| hair & baldness | |||
| hair biology Hair originates from a hair follicle; a structure of about 4-5 mm deep in theskin.Hair follicles connect to thebody through the dermal papilla. Blood vesselsand lymphatic vessels connect with the dermal papilla and sendall importantnutrients, hormones, etc. to the hair follicle for building hair. The hair follicularunit is composedof 1-3 hairs, a sebaceous gland (produces oil), and an apocrinegland (produces odor). Hair grows out from the skin as a group of 1-3 hairs and occasionally 4 hairs. When we get older, the amount of hair in each group declines from 3 to 2 to 1 and the hair's diameter becomes smaller with aging. |
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| hair density Humans have roughly 1 follicular unit/square mm of scalp.  About 2 hairs/unit = 200 hairs/sq. centimeter. 100,000 hairs on the scalp = 500 sq. centimeters of scalp.hairline shape For normal males and females with no baldness or receding hairline, hairline shapes are as follows:  hair direction The normal direction of hair is from the occiput to the nose as a whorl from the back. However, some will still have remnants of vellus hair left on the border of the hairline.
hair cycle Each unit of hair grows and rests in cycles. The growing phase (anagen) lasts 3-5 years. The resting phase (telogen, catagen) lasts 3-6 months. During the catagen and telogen phases, the hair root becomes club hair(dumbbell shaped hair) and is ready to fall out. If you rub or massage your head or comb the hair, club hair will fall out. This is a normal physiological phenomenon. Each day, approximately 100 hairs will fall out in normal men.
hair synthesis Hair stem cells (progenitor or pluripotential stem cells) are located at the orifice of the sebaceous gland and the insertion point of the arrector pili muscle (bulge region). Stem cells will produce daughter cells at each cycle of hair growth. Daughter cells will differentiate to become full follicular units and produce hair. At the end of the hair matrix and papilla interaction, each hair cell becomes a dead cell and piles up through the hair canal to become a hair shaft growing out of the skin. Hair synthesis requires a high metabolic rate in the body. So, in the eventthat anything interrupts this process, hair will stop growing and fall out.Examples include chemotherapy for cancer, after the shock stage,following typhoid fever, and a few months after delivering a baby. |
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