A study of the hair follicle and its melanocytes

Abstract

Besides decreasing the number of secretory melanocytes in the hair follicle, X-irradiation can cause change in the length of dendrites, decrease in the secretion of pigment granules, change in the color of melanin granules, and alteration in the distribution of melanin in the hair. Graying or lightening of hair color is often caused by a shortening of the dendrites, which alters the balance of pigment in the hair. Decreased dendrite length reduces the quantity of pigment in the hair cortex, but does not greatly alter the quantity of pigment in the medulla. This type of graying can be induced by X-irradiation and by genetic change. It also occurs cyclically during the growth of normally pigmented hairs. Melanocytes in the dermal papilla of the dilute-black rabbit follicle give off granules of melanin to the connective tissue ground substance and to the dermal papilla cells. This is a rare example of melanin granule secretion, and possibly cytocrine activity, taking place in the absence of the usual melanocyte-epithelial relationship. In the rabbit hair follicle the secretion of pigment was studied in melanocytes that completely lack dendrites. In these cells, the cytocrine transfer of melanin involves the budding of granules from the smooth surface of the melanocyte, with the simultaneous engulfing of the granules by adjacent epithelial cells. This suggests that the dendrite is not an essential device for carrying out the transfer of melanin. In hair follicles of the rabbit, dendrites appear to have three important functions: they greatly increase the surface area of the secretory membrane and thus facilitate pigment granule transfer, they control hair color intensity by regulating the distribution of pigment in the developing hair, and they may help to anchor the melanocyte to the dermal papilla or “basement membrane.” By tracing the flow of pigment granules secreted by single melanocytes, the relative movements of hair cells and melanocytes can be demonstrated in the growing follicle of the rabbit. This type of study indicates that secretory melanocytes are not positioned in or held by the bulb epithelium, as has previously been believed. Instead, they adhere to the dermal papilla, to the “basement membrane” of the hair follicle bulb, or to both. Actually, there is no stable region of epithelium that can hold or anchor the secretory melanocytes. All the epithelial cells bordering the upper part of the dermal papilla migrate around the melanocytes, pick up granules of pigment, and then differentiate into the medulla of the hair. These observations do not disprove the idea that pigment cells migrate back and forth between the hair follicle bulb and dermal papilla during the growth cycle, but they do indicate that it is unnecessary to postulate such movements. Large changes occur in the morphology and physiology of the melanocytes and epithelial cells of the ahir follicle during the period of hair growth (anagen VI or metagen). When the hair begins to form, a minimum number of melanocytes appears at the bulb-papilla interface, and long dendrites from these cells deliver pigment to both the medulla and cortex of the hair. As the hair increases in width the dermal papilla and hair follicle bulb enlarge, and many new melanocytes start to synthesize the additional melanin needed to maintain the color of the hair. When the narrow basal part of the hair begins to grow, the dermal papilla and hair follicle bulb shrink to a minimal size once more, and many of the melanocytes disappear. The dendrites of the remaining melanocytes shorten, and a minimum amount of pigment is delivered to only the medulla or central part of the hair.