Representative examples of testicular histology from 85 day old hpg mice treated with vehicle (VEH) or testosterone propionate (TP) on postnatal day 2, then receiving either a cholesterol (chol) or a 2% estradiol (E) subcutaneous implant on day 35. Note that the most mature cells types in testes from mice treated with cholesterol (VEH+chol, TP+chol) are type A spermatogonia (SA) and primary spermatocytes (PS); there is little evidence of lumen formation, and Sertoli cells are frequently observed adjacent to the basal lamina and more centrally (St). In contrast, after estradiol treatment (VEH+E, TP+E) the seminiferous tubules are expanded and have developed a lumen (L), Sertoli cells are adjacent to the basal lamina and round and elongating spermatids (ES) are present. Treatment with testosterone propionate in the neonatal period did not affect the testicular response to estradiol in later life. Scale bars represent 30 μm.