Sex Differences in Cell Divisions
As the technique of in vitro fertilization and implantation became widespread in the early 1990s, there were anecdotal reports that women who had undergone this procedure were delivering male babies at a higher than expected rate. Why should in vitro fertilization bias the odds towards having a boy? Studies done by Pergament and colleagues (1994) may explain this presumed phenomenon. They have shown that XY embryos undergo second cell division faster than XX embryos. Indeed, XY embryos tend to develop more rapidly than XX embryos throughout early development. During the in vitro fertilization procedure, several eggs are fertilized, but not all of the resulting embryos are implanted back into the uterus. Moreover, the physicians doing the procedure obviously want to implant healthy looking embryos. Since the XY embryos have more cells per embryo at the time of surgical implantation, these XY embryos might tend to look healthier and be placed into the uterus.
This brings up the question of how it is that males develop faster. It may be because they have less chromatin to replicate, but another possibility is that the XY embryos are activating the cell division apparatus more rapidly than the XX embryos. Fiddler and colleagues (1995) have shown that the human SRY gene, a gene on the Y chromosome that encodes the major determinant of testis formation later in development, is expressed during the zygote and throughout the blastocyst stages. Perhaps it is active in stimulating cell division. Ursula Mittwoch (1988) has noted that male embryos, in general, and male gonads, in particular, divide faster than their female counterparts. She speculates that this difference may be critical in the proper timing of events needed for male-specific differentiation. She concludes that while female embryos may take their time in developing, "Time and tide wait for no man."
Fiddler, M., Abdelrahmam B., Rappolee, D. A., and Pergament, E. 1995. Expression of SRY transcripts in preimplantation human embryos. Amer. J. Med. Genet. 55: 80-84.
Mittwoch, U. 1988. The race to be male. New Sci. 120: 38-42.
Pergament, E., Fiddler, M., Cho, N., Johnson, D., and Holmgren, W. J. 1994. Sexual differentiation and preimplantation cell growth. Human Reprod. 91: 1730-1732.