1.1.1.2　Melanocytes

Melanocytes originate from the neural crest ofthe ectoderm and are located in the stratum basale and hair follicle of the epidermis，accounting for about 10％of the cells in stratum basale.The number of melanocytes varies with different parts of the body and age，and it can increase after repeated exposure to ultraviolet light.There is no significant difference in the number and distribution of melanocytes among different ethnic groups.Therefore，the difference of their skin color is mainly ascribed to the difference in the amount and distribution of melanin produced by melanocytes.In HE stained sections，melanocytes have a small and densely stained nucleus and transparent cytoplasm，so they are also called clear cells.However，not all clear cells seen in conventional slices are melanocytes，because basal cells can occasionally shrink artificially，which make it difficult to distinguish from melanocytes.Melanocytes have the function of forming melanin，thus the dopa response is positive；moreover，they contain melanin，thus the silver staining is positive.Melanin is transported into basal cells through dendritic processes of melanocytes.Under the electron microscope，characteristic melanosomes can be seen in the cytoplasm of melanocytes.Melanosomes are organelles containing tyrosinase and are the place where melanin is synthesized.One melanocyte can provide melanin to the surrounding 10-36 keratinocytes through its dendritic processes，called epidermal melanin unit.Melanin can block and reflect ultraviolet rays，thereby protecting the dermis and deep tissues（Figure 1-2）.(https://www.daowen.com)