Flow cytometry was used to separate and identify Sertoli and germ cell populations in primary rat testicular cultures derived from animals of different ages on the basis of cell size and DNA and lipid content. Multiparameter fluorescent evaluation of each cell preparation resulted in the assignment of specific staining patterns to Sertoli cells (diploid, high lipid content), spermatogonia (diploid, low lipid content), spermatocytes (large, tetraploid, high lipid content), and round spermatids (haploid, low lipid content). Each field was separately analyzed for inhibin and activin binding. Fluorescein isothiocyanate-conjugated activin bound with greatest intensity to spermatogonia, with little binding to leptotene or zygotene spermatocytes. Fluorescein isothiocyanate-conjugated inhibin bound to all stages of germ cells tested. Cross-competition data indicate that at least two and probably three distinct receptors exist for these peptides.
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