Gartner_BRS Cell Biology & Histology, 9e

BRS Cell Biology and Histology

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Clinical PEARLS The production of pheomelanin appears to be dependent on a loss-of-function mutation in the MC1R transmembrane molecule . Thus, individuals with this mutation produce less melanin and more pheom elanin; therefore, their hair is reddish in color and their skin is less pigmented. Individuals with this type of mutation have a greater incidence of melanoma. 3. The average human head has approximately 150 000 hairs, which grow at an approximate rate of 2 mm/d. Hair growth occurs in three phases, anagen (growing phase), catagen (short period of suspension from hair growth), and telogen (terminal resting phase). When the hair falls out, that shed hair is known as a club hair because it possesses a club-shaped root. Approximately 50 to 100 hairs are lost from the head every day. Hairs in some regions of the body last longer than in others; hair on the head stays in place for as long as 7 years, whereas hairs in the armpit fall out in less than a half a year. C. The arrector pili muscle is a strip of smooth muscle that attaches at an oblique angle to the dermal sheath surrounding a hair follicle. It extends superficially to underlie sebaceous glands ( Fig. 12.7 ), passing through the reticular layer of the dermis and inserting into the papillary layer of the dermis. Contraction elevates the hair and is responsible for the formation of goosebumps, caused by depressions of the skin where the muscle attaches to the papillary layer of the dermis.

DC

PN

HN

BC

APM

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FIGURE 12.7. A high-magnification micrograph of a sebaceous gland displaying the basal cells (BCs) that are re sponsible for the regeneration of the gland. Young cells are evident because of their healthy nuclei (HN), and the dying cells display pyknotic nuclei (PN). Cells that are dead (DC) are going to be released, via holocrine secretion, into the follicular canal of the hair follicle. Observe the arrector pili muscle (APM) ( × 270).