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Small Intestine

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Light microscopy image of a cross-section of the jejunum. The image shows prominent villi extending upward, stained with hematoxylin and eosin (H&E), giving them a purple appearance. At the bottom is a muscular layer. The rest of the sample around the villi is a lighter pink. The backdrop is stark white.
Figure 1: Microscopic view of small intestine jejenum tissue showing villi stained purple.

The small intestine is responsible for the absorption of nutrients from ingested food. Its histology is highly specialized to maximize the surface over which it can do this absorption.

Illustration of the cross-section of the small intestine showing circular folds (plicae circulares) and a close-up of two villi with simple columnar cells lining the lumen.
Illustration of the cross-section of the small intestine showing circular folds (plicae circulares) and a close-up of two villi with simple columnar cells lining the lumen.

Mucosa

The innermost layer of the small intestine is lined with simple columnar epithelium, consisting mainly of absorptive enterocytes. These cells have microvilli on their apical surface. The brush border is formed, which greatly increases the surface area for nutrient absorption. Scattered among the enterocytes are mucus cells. They secrete mucus to protect the epithelium from mechanical damage. The mucus also provides lubrication for the passage of food.  Enteroendocrine cells are interspersed throughout the epithelium. They secrete various hormones involved in regulating digestion. These hormones include cholecystokinin (CCK) and secretin.

Invaginations of the epithelium into the underlying lamina propria form intestinal glands, also known as crypts of Lieberkühn. These glands contain stem cells. They continuously replenish the epithelial lining. The glands also have secretory cells that produce mucus and other substances.

Submucosa

The submucosa is rich with arteries bringing blood to the capillary beds absorbing nutrients from the chyme in the lumen.  The veins draining the enriched blood flow into the hepatic portal system. This system transports the blood to the liver. It releases the blood to systemic circulation afterwards.  It also contains the lacteals, specialized lymphatic capillaries responsible for absorbing dietary fats.

Muscularis Externa

The muscularis externa is composed of inner circular and outer longitudinal layers of smooth muscle. It is responsible for peristaltic contractions that propel chyme through the small intestine for digestion and absorption.

Serosa

The outermost layer of the small intestine is covered by serosa. It is a serous membrane made up of mesothelial cells and connective tissue.  It provides protection and facilitates movement within the abdominal cavity.

High-magnification light micrograph showing intestinal villi from the jejunum. The villi exhibit a tall columnar epithelium with basally located, darkly stained nuclei. The lamina propria is visible as the central core of the villi. The tissue is stained to highlight cellular details. The background is light, providing contrast to the violet-stained tissue.
Figure 2: Microscopic view of a small intestine (jejunum) villus with visible nuclei of simple columnar epithelium and mucus cells
Light microscopy image showing small intestine submucosa. Intestinal villi are arranged on the left and right sides, stained with Hematoxylin and Eosin (H&E) to appear purple. Connective tissue, blood vessels, and cellular components are visible between the villi. The composition emphasizes structural details.
Figure 3: Microscopic view of small intestine submucosa, showcasing its structural composition of glands, vessels and lacteals.
Light micrograph of small intestine's muscularis externa. The image is vertically oriented. Top section displays villi, colored in dark pink. Beneath the villi is a layer showing blood vessels. The Muscularis Externa section appears below these sections. Section appears light pink color. At the bottom section is the muscle layers, colored pink. Overall pink-red staining highlights cellular structures.
Figure 4: Small intestine tissue under microscope; shows circular and longitudinal layers of muscularis externa smooth muscle.
Light micrograph of small intestine serosa. Pink-stained collagen fibers exhibit a wavy texture in the bottom left of the compositon as the layer thins moving to the top right of the compositon. The tissue is contrasted by sparse dark pink elliptical shaped nuclei on a cream light white background. A ragged tear is visible on the tissue surface.
Figure 5: Microscopic view of serosa layer in small intestine, showing wavy connective tissue and scattered nuclei stained pink.

Functions

Absorption
Chemical Digestion via Enzymes
Emulsification via Bile

Specializations

The small intestine is highly adapted to increase the surface over which absorption of nutrients can happen.  Circular folds, also called plicae circulare, expand the surface area.  On the circular folds are finger-like projections called villi that further expand the surface are.  Microvilli are actually cellular extensions and not folds of the mucosa.  

Figures 1-3: Jejunum, human, H&E stain, 40X Slide UCSF 246
Figures 4 & 5: Small intestine, ileum, H&E, 20X Slide 170
All by University of Michigan Histology, licensed under CC BY-NC-SA 3.0.

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