Figure 1619

Photomicrograph of an intestinal villus. The surface of the villus consists of columnar epithelial cells, chiefly enterocytes with a striated border. Also evident are goblet cells that can be readily identified by the presence of the apical mucous cup. Located beneath the epithelium is the highly cellular loose connective tissue, the lamina propria. The lamina propria contains large numbers of round cells, mostly lymphocytes. In addition, smooth muscle cells can be identified. A lymphatic capillary called a lacteal occupies the center of the villus. When the lacteal is dilated, as it is in this specimen, it is easily identified. X160.

Tight junctions establish a barrier between the intestinal lumen and the epithelial intercellular compartment

The tight junctions between the intestinal lumen and the connective tissue compartment of the body allow selective retention of substances absorbed by the enterocytes. As noted in the section on occluding junctions (see page 97), the "tightness" of these junctions can vary.

In relatively impermeable tight junctions, as in the ileum and colon, active transport is required to move solutes across the barrier. In simplest terms, active transport systems, e.g. sodium pumps (Na+/K+-ATPase), located in the lateral plasma membrane transiently reduce the cytoplasmic concentration of Na+ by transporting it across the lateral plasma membrane into the extracellular space below the tight junction. This transport of Na+ creates a high intercellular Na+ concentration, causing water from the cell to enter the intercellular space, reducing both the water and Na+ concentrations in the cell. Conse quently, water and Na+ enter the cell at its apical surface, passing through the cell, and exiting at the lateral plasma membrane as long as the sodium pump continues to function. Increased osmolality in the intercellular space draws water into this space, establishing a hydrostatic pressure

submucosa muscularis externa lymphatic nodules circular fold

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