Small Intestine

Object: Examination of Histological Slide of Small Intestine

General Overview

The small intestine is the principal site of digestion and nutrient absorption, extending ~6–7 m from the pylorus to the ileocaecal valve. It is divided into duodenum (25 cm), jejunum (~2.5 m), and ileum (~3.5 m). Its enormous absorptive surface area is achieved through three structural adaptations: plicae circulares, villi, and microvilli (brush border).

Layers of the Small Intestinal Wall

1. Mucosa

• Villi: Finger-like projections of mucosa into the lumen, 0.5–1.5 mm tall, covered by simple columnar epithelium. Each villus contains a central lacteal (lymphatic capillary), blood capillaries, smooth muscle fibres, and a nerve supply. They are tallest in the jejunum and shortest in the ileum.
• Enterocytes (Absorptive Cells): Tall columnar cells with an apical brush border of microvilli (~3000 per cell). Microvilli increase surface area ~20-fold. Contain digestive enzymes (maltase, sucrase, peptidases) in the glycocalyx.
• Goblet Cells: Mucus-secreting unicellular glands scattered among enterocytes. Increase in number from duodenum → ileum.
• Crypts of Lieberkühn: Tubular glands between the bases of villi that extend to the muscularis mucosae. Contain stem cells (base), Paneth cells (base — secrete defensins/lysozyme), goblet cells, enteroendocrine cells, and immature enterocytes migrating upward.
• Lamina Propria: Loose connective tissue filling the core of villi and surrounding crypts; contains capillaries, central lacteals, smooth muscle, lymphocytes, plasma cells, and mast cells.
• Muscularis Mucosae: Inner circular and outer longitudinal smooth muscle — produces villous movement for absorption.

2. Submucosa

• Duodenum only — Brunner's Glands: Branched tubular mucous glands secreting an alkaline mucus (pH 8–9) to neutralise gastric acid entering the duodenum. Pathognomonic of duodenum.
• Contains Meissner's (submucosal) nerve plexus, blood/lymph vessels.

3. Muscularis Externa

Inner circular and outer longitudinal smooth muscle layers with Auerbach's (myenteric) plexus between them for peristalsis.

4. Serosa / Adventitia

Peritoneum covers jejunum and ileum (serosa); duodenum is largely retroperitoneal (adventitia).

Peyer's Patches

Aggregated lymphoid nodules in the ileal submucosa (occasionally extending into mucosa). Overlying epithelium contains M (microfold) cells that sample luminal antigens and deliver them to underlying lymphocytes. Key site of mucosal immunity. Their presence on a slide confirms ileal tissue.

Regional Differences

• Duodenum: Brunner's glands in submucosa; villi broad and leaf-shaped.
• Jejunum: Tallest, most numerous villi; prominent plicae circulares; richest in goblet cells and crypts.
• Ileum: Shorter villi; Peyer's patches in submucosa; most goblet cells; fewer and lower plicae circulares.

Distinguishing Features on Slide

• Long villi projecting into the lumen — key distinguishing feature from large intestine
• Crypts of Lieberkühn at villus bases
• Brush border visible at high magnification on enterocyte apex
• Brunner's glands in submucosa → confirms duodenum
• Peyer's patches → confirms ileum