Reproductive hormones released by the glands of the endocrine system have been identified for nearly a century, yet the influence that they have on sexual behaviors is far from understood or agreed upon. This paper very briefly reviews the physiological effects of hormones on reproduction and development, and describes some of the effects of hormones on reproductive behavior.
Although there are complex physiological loops, feedback and interactions between the hormones of the endocrine system, the most relevant parts of the endocrine system for the purposes of this paper are the ones that secrete sex hormones:
- Tthe ovaries and testes
- Anterior and posterior pituitary
- The hypothalamus
Hormones play an organizational role in sexual maturation and development of the embryo, where gonads arise from tissues of sexually bipotential under the influence of hormonal stimulus. Precursors to both female and male genital structures are present in the embryo until hormones initiate an anti-feminizing effect, in the case of male development, or until the male reproductive system shrivels away, in the case of female embryonic development.
It may be said that nature favors female development, as the least number of resources are required for successful female zygote maturation. The development of testes (rather than the default ovaries) will not take place without the presence of hormones whose release is signaled genetically from instructions on the Y chromosome inherited from the father. Ovum that are fertilized by sperm bearing an X chromosome from the father will be an XX zygote, and the embryo will develop ovaries from the sexually indifferent gonad and female genitalia.
Sperm bearing a Y chromosome result in an XY zygote, with the sexually bipotential embryonic gonad developing into testes, and the Mullerian inhibitory factor (anti-Mullerian hormone) released from them to produce a de-feminizing effect on the male embryo, while testosterone release instructs the Wolffian system to develop into male accessory sex organs. In this case, the Mullerian system, the female sex organs, will degenerate by the third month of male development and testosterone and other androgens secreted from the Leydig cells will stimulate the development and maintenance of male sex characteristics, external genitalia and the male secondary sex organs: the epididymis, vas deferens, seminal vesicles, prostate, and ejaculatory duct. Sperm production also takes place in the testes under the activational influence of follicle-stimulating hormone, FSH, released from the anterior pituitary.
In females, FSH is also present but its release from the anterior pituitary stimulates follicle formation. Lutenizing hormone (LH) is also bi-potent and its release from the anterior pituitary stimulates ovulation in females, and androgen secretion in males. In females, prolactin stimulates post-partum milk production, and oxytocin is reflexively secreted on stimulus by suckling, in a neuroendocrine reflex that stimulates contraction of the milk ducts and ejection of milk from the nipples. Other hormones are also secreted from the anterior pituitary in females, and the development and maintenance of female sex characteristics is stimulated by the release of estrogens by the ovaries and progesterone by the corpeus luteum. Female development of the Mullerian system produces the accessory sex organs, the uterus and fallopian tubes.
Hormones of the hypothalamus, including gonadotrophin-releasing hormone (GnRH) stimulate or inhibit those secreted by the anterior pituitary, as does usually negative feedback from hormones released by the target glands. At puberty, GnRH release is increased to ensure normal reproduction and sexual maturation by in turn stimulating the anterior pituitary to release FSH and LH, gonadotrophic hormones that stimulate the production of sex hormones in the testes and ovaries. Releasing hormones (RH) from the hypothalamus are also important chemical triggers for anterior pituitary hormone release. The placenta also secretes estrogen, progesterone and polypeptide hormones that are similar in action to some anterior pituitary hormones.
Melatonin is the final hormone that is involved in reproduction; although not well understood, it's release is thought to play a role in the onset of human puberty. Melatonin is secreted by the pineal gland and is regulated by the suprechiasmatic nucleus of the hypothalamus that controls circardian rhythms. Unlike the nervous system's electrochemical responses to stimuli, which are rapid, (taking seconds or less), specific to the target tissue, and of short duration as target tissue chemical changes are effected, endocrine system responses are relatively slow, taking seconds, minutes or longer, widespread, as several target tissues respond, and long-lasting as specific chemical changes are brought about in the areas of metabolism, growth or reproduction.