"Dark cells" in normal, hyperplastic, and promoter-treated mouse epidermis studied by conventional and high-voltage electron microscopy.

Dark cells (DC) could be reproducibly demonstrated by differential toluidine-blue staining and electron microscopy (EM) of NYLR/Nya 16- to 19-day embryo and new born skin and phorbol ester-treated or untreated young adult skin. High-voltage electron microscopy on the same or adjacent sections showed that toluidine-blue staining picks out some but not all the DC seen by EM. The ultrastructure of DC was similar in all the above situations, except that phorbol ester-induced DC showed a less contracted nucleus. No support was obtained for DC as stem cells either for basal-cell hyperplasia or for development of hair follicle or gland outgrowths. Most of the severely contracted DC (Types 3 and 4) were assumed to have undergone an apoptotic type of cell death. Two phenomena that may have caused the contraction and apoptosis were observed. Formation of a "contraction vacuole" adjacent to the DC probably led to a loss of intercellular communication. An apparent necrosis of dermal capillaries in areas of abundant follicle downgrowth probably produced local anoxia. Further characterization of DC requires a search for cytochemical or immunologic markers, analysis of intracellular calcium and other elements, and the cloning of subpopulations of basal cells that can be selectively induced to form DC.