T cell receptor synthesis in thymocytes was examined by the differential immunoprecipitation of receptors from the surfaces and interiors of metabolically labeled newborn and adult thymocytes. Precipitated molecules were then analyzed for size, charge, and state of glycosylation. Our experiments identified cells within the thymic cortex that contained a large pool of cytoplasmic-free receptor beta chain. The beta chain in this pool was synthesized and degraded rapidly and bore only high-mannose N-linked oligosaccharides. This pool was found predominantly in cells that lacked surface alpha/beta receptors and appeared in ontogeny before cells expressing surface alpha/beta. These results are consistent with a model in which the progenitor of cells with surface alpha/beta expression is the T cell equivalent of the pre-B cell, which has rearranged and expressed beta chain, but not alpha chain.

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