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The physiological life cycle of melanocytes in human skin is likely very different from mouse skin since murine melanocytes are almost exclusively located in the hair follicle and dermis and not in the epidermis. In contrast to keratinocyte stem cells, melanocyte stem cells in human skin were only very recently identified in the hair follicle. Our laboratory has isolated and characterized a multi-potent progenitor cell from the human hair follicle and maintained it in ‘hair spheres’ that can differentiate not only into melanocytes but also into neuronal and smooth muscle cells. We learned to culture these cells as hair spheres using media conditions developed for human embryonic stem cells, which we can also differentiate into melanocytes. Multi-potent progenitor cells, grown as ‘embryoid bodies’ from embryonic stem cells or hair spheres from hair follicles, will provide the main resource for our studies in identifying the microenvironmental cues for stem cell differentiation in skin.
The key for our understanding of self-renewal and differentiation is composition and physical stiffness of the matrix. Nothing is known about the stem cell niche for human melanocytes outside of the human hair follicle, but on several locations of the human body the skin is normally pigmented although there are no hair follicles. In preliminary studies we have isolated spheres from both epidermis and dermis of foreskin that contain cells capable of differentiating into melanocytes and appear to have multi-potent characteristics because they can also differentiate into neuronal cells, smooth muscle cells and adipocytes.
Yu, H., Fang, D., Kumar, S.M., Li, L., Nguyen, T. K., Acs, G., Herlyn, M., Xu, X.: Isolation of a novel population of multipotent adult stem cells from human hair follicles. Am J Pathol 168:1879-1988, 2006. PMID16723703
The microscope in the image belonged to William E. Horner, M.D., a collaborator with Caspar Wistar, M.D., in the early 1800s.
Dr. Horner, a lecturer at the University of Pennsylvania, was a pioneer of the use of microscopes in anatomical and medical research. He authored Special Anatomy and Histology, a seminal text on the subject.