- Meiosis produces gametes that contain half the number of chromosomes found in somatic cells. For each cell entering meiosis, the testes produce four spermatozoa, whereas the ovaries produce a single ovum.
- Spermatogenesis begins at puberty (sexual maturation) and continues until relatively late in life (above age 70). It is a continuous process, and all stages of meiosis can be observed within the seminiferous tubules.
- Oogenesis begins during embryonic development, and primary oocyte production is completed before birth.After puberty, each month the ovarian cycle produces one or more secondary oocytes from the pre-existing population of primary oocytes. The number of viable and responsive primary oocytes declines markedly over time, until ovarian cycles end at age 45–55.
- Cyclic changes in FSH and LH levels are responsible for the maintenance of the ovarian cycle; the hormones produced by the ovaries in turn regulate the uterine cycle. Inadequate hormone levels, inappropriate or inadequate responses to circulating hormones, or poor coordination and timing of hormone production or secondary oocyte release will reduce or eliminate the chances of pregnancy.
- Sex hormones have widespread effects on the body. They affect brain development and behavioral drives, muscle mass, bone mass and density, body proportions, and the patterns of hair and body fat distribution. As aging occurs, reduction in sexual hormone levels affect appearance, strength, and a variety of physiological functions.
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