Analysis of X-chromosome inactivation in horse embryos

To define the time of X-chromosome inactivation in the horse, 122 conceptuses were collected transcervically between Days 6 and 28 (ovulation = Day 0) and subjected to cytogenetic analysis: 59 of the embryos were divided and in 41 of these separate cytogenetic analysis of the embryonic disc and remaining tissues was possible. Conceptuses were measured and photographed before capsule removal, culture in the presence of 5-bromodeoxyuridine and subsequent fixation for cytogenetic analysis. On average, 15 slides were prepared per conceptus. C-banding was used to determine the sex of each conceptus and, in the females, the effects of 5-bromodeoxyuridine incorporation were revealed by u.v. irradiation which produced R-bands and allowed identification of the inactive X-chromosome. Inactivation of the X chromosome began gradually in the trophoblastic (+/- attached endoderm and mesoderm) cells around Day 7.5 and in the embryonic disc around Day 11.5. Cells with an inactive X-chromosome predominated in the trophoblast by Day 10.5 and in the disc by Day 12.5.