Fig. 1. Oocyte maturation in vitro in sea urchins begins with movement of the germinal vesicle to an asymmetric (future animal pole) position (A), followed by germinal vesicle breakdown and release of first one polar body (B), then a second polar body (C). Frequent aberrations in maturation in vitro include generation of two pronuclei (D, egg matured in vivo; E, egg matured in vitro – note two pronuclei, each transcriptionally active for cleavage stage histone genes), and formation of enlarged pronucleus (F, compare enlarged pronucleus * to normal-sized pronuclei, arrows; all matured in vitro). Scale bars: in A, 25 µm for A-E; in F to 25 µm in F. gv, germinal vesicle; nco, nucleolus; pb(s), polar body(s); pn(s) pronucleus.

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