Immediate pigment darkening phenomenon. A reevaluation of its mechanisms

Proposed mechanisms of immediate pigment darkening (IPD) are controversial. They include photooxidation of "premelanin," changes in the distribution pattern of microfilaments and microtubules, movement of melanosomes to melanocyte dendrites, increased transfer of melanosomes to keratinocytes, and changes in the melanosome distribution pattern in keratinocytes. We investigated the following aspects of IPD: production of IPD by UVA under physiologic and nonphysiologic conditions in fullthickness skin and epidermal sheets; reversibility of IPD in vitro after in vivo and in vitro production; blocking of IPD by disruption of the microfibrillar or microtubular system in vitro; alterations of the cytoskeleton of melanocytes; the melanosome distribution pattern in melanocytes and keratinocytes. The results were as follows: IPD could be elicited in vitro in full-thickness skin and in epidermal sheets. Its production was temperature independent (0 degrees-37 degrees C) and was not inhibited by repeated freezing and thawing, or by formalin fixation. IPD was reversible in vitro under tissue culture conditions but only in viable skin. IPD could not be blocked by substances that disrupt the microfibrillar or microtubular system (cytochalasin B, colcemid, vincristine). As shown with a monoclonal antivimentin antibody, IPD-producing UVA doses did not induce changes in the cytoskeleton of melanocytes. No changes in number and distribution pattern of melanosomes were observed electron-microscopically and by morphometric analysis of EM micrographs. Production of IPD does not depend on the structural and functional integrity of the melanocyte cytoskeletal apparatus and is not confined to viable skin, whereas its reversibility is. The fact that no increased melanosome transfer occurs may explain the lack of a UV protective action.