Absorptive Cells

that drives Na+ and water across the basal lamina into the connective tissue. ,

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 interdig-itate with those of adjacent cells (see Fig. 4.15). 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. 4.1).

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. 16.21). Small secretory vesicles containing glycoproteins destined for the cell surface are located in the apical cytoplasm, just below the terminal web, and along the lateral plasma membrane. Histochemical or autoradiographic methods are needed, however, to distinguish these secretory vesicles from endocytotic vesicles or small lysosomes.

The small intestine also secretes water and electrolytes. This activity occurs mainly in the cells within the intestinal glands. The secretion that occurs in these glands is thought to assist the process of digestion and absorption by maintaining an appropriate liquid state of the intestinal chyme. Under normal conditions, the absorption of fluid by the villus enterocyte is balanced by the secretion of fluid by the gland enterocyte.

Goblet cells represent unicellular glands that are interspersed among the other cells of the intestinal epithelium

As in other epithelia, goblet cells produce mucus. In the small intestine, goblet cells increase in number from the duodenum to the terminal part of the ileum. Also, as in other epithelia, because water-soluble mucinogen is lost during preparation of routine H&E sections, the part of the cell that normally contains mucinogen granules appears empty. Examination with the TEM reveals a large accumulation of mucinogen granules in the apical cytoplasm that distends the apex of the cell and distorts the shape of neighboring cells (Fig. 16.22). With the apex of the cell containing a large accumulation of mucinogen granules, the basal portion of the cell resembles a narrow stem. This basal portion is intensely basophilic in histologic preparations because it is occupied by a heterochro-matic nucleus, extensive rER, and free ribosomes. Mitochondria are also concentrated in the basal cytoplasm. The characteristic shape, with the apical accumulation of granules and the narrow basal stem, is responsible for the name of the cell, as in a glass "goblet." An extensive array of flattened Golgi cisternae forms a wide cup around the newly formed mucinogen granules adjacent to the basal part of the cell (Fig. 16.23). The microvilli of goblet cells are restricted to a thin rim of cytoplasm (the theca) that surrounds the apical-lateral portion of the mucinogen granules. Microvilli are more obvious on the immature goblet cells in the deep one half of the intestinal gland.

microvilli microvilli

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