Serum Gonadotrophin Levels in Pro-Oestrous Rats in Relation to the Resumption of Meiosis by the Oocytes

Summary. Serum levels of lh, fsh and prolactin were determined in pro-oestrous rats by radioimmunoassay, and the time of commitment of oocytes to undergo maturation was defined by explanting follicles at hourly intervals into a hormone-free medium. All three pituitary hormones showed a rise during the afternoon of pro-oestrus. The prolactin level was still elevated on the morning of oestrus, and serum fsh showed a secondary rise at that time. Administration of pentobarbitone-sodium at 14.00 hours abolished the afternoon rise in the levels of lh and fsh, but not of prolactin, and prevented the induction of oocyte maturation. The percentage of oocytes that completed the first meiotic division when explanted within intact follicles increased during the afternoon of pro-oestrus from <5% at 14.30 to 80% at 17.30 hours. This maturation commitment preceded the dissolution in vivo of the germinal vesicle by about 1½ hr. Its attainment closely followed the rising curve of serum lh concentration, indicating that only brief exposure of the follicle to lh is required to initiate ovum maturation.