The first hair follicles are formed from the ectoderm, an epithelial layer that will give rise to the epidermis, and the underlying mesoderm, a mesenchymal layer that will form the dermis. Figure 1 indicates the main stages of follicle development as seen on the back of a mouse, but is representative of hair follicles in most mammals. The numbers assigned to stages will be used in this review. Melanoblasts, of neural crest origin, are usually present among the epithelial cells at the beginning of this period, and will differentiate into melanocytes in the base of the follicle and transfer pigment to the hair. Since they have relatively little influence on follicle development, they are not included in this review.

In most mammals the follicles that produce the pelage hairs-which form the coat of fur, hair or wool-begin to form in the skin during prenatal life at one location, such as the crown of the head, and extend in a wave-like manner over the body surface. In animals with a dense coat this first wave may be followed by further waves of smaller follicles producing smaller hairs. These follicles are arranged in regular patterns, usually in groups with one large primary follicle flanked by two slightly smaller ones, and a group of secondary follicles associated with each trio 1, 2. Whisker (vibrissa) follicles develop earlier, in restricted locations, and have their own regular pattern, for example, in rows on the upper lip.