The fate of the cells of the stratified squamous epithelium of the esophagus was investigated radioautographically in young adult rats at various time intervals after a single injection of thymidine‐H³.

Soon after injection, labeled cells appeared in the basal layer of the epithelium (stratum basale). During the following 12 hours, the labeled cells completed DNA synthesis and mitosis, and mostly remained in the basal layer. After 12 hours, however, the labeled cells arising from the mitoses were transferred to the spinous layer (stratum spinosum) at the rate of 1.2% per hour.

The respective fates of the two daughter cells of a mitosis were then examined using two‐dimensional maps showing the location of the labeled nuclei at 24 and 48 hours after injection. It was assumed that any two nuclei located side by side and overlaid by a similar number of grains are the two daughter cells of a mitosis. Such pairs of labeled daughter cells fell into three categories: (1) Basal pairs, composed of two basal cells; (2) Outgoing pairs, composed of two spinous (or granular) cells; and (3) Mixed pairs, composed of one basal and one spinous (or granular) cell. Since three possibilities were encountered at the two time intervals, the mitoses could not be differential (in which case the pairs of daughter cells would consist of a cell remaining in the basal layer and another migrating to the spinous layer, that is, all pairs would be “mixed”). Instead, the frequency of the three types of pairs was such as to indicate that the transfer of a basal cell to the spinous layer is a chance event which can affect any basal cell (except those undergoing DNA synthesis or mitosis). Accordingly, the transfer of either or both daughter cells of a mitosis would also be due to chance.

The transfer of a cell out of the basal layer is a critical step in the life of the cell, since it precludes further division and appears to trigger differentiation.