In epithelia with more permeable tight junctions, such as those in the duodenum and jejunum, a sodium pump also creates low intracellular Na concentration. When the contents that pass into the duodenum and jejunum are hypotonic, however, considerable absorption of water, along with additional Na and other small solutes, takes place directly across the tight junctions of the enterocytes into the intercellular spaces. This mechanism of absorption is referred to as solvent drag.
Other transport mechanisms also increase the concentrations of specific substances, such as sugars, amino acids, and other solutes in the intercellular space. These substances then diffuse or flow down their concentration gradients within the intercellular space to cross the epithelial basal lamina and enter the fenestrated capillaries in the lamina propria located immediately beneath the epithelium. Substances that are too large to enter the blood vessels, such as lipoprotein particles, enter the lymphatic lacteal.
The lateral cell surface of the enterocytes exhibits elaborate, flattened cytoplasmic processes (plicae) that interdigitate with those of adjacent cells (see Fig. 5.000). These folds  increase the lateral surface area of the cell, thus increasing the amount of plasma membrane containing transport enzymes. During active absorption, especially of solutes, water, and lipids, these lateral plications separate, enlarging the intercellular compartment. The increased hydrostatic pressure from the accumulated solutes and solvents causes a directional flow through the basal lamina into the lamina propria (see Fig. 5.1).
FIGURE 17.19 • 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. 160.
In addition to the membrane specializations associated with absorption and transport, the enterocyte cytoplasm is also specialized for these functions. Elongated mitochondria that provide energy for transport are concentrated in the apical cytoplasm between the terminal web and the nucleus. Tubules and cisternae of the smooth endoplasmic reticulum (sER), which are involved in the absorption of fatty acids and glycerol and in the resynthesis of neutral fat, are found in the apical cytoplasm beneath the terminal web.
Enterocytes are also secretory cells, producing enzymes needed for terminal digestion and absorption as well as secretion of water and electrolytes.
The secretory function of enterocytes, primarily the synthesis of glycoprotein enzymes that will be inserted into the apical plasma membrane, is represented morphologically by aligned stacks of Golgi cisternae in the immediate supranuclear region and by the presence of free ribosomes and rER lateral to the Golgi apparatus (see Fig. 17.21). Small secretory vesicles con-
FIGURE 17.20 • Photomicrograph of Peyer’s patches. This photomicrograph shows a longitudinal section through the wall of a human ileum. Note the extensive lymphatic nodules located in the mucosa and the section of a circular fold projecting into the lumen of the ileum. Lymphatic nodules within the Peyer’s patch are primarily located within the lamina propria, although many extend into the submucosa. They are covered by the intestinal epithelium, which contains enterocytes, occasional goblet cells, and specialized antigen-presenting M cells. 40.