General Features

Originated in yolk sac, endoderm tissue. A tube of muscle lined with mucous membrane.

Muscular wall: inner circular, outer longitudinal (really 2 spiral layers, inner being close-wound), allowing peristaltic action. Is smooth muscle

Mucous membrane has epithelium, lamina propria, and muscularis mucosa, with scattered endocrine cells (produce peptides / amines for neurotransmission / hormones). Are APUD cells.

Oesophagus

Stratified squamous non-keratinising epithelium, thickish muscularis mucosae, with small groups of mucous-secreting glands at the upper and lower ends.

Stomach

Abrupt change to single-layer columnar epithelium, continues to anal canal (though differs)

- epithelium not flat, but dips into lamina propria to form glands, gastric pits in stomach to peptic and parietal cells (pepsin and HCl respectively); also intrinsic factor from parietals.

- At cardia (for ~1cm) glands are shorter and cells secrete mucus.

- In pylorus, cells like coiled test-tubes and again all mucus secreting; G cells (gastrin) and D cells (somatostatin) are also found in this area.

- Enterochromaffin cells in body and pyloris secrete serotonin and endorphin.

Small Intestine

Columnar epithelium dips into glands (Crypts of Lieberkuhn) and thrown up into villi (soft-tissue cores covered in epithelium). Some secrete mucus, others absorb (are enterocytes).

- granular Paneth cells at bases of crypts secrete lysosyme

- enteroendocrine cells in crypts secrete hormones, eg secretin, somatostatin and CCK (ECs too)

- duodenum also has Brunner’s glands (mucus secreting)

- Peyer’s patches in the terminal ileum has groups of lymphoid follicles in the mucosa.

Large Intestine

No villi, many glands and goblet cells. Stratified squamous again in anal canal.