Ultrastructure of meiotic chromosomes in boys undergoing chemotherapy for leukemia.

Meiotic chromosomes from four boys treated with combination chemotherapy for acute lymphoblastic leukemia were analysed ultrastructurally by serial sectioning and reconstruction of spermatocyte chromosomes. Analysis of the 1012 chromosomes in 22 zygotene and pachytene spermatocyte nuclei showed that in most of the cells that reached the pachytene stage, one or more of the following abnormalities were found in the intranuclear structures: decreased condensation of euchromatin, centromeric heterochromatin, diffuse heterochromatin, or abnormal nucleolar morphology. The total number of recombination nodules and -bars was within normal limits, but in some nuclei the ratio between the two types of structure was inappropriate for the stage, indicating developmental asynchrony. With the exception of one nucleus, there was no increase in abnormalities of pairing and synaptonemal complex formation. The number of structural aberrations in autosomal and sex chromosomes was similar to that found in the reference material. The repair system responsible for resolving chromosome interlockings in zygotene was functioning. It is not possible to predict which of the short term effects found will become permanent and thus pose a risk of genetically abnormal conceptions. New means of classifying meiotic chromosomes, of evaluating the synchrony of development, and of analysing the telomere attachment pattern are described.