Generation of cells expressing cytoplasmic and/or surface T-cell receptor beta chains during the development of mouse fetal thymus.

We have used a double immunofluorescence technique to identify newly formed cells expressing cytoplasmic and/or surface T-cell receptor beta chains in mouse embryo thymus. In addition, we have localized these cells to the developing thymic cortex by immunoperoxidase labelling of frozen sections. The first positive cells appear in small numbers in the thymus at Day 15 of gestation and express cytoplasmic but not surface beta chains (C beta+). At Day 16 of gestation, cells appear that express small capped areas of surface beta chains as well as cytoplasmic beta chains (C beta+S beta+ cells). Just prior to birth, a cohort of cells appears that express surface beta chains but no cytoplasmic beta chains detectable by the methods employed (S beta+ cells). These results and vincristine mitosis-blocking studies suggest that a proliferating population of cortical thymocytes accumulate cytoplasmic beta chains before giving rise to T cells that express surface beta chains (presumably in association with alpha chains). When a monoclonal antibody to beta chains is present in organ cultures of embryonic thymus, C beta+ thymocytes continue to be generated but there is an absence of S beta+ cells. The latter appear after the cultures are transferred to fresh medium not containing the antibody.