MENSTRUATION - A NONADAPTIVE CONSEQUENCE OF UTERINE EVOLUTION

Although adaptive explanations for menstruation go back at least twent y-five hundred years, In the last decade two new hypotheses have been advanced. The first suggests that menstruation evolved to cleanse the uterus of pathogens introduced by sperm, and the second argues that th e function of endometrial regression (with the associated menstruation in humans) is to save energy by getting rid of tissue, rather than ma intaining it in the absence of an available blastocyst. Both these sug gestions may be questioned on the grounds that they do not take into a ccount the physiology of the reproductive processes involved. Menstrua tion is not an independent physiological process and is unlikely to ha ve been selected for independently of the evolutionary events that led to it. Furthermore, most primitive menstruating animals would have me nstruated infrequently, and many may have reproduced or died without e ver menstruating In order to provide a context for understanding how m enstruation may have come about, the evolution of the female vertebrat e reproductive tract is briefly reviewed. In late, stages, the coevolu tion of the embryo and uterus resulted in an intimate association betw een the trophoblast and the uterine blood vessels. As the embryo becam e more invasive, the uterus responded with increased cellular growth a nd differentiation of the endometrium to accommodate it. This reached its peak in mammals (such as rodents and humans), where the embryo pas ses through the epithelium into the endometrial stroma, which responds with differentiation of cells and blood vessels. Progesterone, secret ed after ovulation, plays a crucial role in preparation for pregnancy. In addition to its well-known effects on the uterus, progesterone may be important in suppressing the inflammatory reaction that would be e xpected in response to the presence of a foreign body, such as an embr yo. It is also suggested that vascular and cellular differentiation of the endometrial stroma has evolved by adaptation of the inflammatory (granulation tissue) reaction. When progesterone levels fall at the en d of the cycle, there is tissue breakdown and bleeding The uterus then reforms for the next ovulatory cycle. It is shown that the female rep roductive tract has multiple functions that must occur in sequence The coevolution of the embryo and maternal tract thus led to the close co ntact of two genetically different tissues, and problems such as the i nflammatory reaction had to be overcome. Menstruation is a necessary c onsequence of these evolutionary changes, and needed no adaptive value in order to evolve.