Cytogenetic analysis of pig oocytes matured in vitro.

Abstract

AbstractPrimary oocytes from the ovaries of prepubertal pigs were stimulated to undergo maturation by removing them from small follicles and incubating them in a defined culture medium for various periods of time. The cultured oocytes were air‐dried and their chromosomes examined for the stage of meiosis and for the presence of abnormalities. Of 107 oocytes recovered after 26–27.5 hours of culture 41% were at diakinesis‐metaphase I and 2% were at telophase I. Of 96 oocytes recovered after 36–37 hours of culture 46% were at diakinesis‐metaphase I while only 14% were at metaphase II. These observations suggest that the oocytes were delayed at diakinesis‐metaphase I, since it has been shown previously that at 36–37.25 hours after HCG injection only 23% of oocytes are at metaphase I and 45% are at metaphase II in vivo. When recovered after 42.5–55 hours of culture 128 of 232 oocytes (55%) had reached metaphase II.Of 56 cultured oocytes whose chromosomes were analysed at metaphase I, only one exhibited a chromosomal aberration (19 bivalents plus three fragments) and one oocyte contained 18 bivalents plus two monovalents.Of 123 oocytes analysed at anaphase I, telophase I and metaphase II, 33 (27%) exhibited such chromosomal abnormalities as heteroploidy, fragments, chromosomal bridges, tripolar spindles and lagging chromosomes. In addition 31% of 128 oocytes at metaphase II displayed degenerate chromosomes. Therefore, of 232 immature oocytes cultured for 42.5–55 hours only about 20% could be regarded as chromosomally normal, mature secondary oocytes.The delay at diakinesis‐metaphase I may be related to a failure of the supporting granulosa cells to supply some substance, possibly an energy source, required by the oocytes. It is suggested that during a delay at diakinesis‐metaphase I, the chromosomes of pig oocytes which failed to divide, may have become “sticky,” leading to the chromosomal aberrations observed at subsequent stages of meiosis.