Development of Thy 1 positive rat thymocytes: analysis of fetal and neonatal thymocytes by flow microfluorometry

Rat thymocytes of embryos and neonates were analyzed for cell size and amount of Thy 1 by flow microfluorometry. The rat embryonic thymus of 18 days' gestation appeared to be composed of very large cells with relatively low levels of Thy 1. The Thy 1 content was intermediate between rat bone marrow cells (low Thy 1) and the majority of rat adult thymocytes (high Thy 1). As cells became small at the 19th and the 20th day, the Thy 1 content appeared to increase. In the early postnatal thymus of the rat large cells appeared with low Thy 1 levels which were comparable to those in the 18th day fetal thymus. These results indicate that immature thymocytes increase in Thy 1 content as they differentiate into more mature cells from low Thy 1 thymocyte precursors. This conclusion is in close agreement with our recent results regarding rat bone marrow prethymic lymphocyte progenitors and the rat immature thymocytes.