Q1 LONG ANSWER 10M ★ High-yield

Describe the general plan of the gastrointestinal tract. Describe the histology of the stomach, small intestine and large intestine with their distinguishing features.

Anatomy · 1st Professional — Pre-Clinical

The whole gastrointestinal tract from oesophagus to anal canal is built on four concentric coats, modified region by region.

Coat	Components
Mucosa	Epithelium; lamina propria (loose connective tissue with glands, vessels and lymphoid tissue); muscularis mucosae
Submucosa	Dense irregular connective tissue with vessels and the submucosal (meissner) plexus
Muscularis externa	Inner circular and outer longitudinal smooth muscle, with the myenteric (auerbach) plexus between them
Serosa or adventitia	Serosa where the gut is intraperitoneal; adventitia where it is retroperitoneal or in the thorax

The muscularis mucosae belongs to the mucosa, not to the muscularis externa — a distinction constantly confused, and the one that defines whether a carcinoma is invasive

Epithelium — stratified squamous non-keratinised
Mucous glands in the submucosa (oesophageal glands proper) and in the lamina propria at the two ends (cardiac glands)
Muscularis externa is the distinguishing feature: upper third skeletal muscle, middle third mixed, lower third smooth
Adventitia, not serosa, except the short intra-abdominal part

Feature	Detail
Epithelium	Simple columnar, entirely mucus-secreting; NO goblet cells
Surface	Gastric pits (foveolae) into which the glands open
Rugae	Folds of mucosa and submucosa
Muscularis externa	Three layers — inner oblique, middle circular, outer longitudinal

Cell	Position in the gland	Appearance	Secretes
Mucous neck cell	Neck	Pale	Mucus
Parietal (oxyntic) cell	Upper half (body of gland)	Large, round, eosinophilic, with a central round nucleus	HCl and intrinsic factor
Chief (peptic) cell	Base of the gland	Basophilic, pyramidal	Pepsinogen
Enteroendocrine cells	Base	Clear, granules at the base	Gastrin (G cells in the antrum)

Three devices increase the surface area — plicae circulares (mucosa and submucosa), villi (mucosa), and microvilli (cell surface); together about 600-fold
Villi — finger-like projections with a core of lamina propria containing a central lacteal and smooth muscle
Crypts of Lieberkuhn between the villi contain PANETH cells at their base — large eosinophilic granules, secreting lysozyme and defensins
Cells of the epithelium — enterocytes with a striated border, goblet cells, Paneth cells, enteroendocrine cells and stem cells

Region	Distinguishing feature
Duodenum	Brunner glands in the submucosa — the only part of the gut with submucosal glands below the oesophagus; they secrete alkaline mucus
Jejunum	Tallest villi and the most prominent plicae circulares; no distinctive glands or lymphoid aggregates
Ileum	PEYER patches — aggregated lymphoid follicles in the lamina propria and submucosa, on the antimesenteric border; villi shorter, goblet cells more numerous

NO villi — the single most reliable feature; the surface is flat
Straight, closely packed crypts with abundant goblet cells
The outer longitudinal muscle is gathered into three taeniae coli
Appendices epiploicae on the serosa
Appendix — a small irregular lumen, often containing debris, with abundant lymphoid tissue encircling the whole wall; few crypts

Feature	Duodenum	Jejunum	Ileum
Villi	Broad, leaf-like	Tallest, finger-like	Shorter, club-shaped
Plicae circulares	Present	Most prominent	Sparse, absent distally
Submucosal glands	Brunner glands — diagnostic	Absent	Absent
Lymphoid tissue	Scattered	Solitary follicles	PEYER patches — diagnostic
Goblet cells	Fewest	Intermediate	Most numerous
Lumen and wall	Widest, thickest	Wide	Narrowest, thinnest

Goblet cells increase from duodenum to rectum, as the need for lubrication rises and for absorption falls — a gradient worth remembering as a single rule

Barrett oesophagus — the stratified squamous epithelium is replaced by columnar with goblet cells (intestinal metaplasia) in chronic reflux; premalignant for adenocarcinoma
Coeliac disease — villous atrophy with crypt hyperplasia and intra-epithelial lymphocytes on duodenal biopsy; the surface area is lost and malabsorption follows
Peptic ulcer — parietal cell mass and Helicobacter pylori determine acid secretion; the organism lives in the surface mucus of the antrum
Pernicious anaemia — autoimmune destruction of parietal cells removes intrinsic factor, so vitamin B12 cannot be absorbed
Hirschsprung disease — absence of ganglion cells in the myenteric and submucosal plexuses; diagnosed on rectal biopsy, which must include submucosa
Typhoid ulcerates the Peyer patches of the ileum along the long axis of the gut, which is why perforation and haemorrhage occur in the third week

All questions in Systemic Histology