Reproductive biology includes both sexual and asexual reproduction. Reproductive biology includes a wide number of fields: Reproductive systems Endocrinology Sexual development (Puberty) Sexual maturity Reproduction Fertility Human reproductive biology Endocrinology Human reproductive biology is primarily controlled through hormones, which send signals to the human reproductive structures to influence growth and maturation. These hormones are secreted by endocrine glands, and spread to different tissues in the human body. In humans, the pituitary gland synthesizes hormones used to control the activity of endocrine glands. Reproductive systems Internal and external organs are included in the reproductive system. There are two reproductive systems including the male and female, which contain different organs from one another. These systems work together in order to produce offspring. Female reproductive system The female reproductive system includes the structures involved in ovulation, fertilization, development of an embryo, and birth. These structures include: Ovaries Oviducts Uterus Vagina Mammary Glands Estrogen is one of the sexual reproductive hormones that aid in the sexual reproductive system of the female. Male reproductive system The male reproductive system includes testes, rete testis, efferent ductules, epididymis, sex accessory glands, sex accessory ducts and external genitalia. Testosterone, an androgen, although present in both males and females, is relatively more abundant in males. Testosterone serves as one of the major sexual reproductive hormones in the male reproductive system However, the enzyme aromatase is present in testes and capable of synthesizing estrogens from androgens. Estrogens are present in high concentrations in luminal fluids of the male reproductive tract. Androgen and estrogen receptors are abundant in epithelial cells of the male reproductive tract. Animal Reproductive Biology Animal reproduction occurs by two modes of action, including both sexual and asexual reproduction. In asexual reproduction the generation of new organisms does not require the fusion sperm with an egg. However, in sexual reproduction new organisms are formed by the fusion of haploid sperm and eggs resulting in what is known as the zygote. Although animals exhibit both sexual and asexual reproduction the vast majority of animals reproduce by sexual reproduction. In many species, relatively little is known about the conditions needed for successful breeding. Such information may be critical to preventing widespread extinction as species are increasingly affected by climate change and other threats. In the case of some species of frogs, such as the Mallorcan midwife toad and the Kihansi spray toad, it has been possible to repopulate areas where