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  • The gonads possess a dual function: an endocrine function involving the secretion of sex hormones and a nonendocrine function relating to the production of germ cells (gametogenesis).

  • Gametogenic and secretory functions of either the ovary or testes are dependent on the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary.

  • The blood–testis barrier between the lumen of an interstitial capillary and the lumen of a seminiferous tubule impedes or prevents the free exchange of chemicals/drugs between the blood and the fluid inside the seminiferous tubules.

  • Xenobiotics can act directly on the hypothalamus and the adenohypophysis, leading to alterations in the secretion of hypothalamic-releasing hormones and/or gonadotropins.

  • Steroid hormone biosynthesis can occur in several endocrine organs including the adrenal cortex, ovary, and the testes.

  • Female reproductive processes of oogenesis, ovulation, the development of sexual receptivity, coitus, gamete and zygote transport, fertilization, and implantation of the conceptus may be sites of xenobiotic interference.

  • Xenobiotics may influence male reproductive organ structure, spermatogenesis, androgen hormone secretion, and accessory organ function.

Chemicals can adversely affect reproduction in males and females, impacting the viability and quality of life of their potential offspring and even later generations. Recent trends in human fertility, fecundity—changing social influences (age at which women have their first child), and the knowledge that populations in many western countries are no longer self-sustaining, point to potential declines in normal human reproduction. Underlying these issues with human reproductive performance is the concept that exposure to environmental chemicals and drugs may be contributing to these declines. The reproductive cycle is outlined in Fig. 21–1.


The reproductive cycle.


Numerous complex processes must be orchestrated in a precise, sequential order for optimal performance at different stages of the life cycle of animals and humans. Following fertilization of an egg by a sperm, the resulting zygote must be transported along the oviduct while maturing into an early embryo. This embryo must implant in the uterus successfully, such that the developing conceptus can differentiate, produce a placenta, and undergo normal embryogenesis and fetal development. Parturition must occur at the correct time with birth of the neonate, lactation, and weaning following sequentially.

Acquisition of sexual maturity involves generation of gametes by the gonads. For the parental animals, once their reproductive life span has finished, the process of reproductive senescence occurs. These processes involve complex interplay between tissues and cells, under hormonal control that provides the critical signals and precise timing of these events. All these processes can be targets for chemicals that can disturb these events leading to adverse effects on reproduction. The dose of the toxic chemical and its resultant effects will depend on when in the life stage of the organism that the chemical is administered and evaluated. Table 21–1 provides basic ...

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