Non-Random Segregation during Mammalian Oogenesis

Abstract

INVESTIGATIONS of the breeding performance and early embryo-genesis of the XO female mouse^1,2 by Cattanach³ and Morris⁴ were difficult to explain because the number of XO females born to XO mothers was considerably less than predicted. Cattanach³ suggested there was either a preferential loss of the chromosome sets lacking an X to the polar bodies during meiosis, or death during early embryonic development. Morris⁴ and others^5,6 concluded that YO was inviable. The proportion of oocytes with and without an X chromosome was determined at metaphase II. The sex-linked Tabby gene was used as a marker in the XO mice, which were of similar stock to those used by Cattanach³ and Morris⁴. XO females in succeeding generations were mated to +/y and Ta/y males. Ta/O females were superovulated and autopsied 16–20 h after the human chorionic gonadotrophin (HCG) injection. The freshly ovulated oocytes were liberated from the ampullae and ovarian oocytes from the larger intact follicles. Oocytes at the germinal vesicle stage were cultured for 18–24 h in modified Krebs-Ringer bicarbonate medium⁷, after which most reached metaphase II. The metaphase II cells were placed in 1% sodium citrate solution for 10–20 min. Air-dried preparations were made by the method described by Tarkowski⁸ and stained with 2% Giemsa at pH 7.0 for 45 min. The slides were washed briefly with distilled water, allowed to dry and then dehydrated with xylol and mounted with ‘Clearmount’.