The Occurrence and Morphogenesis of Melanocytes in the Connective Tissues of the PET/MCV Mouse Strain1

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Abstract
Mice of the PET/MCV strain are characterized by having melanocytes scattered in the connective tissues of most of the body. The ‘internal’ pigment cells generally first appear in fetuses as spherical melanocytes which later produce branches and lay down melanin. By contrast, pigment cells within the skin branch prior to the onset of melanogenesis. Spherical melanocytes of the serosae of the PET/MCV mouse proliferate within the coelomic lining of host chick embryos. However, the degree of proliferation and spread is considerably less than that of melanoblasts under similar conditions. Under the conditions of experimental grafting, both spherical ‘internal’ melanocytes and melanoblasts from the dermis showed a similar pattern of differentiation. The time at which donor melanocytes produce branches within the host chick embryos is in accord with their location and the specific time expected of the host and not of the donor. The data suggest that the branching of a pigment cell is evoked by a morphogenic substance which is produced by certain tissues of the body, particularly by the epidermis. The effect, at least with regard to the mouse and chick, is not species specific, since grafted mouse pigment cells react in keeping with the chick environment.