Neural crest-derived melanocytes are scattered among the basal cells of the stratum basale
During embryonic life, melanocyte precursor cells migrate from the neural crest and enter the developing epidermis. A specific functional association is then established, the epidermal-melanin unit, in which one melanocyte maintains an association with a given number of keratinocytes. This ratio varies in different parts of the body.
The epidermal melanocyte is a dendritic cell that is scattered among the basal cells of the stratum basale (Fig.
14.7). They are called dendritic cells because the rounded cell body resides in the basal layer and extends long processes between the keratinocytes of the stratum spinosum. Neither the processes nor the cell body forms desmosomal attachments with neighboring keratinocytes. However, melanocytes that reside close to the basal lamina have structures that resemble hemidesmosomes. The ratio of melanocytes to keratinocytes or their precursors in the basal layer ranges from 1:4 to 1:10 in different parts of the body and is constant in all races. In routine hematoxylin and eosin (H&E) preparations melanocytes are seen in the stratum basale with elongated nuclei surrounded by a clear cytoplasm. With the TEM, however, they are readily identified by the developing and mature melanin granules in the cytoplasm (Fig. 14.8). Melanocytes maintain the capacity to replicate throughout their life, although at a much slower rate than keratinocytes, thus maintaining the epidermal-melanin unit.
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