Evidence from chimaeras for the pattern of proliferation of epidermis in the mouse

The recessive mutant gene downless (dl) causes abnormal texture of the coat and absence of hair on the tail. The dl locus had previously been shown to act in the epidermis and not in the dermis. To obtain evidence on the pattern of proliferation of epidermis, downless ↔ normal chimaeras were produced by embryo aggregation, and the pattern of normal and mutant hair in the coat was examined. The chimaeras showed a pattern of narrow transverse stripes of normal and abnormal hair. This pattern was similar to that found in mice chimaeric for alleles at the agouti locus known to act in the dermis. This evidence supports the conclusion that the pattern of cell proliferation is similar in dermis and epidermis, and is compatible with the hypothesis that both tissues proliferate by lateral coherent clonal growth from a randomly mixed array of longitudinally arranged cells.