Melanocytes
The hair follicle, a portion of the skin where hair is formed, is divided into several specialized regions that are responsible for the development, pigmentation, and growth of hair. At the very base of the hair follicle is a region known as the hair bulb.
It is in the hair bulb that the melanocytes are found, cells that contain the melanin responsible for hair pigmentation. There are two main types of melanin – red/yellow and brown/black.
This molecule is found in the form of granules that are exported from the melanocytes to the keratinocytes nearby.
The keratinocyte is responsible for the formation of hair and the deposition of melanin, and the differences in hair pigmentation arise from variations in the number, size, composition, and distribution of these granules.
Melanocytes originate from melanoblasts, cells that arise from the neural crest starting in the fourth week of embryonic development. From this period, melanoblasts migrate to the hair follicle where they differentiate and transform into melanocytes.
The Renewal Cycle
The hair follicle renews itself through development cycles known as anagen, catagen, and telogen. During the anagen phase, the undifferentiated keratinocytes present in the hair bulb proliferate in response to stimuli, leading to hair growth. Catagen is a period of regression of activity, and telogen is the period of minimal activity.
Due to this cellular proliferation activity in the hair bulb, it was believed that this region was composed of stem cells responsible for hair growth. Currently, this region of stem cell proliferation in the hair follicle is referred to as the follicular bulge, located above the hair follicle region.
The biggest discovery regarding the follicular bulge is the fact that it can differentiate into epidermal cells and keratinocytes, as a research group from the University of Pennsylvania School of Medicine in the United States discovered in 2000. In this study, the researchers used histological sections of mouse skin and performed labeling with bromodeoxyuridine, which allows for the observation that these cells are capable of dividing. From this result, the researchers concluded that this region contains bipotent cells, which may be a significant source for skin studies.
Stem Cells
The stem cells stem, like bipotent cells, are still a significant source of studies related to the development of the different tissues that make up the human body, and how such discoveries can lead to a greater understanding of diseases and physiological processes. Thus, the development of new medications and cosmetics that can benefit societal well-being still depends on the meticulous study of how living beings function at the cellular and molecular level.
References:
Ohyama M. 2007. Hair follicle bulge: A fascinating reservoir of epithelial stem cells. J Dermatol Sci. 46(2), 81-89.
Jensen PJ, Sun TT, Lavker RM. 2000. Involvement of Follicular Stem Cells in Forming Not Only the Follicle but Also the Epidermis. Cell, Vol. 102, 451–461.
Lin JY and Fisher DE. 2007. Melanocyte biology and skin pigmentation. Nature 445, 22.
