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Fig. 1. The timing of mature gamete production from mouse primordial germ
cells. Primordial germ cells (PGCs) are specified at 
7.2 dpc and
subsequently proliferate and migrate towards the bipotential gonads, which
they occupy at 10.5 dpc. In the ovary (pink), at 13.5 dpc, PGCs
begin to enter meiosis I (the first meiotic division). Primary oocytes pass
through the leptotene, zygotene and pachytene stages before entering
diplonema/dictyate arrest at around birth. Once sexual maturity is reached, at
each ovulation a cohort of arrested oocytes is stimulated to resume and
complete meiosis I and enter meiosis II. The released mature egg is arrested
in metaphase II, completing meiosis II after fertilization. In the testis
(blue), germ cells do not enter meiosis at 13.5 dpc, but arrest mitotically.
Immediately after birth, germ cells re-enter the mitotic cycle and meiosis I
is initiated several days later. Both divisions are completed rapidly to
produce spermatozoa (sperm). The process is repeated many times throughout
life to ensure a continuous supply of mature sperm.
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