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Pigmentation Revisited

By: Peter T. Pugliese, MD
Posted: February 26, 2009, from the March 2009 issue of Skin Inc. magazine.

page 2 of 12

Melanocytes originate in the neural crest, but they are also found in a few other places: the eyes, hair, ears and leptomeningesb of the brain all contain melanocytes. What is interesting about this is that they must have different functions other than environmental protection, as do epidermal melanocytes. This is a different topic and, although it is exciting, little is known about these functions.

The pigmented cells migrate as melanoblastsc, which are the precursors of melanocytes. Eventually, the skin melanoblasts end up in the basal layer of the epidermis. Keep in mind that the migration of the melanoblasts occurs in the embryo, and that these cells must move from the neural crest to the dermis of the skin. Briefly, that pathway follows the lateral structures of the embryo, namely the space between the somites and the ectoderm to eventually reach the basal layer of the epidermis via the dermis. If you look at this pathway in Figure 2 you will see the structures clearly. It is important to understand this because the melanoblasts must pass through many types of tissues and proteins and need to be protected from attack by these structures. As a blast cell, it is not fully differentiated and thus does not have the antigenicityd of fully developed cells. Although the immune system is poorly developed in a fetus, it is almost nonexistent in the embryo.e

To review this pathway, all melanocytes originate in the neural crest, make their way to the epidermis through the embryonic tissues and land in the dermis. From here, they enter the epidermis and reside in the basal layer. Investigators say this migration is nearly complete by the seventh week of gestation and is fully completed by week 11, though some say sooner.1

Making melanosomes

Now that melanoblasts are in the epidermis, they will fully differentiate and do their job. The major function of melanocytes is to make melanosomes and then transfer them to the keratinocytes so they will act as a shield against ultraviolet (UV) radiation. Just these two topics alone would fill hundreds of pages , but this article will coveronly the essential steps for each process. f

The formation of melanin in a melanosome occurs in four stages.