Hair Development, Growth, and Loss 50 synchronized in small domains. Overall, the adult skin displays a mosaic pattern of hair follicles belonging to the different cycling phases as the mouse ages.42 In contrast, synchronized human fetal hair cycle becomes asynchronous soon after birth, when each hair follicle starts to follow its own cycling clock. With each additional cycle, hair follicle cycling slows down and telogen duration extends considerably.43 Postnatal hair follicle cycling in mice is actually initiated around postnatal day 17 or 18 when hair follicles become mature and subsequently enter rapid regression (catagen). During this anagen- to-catagen transition, proliferation and differentiation of matrix cells decline. The timing of catagen onset varies between mouse strains as well as different skin regions. In general, the first two catagens begin in a wave spreading from the head towards the tail, and from the midline of the body laterally down the sides of the animal. First catagen lasts three to four days in mice, and can be identified by observing the color of the skin, which changes from black to pink. During catagen the lower two-thirds of the hair follicle rapidly regresses, which is mainly caused by apoptosis of keratinocytes in the bulb, IRS, and ORS. As the lower cycling portion of the hair follicle recedes, the bottom of the hair shaft seals off and forms a rounded structure called the hair club. The hair club will move upward until it reaches the permanent, non-cycling portion of the hair follicle. During this process, a temporary epithelial strand is formed and connects the DP to the upper portion of the hair follicle. The epithelial strand mainly contains apoptotic cells and is eliminated when the DP moves into close proximity of the bulge. Bulge hair follicle stem cells escape apoptosis during the catagen.8,13 The first catagen is followed by a short telogen phase when follicles remain dormant. The first hair follicle telogen lasts only one to two days, while the second telogen usually takes more than two weeks. Although traditionally described as the resting phase of the hair follicle cycle, there are major changes in transcriptional activities of genes during the telogen stage. Therefore, telogen is not at all resting, and probably represents a key stage in controlling hair
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