Estrogen receptors and estrogen sensitivity of fetal thymocytes are restricted to blast lymphoid cells

We have observed [3H]estradiol binding sites in the cytosol of lymphoid cell suspensions obtained from the fetal thymus of guinea pig. Scatchard analysis [dissociation constant (Kd), 0.5 +/- 0.02 (SE) nM], binding specificity, and diethylaminoethyl chromatography of these [3H]estradiol binding sites are similar to those described for estrogen receptors. Estrogen receptor levels in the thymic lymphoid cell population have been studied after fractionating cells by Percoll discontinuous density gradients into large and low density cells (alpha-cells) and small and high density cells (beta-cells). Estrogen receptor levels are higher in alpha-cells [1002 +/- 200 (SE) sites per cell] than beta-cells [61 +/- 6 (SE) sites per cell]. Electron microscopy shows that the ultrastructural characteristics of 95% alpha-cells are consistent with their lymphoblastoid nature, whereas 90% of the beta-cells represented here can be categorized as typical small lymphocytes. To study whether cellular estrogen receptor expression was related to blastogenesis, fetal thymocyte suspensions were cultured throughout 48 h in the presence of the mitogen Concanavalin A. A significant increase in estrogen receptor levels was observed in thymic lymphoid cells cultured in the presence of the mitogen [1500 +/- 193 (SE) sites per cell] with respect to cells cultured in the absence of the mitogen [400 +/- 54 (SE) sites per cell]. One-day and 3-day in vivo estrogen treatments decrease significantly the [3H]thymidine uptake [by 40 +/- 1% (SE) and 66 +/- 5% (SE), respectively], the mitotic index [by 82 +/- 8% (SE) and 96 +/- 2% (SE), respectively] and the frequency of lymphoblastoid cells [by 15 +/- 4% (SE) and 50 +/- 2% (SE), respectively] in fetal thymocyte suspensions. A 29 +/- 0.7% (SE), 31 +/- 0.8% (SE), and 35 +/- 2% (SE) decrease of [3H]thymidine incorporation in the DNA of alpha-cells cultured throughout 24 h, 48 h, and 72 h, respectively, in the presence of estradiol (10 nM) was observed with respect to untreated alpha-cells (P less than 0.01), while no effect of estradiol was observed on [3H]thymidine uptake by beta-cells. The mitotic index of cultured alpha-cells was significantly decreased [by 67 +/- 5% (SE)] 48 h after the addition of estradiol to the culture medium. It is concluded that: 1) estrogen receptors and estrogen sensitivity are restricted to lymphoblastoid thymic cells, and 2) estrogen receptor levels are increased after lymphoid blast transformation.