Direct analysis of the chromosome constitution of human spermatozoa

Abstract

ALL available information on the chromosome constitution of human gametes is speculative, having been obtained by inference from the chromosome constitution of conceptuses that survive sufficiently long to produce a clinically recognisable pregnancy. A minimum of 10% of all recognised human conceptions are chromosomally abnormal, and it has been estimated that 1–2% are the result of fertilisation by a spermatozoon with a chromosome abnormality¹. Cytological evaluation of the chromosome constitution of human spermatozoa has been restricted to the staining of fixed smears of whole sperm^2–13. Certain chromosome regions with peculiar staining properties, such as the long arm of the Y chromosome and the heterochromatic region of chromosome 9, are presumed to be represented in appropriately stained sperm nuclei by differentially staining spots. By counting the number of these spots per nucleus, the frequency of aneuploidy in the sperm of normal males has been estimated to be around 40% (refs 5, 9–11). The precision of the data obtained from stained whole sperm is dubious, because several factors must be taken into consideration when blobs are counted in sperm head nuclei, all of which could contribute to biased estimates of nondisjunction^7,8,12. For this reason, some claims have now been retracted^12,13. To investigate the true contribution of male gametes to the production of chromosomally abnormal conceptuses and the factors influencing the production and survival of chromosomally abnormal sperm, it is necessary to analyse the sperm chromosomes directly. However, after meiotic metaphase II sperm chromosomes do not reappear until the male and female pronuclei of the fertilised egg prepare for the first cleavage division. We report here the use of hamster eggs to activate human sperm to the point where their chromosomes can be studied directly.