Evolution of the meiotic prophase and of the chromosome pairing process during human fetal ovarian development

Studies on human oocytes in prophase I are limited due to the difficulty in obtaining the sample. However, a complete study of meiotic prophase evolution and the homologue pairing process is necessary to try to understand the implication of oogenesis in the origin of human aneuploidy.

A complete analysis of meiotic prophase progression comprising the long developmental time period during which meiotic prophase takes place, based on the analysis of a total of 8603 oocytes in prophase I from 15 different cases is presented. The pairing process of chromosomes 13 and 18 is also described.

The findings significantly relate for the first time the evolution of meiotic prophase to fetal development. Although for both chromosomes 13 and 18 a high pairing efficiency is found, pairing failure at the pachytene stage has been observed in 0.1% of oocytes. However, errors at the diplotene stage are substantially increased, suggesting that complete, premature disjunction of the homologues commonly occurs. Moreover, pre-meiotic errors are also described.

Our findings show that homologous chromosomes pair very efficiently, but the high frequency of complete, premature homologue separation found at diplotene suggests that mechanisms other than the pairing process could be more likely to lead to the high aneuploidy rate observed in human oocytes.