Human Physiology, 14th edition (2016)

630 Chapter 18

and are shed into the lumen. In this way, the intestinal epithe-
lium is renewed every five to seven days.
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. 18.11 ).

Intestinal Enzymes

In addition to providing a large surface area for absorption, the
plasma membranes of the microvilli contain digestive enzymes
that hydrolyze disaccharides, polypeptides, and other substrates
( table  18.1 ). These brush border enzymes are not secreted into
the lumen, but instead remain attached to the plasma membrane
with their active sites exposed to the chyme ( fig. 18.12 ). One brush

Figure 18.10 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.

Simple columnar

epithelium

Lacteal

Capillary network

Goblet cells

Intestinal crypt

Lymph vessel

Arteriole

Venule

Villus

Figure 18.11 Electron micrograph of microvilli in

the small intestine. In this colorized image, the microfilaments

within each villus can be seen. The microvilli increase the surface

area for food absorption, and their plasma membranes contain

digestive brush border enzymes.

Microvilli

Plasma

membrane

Microfillaments

CLINICAL APPLICATION

Lactose intolerance, a condition with greatest prevalence

among those with African, Asian, Hispanic, and Native

American heritage, refers to an inability to digest the disac-

charide lactose into glucose and galactose for absorption by

the small intestine. Digestion of lactose requires the enzyme

lactase, one of the brush border disaccharidases. Most peo-

ple with lactose intolerance can produce lactase when they

are babies and young children, but gradually lose this ability

with age. Accumulation of lactose in the intestine produces

such symptoms as abdominal gas and bloating, diarrhea,

and nausea. Avoiding cow’s milk and choosing dairy prod-

ucts low in lactose, such as hard cheeses, helps to avoid

these symptoms. Yogurt, though relatively high in lactose, is

usually well tolerated because it contains bacteria with the

lactase enzyme that can digest lactose in the intestine.