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Fig. 2. Regulation of germ cell entry into meiosis in the developing gonads.
(A) In the bipotential (11.5 dpc) mouse gonad, germ cells are present
and retinoic acid (RA) is produced in the mesonephric duct and tubules.
Cyp26b1 is expressed at low levels in the gonad of both sexes. The
mesonephric tubules, which produce RA, are physically connected with the
anterior (Ant) end of the gonad during this time. (Ba) Once
Sry is expressed in the male gonad (at 11.5 dpc), Cyp26b1
expression is upregulated, probably in both Sertoli and interstitial cells.
The testis cords, which form around germ cell clusters, might concentrate the
enzyme in these regions, thereby protecting germ cells from the actions of RA.
(Bb) Germ cells in the male gonad do not enter meiosis at 13.5 dpc, and
continue to express the pluripotency marker Pou5f1. (Ca) In
the female gonad, Cyp26b1 expression is detectable at 11.5 dpc, but
disappears by 12.5 dpc. Germ cells at the anterior end of the gonad begin to
express Stra8 at 12.5 dpc; (Cb) by 13.5 dpc, later markers of
meiosis, such as Sycp3 and Dmc1, are expressed strongly.
Meiosis markers are upregulated in an anterior-to-posterior (Post) wave over
3 days. Pou5f1 is no longer expressed by germ cells once they
enter meiosis. Germ cells at the anterior end of the gonad might be exposed to
RA earlier than those at the posterior end, or the RA concentration might be
greater at the anterior end than the posterior end. See text for more
details.
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