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The hair follicles of the mouse originate in the dermis and traverse the epidermis. They produce hairs and are associated with sebaceous glands. In mice (with the exception of albino strains), hair follicles and hairs contain a pigment produced by melanocytes, melanin. The morphology of the hair follicles changes with the follicle cycles, anagen (active growth), catagen (controlled regression through apoptosis), and telogen (resting phase). In contrast to the mosaic pattern of hair follicle cycling in humans, mouse hair follicles cycle in waves from the skull to the tail. During the hair follicle cycle the thickness of the hypodermal fat layer changes (e.g., it becomes thicker in anagen), but the thickness of the dermis and the epidermis remain unchanged.
The micrographs are cross sections of mouse dorsal skin (albino strain) in the three hair follicle stages. In anagen, the hair follicles are long (fully formed) with their dermal papilla close to the muscle layer and the hypodermal fat layer (adipose tissue) is thick. In catagen, the follicles have become smaller, whereas the layer of adipose tissue has decreased in thickness. In telogen, the hair follicles are very small and the layer of adipose tissue is thin.
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