Xenopus oocyte maturation

From a cell cycle point of view, oocyte maturation has been an interesting system for many years. In the ovary all post vitellogenic oocytes are physiologically arrested in late diplotene at the G2/prophase border in meiosis I. The meiotic genome of the resting oocyte is transcriptionally active, especially for ribosome synthesis, but growth over several months occurs via uptake of vitellogenin from the blood. The oocyte is one of the clearest cases where cell growth is completely uncoupled from the cell cycle. Biochemically the resting oocyte is known to have relatively high activity of both cAMP-dependent protein kinase and p70 S6 kinase (Maller et al 1979, Schwab et al 1999). p70 promotes the translation of mRNAs containing a 5' tract of pyrimidines (5'-TOP), and many of the ribosomal proteins being synthesized during vitellogenesis have 5'-TOP sequences (Amaldi & Pierandrei-Amaldi 1997). The cell cycle quiescence of the resting oocyte is broken by the action of progesterone secreted by the surrounding follicle cells in response to pituitary hormones. The ultimate effect of the steroid is to stimulate progression through meiosis I and II (MI and MII) and arrest as an unfertilized egg. Oocyte maturation can thus be considered the first major transition in early development and results in a cell poised to undergo rapid cycles of DNA synthesis and mitosis in the absence of transcription, fundamentally different processes than those occurring in the oocyte. The conversion ofan oocyte into an egg is an example of a developmental switch triggered by a hormone that acts to alter the cell cycle.

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