Immunoglobulin gene expression is a normal differentiation event in embryonic thymocytes.

By in situ hybridization to frozen sections of mouse embryos, we have localized cells transcribing the Ig C mu gene during ontogeny. Transcripts were detected from before day 14 of gestation in individual pre-B cells in the liver and, surprisingly, in a large proportion of thymocytes between days 15 and 18. The level of mu RNA sequences in the thymus at day 17 was much higher than has been observed for adult thymocytes; from grain counts, the amount of mu RNA was similar to that observed for Ti gamma RNA. These findings suggest that Ig and Ti genes are under similar transcriptional controls during Ti gene recombination and that elevated mu RNA production is a normal event early in the intrathymic differentiation of T cells.