ABSTRACT
Ribonuclease protection assays have been used to quantitatively assess changes in steady-state levels of specific mRNAs during oogenesis and early embryogenesis in mice. The mRNAs encode ZP3 (a glycoprotein that serves as a sperm receptor), LDH-B (heart-type lactate dehydrogenase), and MOM-1 (a protein of unknown function). MOM-1 and LDH-B are expressed in a variety of adult mouse tissues and midgestation embryos, whereas ZP3 expression is restricted completely to oocytes. All three mRNAs are expressed by growing mouse oocytes and accumulate to unusually high levels in fully grown oocytes as compared to somatic cells; 240000, 200000 and 74000 copies mRNA per fully grown oocyte for ZP3, LDH-B and MOM-1, respectively. Steady-state levels of LDH-B and MOM-1 mRNA undergo a modest decline (∼20–40 %) during ovulation when fully grown oocytes become unfertilized eggs and, in general, mirror the reported change in poly(A)+RNA levels during this period of development. On the other hand, the level of ZP3 mRNA declines dramatically (∼98%) during ovulation, from ∼240000 copies per oocyte to ∼5000 copies per unfertilized egg, and ZP3 mRNA is undetectable in fertilized eggs (<1000 copies per fertilized egg). MOM-1 mRNA is expressed at relatively low levels in morulae ∼2000 copies per embryo) and blastocysts ∼5000 copies per embryo), whereas ZP3 mRNA remains undetectable (<1000 copies per embryo) at these stages of preimplantation development. These findings are discussed in the context of overall gene expression during oocyte growth, meiotic maturation and early embryogenesis in mice.
