Sebaceous glands secrete sebum that coats the hair and skin surface.
Sebaceous glands develop as outgrowths of the external root sheath of the hair follicle, usually producing several glands per follicle (Fig. 15.14 and Plate 45, page 521). The oily substance produced in the gland, sebum, is the product of holocrine secretion. The entire cell produces and becomes filled with the fatty product while it simultaneously undergoes programmed cell death (apoptosis) as the product fills the cell. Ultimately, both the secretory product and cell debris are discharged from the gland as sebum into the infundibulum of a hair follicle, which forms the pilosebaceous canal with the short duct of the sebaceous gland. New cells are produced by mitosis of the basal cells at the periphery of the gland, and the cells of the gland remain linked to one another by desmosomes. The basal lamina of these cells is continuous with that of the
FIGURE 15.14 • Photomicrograph of a sebaceous gland. a.
This micrograph shows the secretory lobules and their pilosebaceous canal of two sebaceous glands. The canal of the gland on the left is about to enter the hair follicle seen at the top of the micrograph. The canal of the sebaceous gland on the right has been sectioned in a manner that shows mostly the wall of the canal. 60. b. The secretory component of the lobule in the lower box of Figure a is shown here at higher magnification. Note the light staining of the secretory cells due to the lack of staining of the sebum that they contain. These cells are actively producing sebum. The basal cells at the periphery of the lobule proliferate producing new sebum-producing cells. 120. c. The secretory component of the lobule in the upper box of Figure a is shown here at higher magnification. The sebum-filled cells are now within the canal. Note their pyknotic nuclei, signifying the death of the cell. 120.
Although many neural and emotional factors can alter the composition of sweat, altered sweat composition can also be a sign of disease. For example, elevated sodium and chlo-ride levels in sweat can serve as an indicator of cystic fbro-sis. Individuals with cystic fibrosis have two to five times higher than normal amounts of sodium and chloride in their sweat. In pronounced uremia, when the kidneys are unable to rid the body of urea, the concentration of urea in sweat increases. In this condition, after the water evaporates, crystals may be discerned on the skin, especially on the upper lip. These include urea crystals and are called urea frost.
 FOLDER 15.5 Clinical Correlation: Sweating and Disease epidermis and the hair follicle. The process of sebum production from the time of basal cell mitosis to the secretion of the sebum takes about 8 days.
The basal cells of the sebaceous gland contain smooth-surface endoplasmic reticulum (sER), rER, free ribosomes, mitochondria, glycogen, and a well-developed Golgi apparatus. As the cells move away from the basal layer and begin to produce the lipid secretory product, the amount of sER increases, reflecting the role of the sER in lipid synthesis and secretion. The cells gradually become filled with numerous lipid droplets separated by thin strands of cytoplasm.