spacing of crypts that extends into the submucosa. It reflects a larger number of microorganisms present in the colon lumen. It has a 5μm thick layer of collagen and proteoglycans, between the basal lamina of the epithelium and the fenestrated absorptive capillaries. It participates in regulation of water and electrolyte transport, and can be up to 15 μm thick in colon cancer. There are no lymphatic vessels present in the core of the lamina propria between colonic crypts, which influence the slow rate of metastasis of colon tumours.
The large intestine is composed of:
- Cecum forms a blind pouch distal to the ileocecal valve (the histology of the cecum resembles the remaining colon). The appendix is a thin finger-like extension of the cecum. It’s different in that it has a complete layer of longitudinal muscle in the muscularis. Conspicuously, large numbers of lymphatic nodules are located in the wall of the appendix resulting in loss of the normal structure. Much scar tissue and lymph nodules are present, also producing trash in the lumen.
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
- Anal canal, where the submucosa has terminal branches of superior rectal artery and rectal venous plexuses. Enlargements of submucosal veins constitute internal hemorrhoids, related to elevated venous pressure of the portal circulation. The inner circular layer of muscularis externa is modified to form the internal anal sphincter. Muscles of the pelvis form the external anal sphincter.