Fig. 2. Alk3 is specifically expressed in PGCs and postnatal spermatogonia. Testes from 4 and 7 dpn mice were analyzed for Alk3 expression by immunofluorescence. (A) Representative images of Toluidine Blue stained frozen sections from 4 and 7 dpn testes (arrowheads point to spermatogonia within the tubules; 400x magnification). (B) Alk3 immunostainings in testes of the same ages as in (A) (arrowheads point to spermatogonia within the tubules; 400x magnification). (C) Alk3 immunostainings in isolated spermatogonia at 7 dpn (1000x magnification). Two classes of Alk3-positive cells are shown: strongly labeled cells with decondensed chromatin (arrowheads) and weakly labeled cells with more condensed chromatin. (D) Western blot analysis (20 µg of proteins) shows that 7 dpn spermatogonia, but not spermatocytes, express Alk3 protein. (E) Immunofluorescence analysis for Alk3 in frozen sections of genital ridges from a 10.5 dpc embryo: left panel shows alkaline phosphatase (APase) histochemistry to identify PGCs (200x magnification; the inset shows the area where the immunostainings have been performed); right panel shows Alk3 immunostaining only in PGCs (arrowheads) (400x magnification). Kit immunostainings in (B) and (E) were included to identify spermatogonia and PGCs, respectively (400x magnification).