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Fig. 1 cyp19a2 expression in the head of 20 days postfertilization (dpf) fish and histological evaluation of gonadal development. A: Mean normalized expression (MNE) of cyp19a2 expression in 20 dpf zebrafish. There are clearly two populations, namely the high and low expressing fish. B: Histological evaluation of the gonadal status of these fish. The germ cells are represented mainly by gonial cells (arrows) possessing a rather homogeneous, basophilic cytoplasm with usually one, centrally located nucleolus; the cytoplasm is restricted to a relatively small rim around the nucleus. These germ cells will probably differentiate into oocytes, because the gonads of juvenile zebrafish initially develop as nonfunctional protogynic ovaries (Maack and Segner, [2003]). However, because oocytes are not yet present, these gonads were classified as undifferentiated. Scale bar = 50 μm.