The X bivalent in fetal bovine oocytes

Wholemount preparations of oocytes from fetal bovine ovaries were examined in an attempt to study the incidence and type of chromosomes involved in pairing irregularities during different stages of early ovarian differentiation. Synaptonemal complexes exhibiting pairing irregularities were noted in meiocytes of all age groups. However, asynapsis and partial synapsis were more frequently noted in X chromosomes at the onset of meiosis in fetal bovine ovaries while the frequencies of similar errors in autosomes were relatively low and remained unchanged in the age groups included in this study. The significance and mechanisms of X chromosome asynapsis in excess of that expected on the basis of numerical ratio of the X to the bovine meiotic complement, are not known at present. We hypothesize that changes in the transcriptional status involving activation, inactivation, and reactivation of the X chromosomes during embryonic and ovarian differentiation on the conceptus, in addition to the inactivation undergone by the paternal X chromosome prior to fertilization, could be a factor rendering them susceptible to structural changes which, in turn may increase the incidence of sex chromosome asynapsis at the onset of meiosis in female fetuses.