Studies on Thymocyte Subpopulations in Guinea Pigs VI. Differentiation of Precursor Cells in vivo and in Vitro

Abstract

The relation between six subpopulations of guinea pig thymocytes, separated on the basis of PNA binding and buoyant density, was studied. Incubation in vitro of either unseparated thymocytes or isolated large, low density cells, caused a shift towards smaller cells with high buoyant density. In order to determine whether these changes reflect a normal differentiation we pulsed labelled thymocytes with 3H-Thymidine and traced the labelling in the separate subpopulations after different intervals by autoradiography. Both in vivo and in vitro, the thymidine was initially incorporated mainly into PNA + cells (85 % of all labelled cells), particularly into PNA + low-density cells (called PNA +Ia), which constitute 14 % of all thymus cells (1). Correlation with autoradiography sections of the thymus indicated that these cells were mainly located in the subcapsular and juxtamedullary cortex. At 24 h after labelling in vivo, more than 95 % of the labelled cells were smaller, high density cells, both PNA + and PNA -. At this time, labelled cells were present throughout the thymus with no preferential localization. Labelling and incubation in vitro were accompanied by similar changes after a 24-h time period when cells were cultured in the presence of serum. We conclude that the labelled PNA +Ia cells represent precursor cells, located in the subcapsular and 1uxtamedullary parts of the cortex. Within 24 h these cells differentiate into high density cells, some of which are PNA -. The corresponding results obtained in vitro indicate that these differentiation steps may also occur in the absence of a thymic microenvironment, and the study of histological sections indicate that these steps were not associated with any mayor relocalization within the thymus.