The hair follicle is a complex but small organ, which contains nerve fibers and blood vessels around the actual hair follicle. About 80% of hair follicles are paired (or come in clusters of two) .
The rest are either singles, triples or an occasional quad .These clusters are called follicular units because they share a common blood and nerve supply .
When doing hair transplantation and dissecting the donor area, it is important not to cut these follicular units apart because this generally results in miniaturized transplanted hairs rather than the full-sized hairs that are desired .
Note that human hairs generally grow for two to six years in the anagen phase, and then shift into the catagen and subsequently telogen phase .The telogen phase usually lasts about three months . During telogen, the hair becomes loose and falls out, and the hair follicle withers up and virtually disappears .
This is shown in the illustration of return to anagen .Miraculously, after about three months, the hair follicle regenerates (from the interaction of two types of stem cells, those from the bulge area around the sebaceous gland, and others from the epithelial layer .) Following this, the regenerated hair follicle generates a new hair, which then grows for two to six years.