Dr  Sampurna Roy  MD

The small intestine consists of duodenum, jejunum and ileum and is the principal site of absorption of food products from gastrointestinal tract. (Total length in man= 4 to 6m )

The epithelial component of the small bowel is composed of villi (finger like projections) and crypts (crypts of Liberkuhn).
Normal  villous to crypt length length ratio is approximately 3:1 to 5:1.
When 4 normal villi are identified in a specimen, it usually indicates that the entire specimen has a normal villous architecture (although presence of focal lesion can not be completely ruled out).
Due to high rate of cell turnover  numerous mitotic figures are noted in the crypts.

Cells in the villi and crypts:
The villous epithelium is composed of columnar absorptive cells (Enterocytes) & mucin secreting cells (Goblet cells).
The enterocytes are characterized by basally located nuclei which are evenly aligned.
PAS positive brush border is present on the luminal surface of enterocytes.
Goblet cells are scattered among the enterocytes. Goblet cells contain Alcian blue positive sialomucin.
The third cell type are the Paneth cells, present in the crypts. These are characterised by brightly eosinophilic, supranuclear, cytoplasmic granules.
Endocrine cells are present in the crypts as single cells or in clusters. Unlike paneth cells these are present in the basal part of the cells (infranuclear position).

Inflammatory cells:
These are present in the lamina propria and usually consists of plasma cells and lymphocytes. Intraepithelial lymphocytes occur in the ratio of 20 lymphocytes per 100 enterocytes (1 lymphocyte per 5 enterocytes).
IELs are T-lymphocytes and express CD3, CD5 & CD8.  CD4 is not expressed.

Duodenum:
The villi are shorter and broader than jejunal villi, with branching extensions.
Brunner's glands are lobular collections of mucin secreting glands that empty into crypts through ducts. Brunner's glands contain neutral PAS positive mucin.
Brunner's glands are abundant in the first part of duodenum, less prominent in second part and not present in the third and fourth parts.
In duodenum there is gradual transition of the epithelium across the gastroduodenal junction.
Antral-type gastric epithelium extends into the proximal duodenum (upto 6mm).
In proximal duodenum the villi are lines by cells showing features of both antral and intestinal mucosa.
More numbers of mononuclear cells are normally present in the duodenum than rest of the small intestine.

Ileum:
Increased number of goblet cells.
Villi are shorter more finger-like in shape.
Lymphoid nodules (Peyer's patches) are prominent in the ileum.

[Segal GH, Petras RE. Small intestine in Sternberg SS editor. Histology for pathologists 2nd edition Lippincott-Raven, Philadelphia 1997, p 495-515.]