MATURATION OF MOUSE OOCYTE IN VITRO .I. SEQUENCE AND TIMING OF NUCLEAR PROGRESSION

Liberation of mouse follicular oocytes into a chemically defined Krebs‐Ringer salt solution with pyruvate is followed by resumption of meiosis and completion of the first maturation division. Examination of fixed whole mount preparations by light microscopy reveals that 90–95% of the germinal vesicles breakdown and proceed at least as far as metaphase I where about 5% arrest; the remainder go on to metaphase II. Three stages of chromatin condensation from the dictyate (germinal vesicle) stage are seen, first with filament shortening forming a chromatin lattice‐work followed by condensation about the nuclear and nucleolar periphery and finally the appearance of discrete bivalents. Just prior to appearance of the spindle, the bivalents are circularly arranged and reach their maximum shortness. Spindle formation is followed by prometaphase I, metaphase I, anaphase I, telophase I with first polar body formation, the chromatin mass stage in which the chromatin assumes a half‐moon shape, prometaphase II and finally metaphase II, the stage at which oocytes arrest both in vitro and in vivo. Examination at one‐two hour intervals reveals first meiotic spindle formation beginning after three hours of culture and metaphase I first appearing at 4 and one‐half hours. After nine hours of culture the majority of the oocytes are in metaphase I and anaphase I first appears at this time. There appears to be a six hour asynchrony in this division, some germinal vesicles requiring six hours to breakdown and the metaphase II stage accumulating between 11 and 17 hours of culture. Copyright © 1968 Wiley‐Liss, Inc., A Wiley Company