 The lipid envelope is a 5-nm-thick layer of lipids attached to the cell surface by ester bonds. The major lipid components of the lipid envelope are ceramides, which belong to the class of sphingolipids; cholesterol; and free fatty acids. However, the most important component is the monomolecular layer of acylglucosylceramide, which provides a “Teflon-like” coating on the cell surface. Ceramides also play an important role in cell signaling and are partially responsible for inducing cell differentiation, triggering apoptosis, and reducing cell proliferation. As the cells continue to move toward the free surface, the barrier is constantly maintained by keratinocytes entering the process of terminal differentiation. Lamellae may remain as recognizable discs in the intercellular space or may fuse into broad sheets or layers.
Experiments have shown that the epidermis of animals with induced essential fatty acid defciency (EFAD) is more permeable than normal to water. The membrane-coating granules also have fewer lamellae than normal. Destruction of the epidermal barrier over large areas, as in severe burns, can lead to life-threatening loss of ﬂuid from the body.
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. In adults, a pool of undifferentiated melanocyte stem cells resides in the area of the hair follicle called the follicular bulge. Differentiation of the melanocyte stem cell is regulated by the expression of the Pax3 gene that belongs to the paired box (PAX) family of transcription factors. Pax3 activates expression of the microphthalmia transcription factor (MITF), which is critical for the development and differentiation of melanocytes (melanogenesis).
The epidermal melanocyte is a dendritic cell that is scattered among the basal cells of the stratum basale (Fig. 15.7). 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 (see Fig. 15.7). Melanocytes maintain the capacity to replicate throughout their life, although at a much slower rate than keratinocytes, thus maintaining the epidermal-melanin unit.
FIGURE 15.6 • Electron micrographs of keratinocytes. a. Much of the keratinocyte cytoplasm is filled with tonofilaments. One keratinocyte exhibits a keratohyalin granule (KG). Near the plasma membrane closest to the surface (upper left), two keratinocytes display lamellar bodies (arrowheads). 8,500. b. A lamellar body at higher magnification. 135,000. c. Part of a keratinized cell and the underlying keratinocyte. Located between the cells are the contents of the lamellar bodies, which have been discharged into the intercellular space (arrow) to form the lipid envelope. 90,000. (Courtesy of Dr. Albert I. Farbman.)