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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.]
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