Cell proliferation and surface alloantigen expression were monitored during development of the fetal mouse thymus in vivo from day 13 of gestation until birth, and in organ cultures established at day 13 of gestation. Expression of Thy-1, Ly-1, Ly-2, Ly-3, TL, GIX, and H-2b determined alloantigens was assessed with specific antisera in one- and two-stage complement-dependent microcytotoxicity assays. A series of C57BL/6-congenic strains was employed to control for effects of the genetic background. Results indicate that at day 13 of gestation alloantigen expression is too low for detection by the cytotoxic assay. From day 14 until birth, exponential cell proliferation is accompanied by gradual increase in alloantigen expression, and by day 19, Ly-1, Ly-2, Ly-3, Thy-1, and GIX are expressed at adult levels, whereas TL expression is higher and H-2b expression is lower than is seen in the adult thymus. In fetal thymus organ cultures established at day 13 of gestation, cell proliferation was not so extensive as was seen in vivo, and by 6 to 8 days in culture, Thy-1 was expressed on essentially all the cells. The other alloantigens were detectable on only 30 to 45% of the cells. Both in vivo and in vitro, the Ly-1, Ly-2, and Ly-3 alloantigens appeared simultaneously on the same population of thymocytes. Results are compared with the reported kinetics of alloantigen expression induced in prothymocytes upon exposure to thymopoietin.

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This work was supported in part by United States Public Health Service Research Grant CA15808 from the National Cancer Institute to Paul D. Gottlieb, and by United States Public Health Service Research Grant CA14051 from the National Cancer Institute to the Massachusetts Institute of Technology Center for Cancer Research.

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