Autoradiographic determination of the number and distribution of replicating mesenchymal cells in rabbit dermis was carried out using animals sacrificed between two hours and 21 days after a multiple-dose-pulse of 3H-thymidine. Initial labeling of mesenchymal cells occurred principally in a zone immediately subjacent to the germinative epithelium. This was confirmed by in vitro studies of the uptake of 3H-Tdr by rabbit skin during one hour of incubation. Five days after the in vitro pulse of 3H-Tdr there was a fifteen-fold increase in the number of labeled cells, considerable dilution of label and migration of labeled cells so that they were evenly distributed throughout the full thickness of the dermis. At 21 days the number of labeled cells was similar to that found at five days, but there was further migration of labeled cells toward the deep dermis.
This tissue provides an example of appositional replication and divergent migration of an epithelial-mesenchymal system.