Normally, the body loses -600 mL of water a day through evaporation from the lungs and skin. Under conditions of high ambient temperature, water loss is increased by an increased rate of sweating. This thermoregulatory sweating first occurs on the forehead and scalp, extends to the face and the rest of the body, and occurs last on the palms and soles. Emotional sweating, however, occurs first on the palms and soles and in the axillae. Sweating is under both nervous control through the autonomic nervous system and hormonal control.
Sebaceous glands secrete sebum, an oily substance that coats the hair and skin surface. Sebaceous secretion is a holocrine secretion; the entire cell produces, and becomes filled with, the fatty secretory product while it simultaneously undergoes progressive disruption, followed by necrosis, as the product fills the cell. Both secretory product and cell debris are discharged into the pilosebaceous canal.
Figure 1, skin, human, H&E X1000.
This section through a sweat gland shows live profiles of the ductal portion (D) and two profiles of the secretory portion (SG). The larger secretory segment is through a region either just below or above where a U turn was made; therefore, it shows two luminal profiles. The lumina of both the ductal and the secretory units are marked by asterisks.
The glandular unit of the eccrine sweat gland contains two epithelial cell types and myoepithelial cells (M). Arrowheads show small cross sections of myoepithelial cell cytoplasm; large arrows show where more elongate profiles of myoepithelial cytoplasm are evident. The epithelial cells are of two types, designated dark cells and clear cells. Unfortunately, the characteristic dark cytoplasmic staining of the dark cells is not evident unless special precautions are taken to preserve the secretory granules in their apical cytoplasm. Nevertheless, note that the dark cells are closer to the lumen, whereas the clear cells are closer to the base of the epithelial layer, making contact with either the basal lamina or, more frequently, the myoepithelial cells. In addition, the clear cells are in contact with intercellular canalic-uli. Several such intercellular canaliculi are shown in the secretory units (small arrows). This figure also shows that the duct consists of two layers of small cuboidal cells.
Sebaceous glands develop from the epithelial cells of the hair follicle and discharge their secretion into the follicle, from where it reaches the skin surface. The sebaceous secretion is rich in lipid, and this is reflected in the cells of the sebaceous gland. A section of a sebaceous gland and its related hair follicle is shown in this figure. At this level, the hair follicle consists of the external root sheath (RS) sur rounding the hair shaft. The sebaceous gland (Seb) appears as a cluster of cells, most of which display a washed-out or finely reticulated cytoplasm. This is because these cells contain numerous lipid droplets and the lipid is lost by dissolution in fat solvents during the routine preparation of the H&E-stained paraffin section. The opening of the sebaceous gland through the external root sheath (eRS) and into the hair follicle is shown in the lower right.
The same sebaceous gland as in Figure 2 is shown here at higher magnification. Numbers 1 to 4 show a series of cells filled with an increasingly greater amount of lipid and progressively closer to the opening of the gland into the hair follicle. The sebaceous secretion includes the entire cell, and therefore, cells need to be replaced constantly in the functional gland. Cells at the periphery of the gland are basal cells (BC). Dividing cells in the basal layer replace those that are lost with the secretion.
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