Fetal sexual development

Still other constructs have been mired in biological ignorance. Consider, for example, fetal sexual development and certain attitudes about it. The ancient Greeks believed that the sex of the fetus is determined by the heat of the male partner during copulation—the more heated the passion, the greater the probability of a male fetus. Aristotle advised men to copulate in the summer if they wanted male heirs. The idea became... 

Can we expect environmental endocrine disruptors to affect the sexual development of the human fetus In my opinion, the answer is an absolute yes, if only because we know that endocrine disruptors act like foreign hormones and thereby disrupt natural hormone physiology. Hormones are critical in fetal sexual and brain development. Exposure of the fetus to hormones can produce profound changes in development. In the case of androgenic hormones, for example, exposure diverts a genetic female to take on the phenotypic appearance of a male, and these hormones change the areas of the brain that ordinarily differ between the sexes (sexually dimorphic areas). 

Meanwhile, the sexual development of the human fetus is shaped by natural hormonal programming, a consequence of genetics, epigenetics, and a fetal environment that is often malevolent. 

Testosterone, the principal male sex steroid hormone, is synthesized in five steps from cholesterol, as shown below. In the last step, five isozymes catalyze the 17/3-hydroxysteroid dehydrogenase reactions that interconvert 4-androstenedione and testosterone. Defects in the synthesis or action of testosterone can impair the development of the male phenotype during embryogenesis and cause the disorders of human sexuality termed male pseudohermaphroditism. Specifically, mutations in isozyme 3 of the 17/3-hydroxysteroid dehydrogenase in the fetal testis impair the for-... 

In some animals, consumption of a phytoestrogen-rich diet can cause temporary infertility and reproductive system disorders (Irvine, 1999). In humans, lower testosterone levels and a decline in human semen quality over the past century have been luiked to increased exposure to environmental endocrine disrupters (EDCs) (Sharpe and Skakkebaek, 1993). Furthermore, cases of sexual impotence have been reported in males exposed to synthetic estrogens in the pharmaceutical industry (Mattison et al., 1990). If this might be the case, the fetal-prepubertal period and Sertoli cell development would be of critical importance (Sharpe and Skakkebaek, 1993). However, an adverse effect of phytoestrogens on male fertility has yet to be proven. Recent work (Mitchell et al., 2001) addressing this point led to the conclusion that up to 40 mg/day of isoflavones over a two-month period had no effects on gonadotrophin and... 

Fig. 1. Diagram of the major reproductive life cycle phases, commencing with sexual maturation and moving through fertilization, fetal development, parturition and postnatal development and ending with a sexually mature individual capable of starting the cycle over again.

Two important aspects of early development of the reproductive tract are that the fetal gonad is structurally indifferent in male and female embyros and that the fetal reproductive system can therefore develop as male or female. Thus, the first major step in development of the reproductive system is establishing gonadal sex. Sex of the embryo depends on whether the spermatozoon carries an X or Y chromosome, and sexual differentiation of the indifferent structures in the gonad is necessary to form the male or female reproductive tract. The SRY gene on the Y chromosome is needed for testicular... 

Sexual differentiation of the central nervous system also occurs during fetal development, and it is equally fundamental to reproductive function during adulthood. Adult males exhibit a relatively continuous pattern of gonadotropin secretion from the hypothalamic-pituitary axis and are therefore fertile throughout their reproductive lifetime. In contrast, adult females exhibit a cyclical pattern of gonadotropin secretion and are fertile only during the transient period that follows ovulation during each reproductive cycle. This fundamental difference is due to irreversible... 

Normal reproduction and development require signaling within and between a variety of diverse organs, and, in sexual reproduction and mammalian pregnancy, critical communication even takes place between distinctly different organisms (i.e. male and female and mother and offspring, respectively) (Evans, 2007). It should be remembered that premature parturition or abortion can be induced by any circumstances which cause fetal or, potentially, maternal stress and initiate the cascade of endocrine and neural signaling events which would normally lead to parturition. Any sublethal intoxication or emotionally traumatic event in a pregnant woman or animal has the potential to threaten fetal survival. 

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