Reproductive and Developmental
The reproductive system is a complex, interconnected set of organs, tissues, and hormones that together make possible the birth of new generations. Not only the structure but also the function of the reproductive system may be harmed by exposure to environmental agents. Reproductive toxicants may directly damage the ovaries, testes, or other critical organs and cause abnormalities in sperm, ovulation, or hormone levels. The genetic makeup of an individual (genotype) only partially determines the course of fetal and childhood development. Ultimately, the outward expression of genetic makeup (phenotype) is a result of interactions between the genotype and the environment. Embryonic or fetal development may be altered by developmental toxicants, resulting in problems that range from birth defects to neurobehavioral disorders to cancer.
We have learned about normal and abnormal reproduction and development from observations in humans and other mammalian species with remarkably similar processes. From the earliest moments of its development, the reproductive system is exquisitely fine-tuned, and at times it is quite vulnerable. Similarly, normal fetal development depends on an orchestrated cascade of events characterized by periods of vulnerability to disruption. Communications among organs or tissues, essential for normal reproduction and development, are mediated by chemical messengers, including hormones.
The following brief summary of normal human reproduction and development, and their vulnerability to hazardous exposures, sets the