Heterochromatin decondensation in chromosomes from chorionic villus samples.

The spontaneous decondensation of constitutive heterochromatic regions of chromosomes 1, 9, 16, and Y has been observed in 46.6 per cent of chorionic villus samples. This type of decondensation is occasionally observed in amniotic fluid cells (9 per cent) and has never been found in fetal lymphocytes. The phenomenon is similar to that described in spermatogonial metaphases, in primary and secondary spermatocytes and in human sperm chromosomes, although decondensation of the heterochromatin of chromosome 15 has never been recorded in chorionic villus samples.