Melanocytes produce and distribute melanin into keratinocytes.
The epidermal melanocytes produce and secrete the pigment melanin. The most important function of melanin is to protect the organism against the damaging effects of non-ionizing ultraviolet irradiation. Melanin is produced by the oxidation of tyrosine to 3,4-dihydroxyphenylalanine (DOPA) by tyrosinase and the subsequent transformation of DOPA into melanin. These reactions initially occur in membrane-bounded structures called premelanosomes, which are derived from the Golgi apparatus (Fig. 15.8). Premelanosomes and the early melanosomes, which have a low melanin content, exhibit a finely ordered internal structure with the TEM, reflecting their content of tyrosinase molecules. As more melanin is produced by the oxidation of tyrosine, the internal structure of the premelanosome becomes obscured until the mature melanin granule, the melanosome, is formed and then appears as an electron-opaque granule. Premelanosomes are concentrated near the Golgi apparatus; nearly mature melanosomes at the bases of the cell processes; and mature melanosomes most commonly in and at the ends of the processes (see Fig. 15.8). Developing melanosomes and their melanin contents are transferred to neighboring keratinocytes by pigment donation. This process, which involves the phagocytosis of the tips of the melanocyte processes by keratinocytes, is a type of cytocrine
FIGURE 15.7 • Diagram of the epidermis and electron micrograph of a melanocyte. a. This diagram shows a melanocyte interacting with several cells of the stratum basale and the stratum spinosum. The melanocyte has long dendritic processes that contain accumulated melanosomes and extend between the cells of the epidermis, which are also visible on the electron micrograph. The Langerhans’ cell is a dendritic cell often confused with a melanocyte but is actually part of the mononuclear phagocytotic system and functions as an antigen-presenting cell of the immune system in the initiation of cutaneous hypersensitivity reactions (contact allergic dermatitis). b. The melanocyte reveals several processes extending between neighboring keratinocytes. The small dark bodies are melanosomes. 8,500. (Courtesy of Dr. Bryce L. Munger.)