Structure of the Alimentary Tract
Structural Overview of the Alimentary Canal
The alimentary canal's wall is made up of four layers:
1. The serous layer or serosa: The belly's organ holder, a sheet, acts like skin to
keep things snug inside.
2. The muscular layer or muscularis mucosae:Comprising of two layers of
smooth muscle - the internal circular layer and the external longitudinal layer.
Contraction of the circular muscle narrows the lumen of the alimentary canal,
while contraction of the longitudinal muscle shortens a segment of the canal.
Between these layers lies the myenteric plexus (Auerbach's plexus), which is
under extrinsic autonomic control.
3. Submucous layer: Has lymphatics and blood vessels. Between this layer and
the inner circular muscle is the submucous plexus (Meissner's plexus), which
connects to the Auerbach's plexus and outside autonomic nerves.
4. Mucous layer: Consists of glands and specialized epithelial cells for absorption
and secretion. The surface area of the epithelium is increased by foldings that
form villi and crypts. Villi are finger-like projections, while crypts are infoldings
between the villi.
Within the Mucosal Layer:
In the villi and crypts of the gut we find enterocytes which have microvilli on what
is known as the luminal surface and these form a brush border which in turn is
very enzyme rich.
The brush border enzymes are membrane-bound proteins responsible for the
final steps of nutrient digestion.
The inner lining has a sugary coat and still surface where stuff must seep into
cells on the moist surface.
Note: Small intestinal epithelium has both crypts and villi, therefore its absorptive
ability is greater than that of the colon, which lacks villi but has crypts.
Features of the Mucosal Lining in the Intestinal Tract
The mucosal lining of the intestinal tract exhibits various features and cell types:
● Tight Junctions: Cell intersections are tight amongst them.
● Smooth Muscle Cells: Linked to one another by gap junctions
● Crypts of Lieberkuhn: Little tubular holes between the villi hold intestinal
glands.
● Folds of Kerckring (Plicae Circulares): Circular folds in the mucosa that
further increase the luminal surface area.
● Base of the Crypts: Contains Paneth cells (secreting antimicrobial
peptides) and stem cells responsible for continual turnover of the crypt and
villus epithelium.
● Interposed Among Enterocytes: Goblet cells secrete mucus, while
enterochromaffin cells secrete hormones (e.g., serotonin) in response to
luminal contents.
● Brunner's Glands: Located in the duodenum's submucous layer, these
glands secrete HCO3- into the duodenal lumen.
● Colon: Features three bands of longitudinal muscle known as taenia coli.
● Lymphoid Tissue and Drainage: Totals of lymphoid tissue, such as Peyer's
patches within the ileum, are show within the GI divider. Each villus
contains a central lacteal, a lymphatic vessel vital for transporting
lipid-soluble substances ingested from the intestine.
