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By: Peter T. Pugliese, MD
Posted: February 26, 2009, from the March 2009 issue of Skin Inc. magazine.
Estheticians are faced with problems of pigmentation every day. Some clients want to be lighter, some darker; some have white spots, dark pigmented spots, freckles or melasma. It is difficult to know what to do for most of these clients without having a good foundation in the science of pigmentation. It takes years to learn all there is to know about human pigmentation, and even longer to put it into practical use. Some additional background is needed in the basic science of pigment-formation, as well as more information about the melanocyte, how melanin is formed and the different types of melanin. Armed with this information, you can judge what treatments and products can and cannot do for your clients. This article is not a how-to article—not a cookbook series of recipes—but it is solid information about melanin and melanocytes that will provide a good foundation resulting in a better understanding of human pigmentation.
Skin color and the melanocyte
Skin color in humans is produced by three compounds: hemoglobin, carotene and a pigment called melanin, which is the topic of this article. There is much new information in the scientific literature about pigmentation and melanin in particular, but some of it is controversial. A clear understanding of what melanin is, where it comes from and how it is regulated is essential, as this knowledge will help form the basis of a rational treatment program when faced with pigmentation problems.
The melanocyte is a dendritic cell with finger-like projections and is found in the basal layer of the epidermis. It is a free-ranging cell without desmosomes or tonofilaments. In appearance, microscopically it has a small dark nucleus and a clear cytoplasm, depending on the histological stain used to visualize it. Melanin is made in solid structures known as melanosomes. Melanosomes are organelles bound to membranes located in the cytoplasm of melanocytes that carry the tyrosinase enzyme. Melanosomes are responsible for melanin synthesis and pigment transfer from the melanocyte to the associated keratinocytes by a process called phagocytosis. In this article, the origin of the melanocyte, how the melanosome and melanin are formed, and how the melanosome is transferred from the melanocyte to the keratinocyte will be discussed.
One week after conception until the end of the second month, a developing child is known as an embryo. After the eighth week, the developing child is called a fetus. It is in the early stages of embryonic development that melanocytes appear. Cellular division yields a small hollow ball of cells, which then invaginates to form a tube within a tube in a process known as gastrulation. The all-powerful embryonic cells now undergo an incredible change and differentiatea into three cell types that are called germ layers. The outer layer is the ectoderm, the middle layer is the mesoderm and the inner layer is the endoderm. This is an important bit of biological information because it is from these three cell layers that all the rest of a human’s cells, tissues and organs originate.
As the gastrulation process progresses, the mesodermal cells form a structure called the notochord. As the notochord develops, it marks the midline of the embryo, and just above the notochord, the ectoderm begins to lay down the neuroectoderm, which will become the entire nervous system. This structure will give rise to the neural crest, from which will come all the melanocytes, and that is why you need to know about embryonic development. This relationship is illustrated and diagramed in Figure 1; however, it is simplified to include only the essential structures you need to know.