Villi and Microvilli

Each villus is a fingerlike fold of mucosa that projects into the intestinal lumen (fig. 18.11). The villi are covered with columnar epithelial cells, among which are interspersed mucus-secreting

Villus

(lined with simple columnar epithelium)

Lamina propria

Muscularis mucosa Duodenal glands

■ Figure 18.11 The histology of the duodenum. Note the duodenal (Brunner's) glands. These exocrine glands, unique to the duodenum, extend into the submucosa and produce a bicarbonate-rich, alkaline secretion.

Villus

(lined with simple columnar epithelium)

Lamina propria

Simple columnar epithelium

Capillary network

Goblet cells

Intestinal crypt

Lymph vessel

Simple columnar epithelium

Capillary network

Goblet cells

Intestinal crypt

Lymph vessel

■ Figure 18.12 The structure of an intestinal villus. The figure also depicts an intestinal crypt (crypt of Lieberkühn), in which new epithelial cells are produced by mitosis.

goblet cells. The lamina propria, which forms the connective tissue core of each villus, contains numerous lymphocytes, blood capillaries, and a lymphatic vessel called the central lacteal (fig. 18.12). Absorbed monosaccharides and amino acids enter the blood capillaries; absorbed fat enters the central lacteals.

Epithelial cells at the tips of the villi are continuously exfoliated (shed) and are replaced by cells that are pushed up from the bases of the villi. The epithelium at the base of the villi invaginates downward at various points to form narrow pouches that open through pores to the intestinal lumen. These structures are called intestinal crypts, or crypts of Lieberkühn (fig. 18.12).

Microvilli are formed by foldings at the apical surface of each epithelial cell membrane. These minute projections can be seen clearly only in an electron microscope. In a light microscope, the microvilli produce a somewhat vague brush border on the edges of the columnar epithelial cells. The terms brush border and microvilli are thus often used interchangeably in describing the small intestine (fig. 18.13).

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Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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