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The Structure of the Hair Follicle

The average adult has 100,000 hair follicles on her scalp. Brunettes have the highest number of scalp follicles (about 155,000) followed by blondes (140,000) and redheads (85,000).

The hair follicle is divided into three anatomic and functional segments:

1. The infundibulum,
2. The isthmus and
3. The lower follicle, which contains the hair bulb.

The infundibulum includes the area from the skin surface to the opening of the sebaceous duct. The isthmus extends from the sebaceous duct to the insertion of the arrector pili muscle and is the site of the hair bulge. The lower follicle includes the deepest straight portion of the hair follicle and the hair bulb. The hair bulb is composed of the hair matrix, which in turn surrounds the follicular papilla.

The infundibulum and the isthmus are the permanent portion of the hair follicle, as they do not undergo any structural changes during the hair growth cycle. The lower follicle segment, in contrast, is the transient portion of the hair follicle.