The Reproductive System
This page outlines common features of both the male and female reproductive
systems. Separate pages describe the male reproductive
system and the female reproductive system.
- The reproductive systems of both sexes share the same basic
- The gonads produce gametes (eggs or sperm) which
unite during fertilization. They also produce steroid hormones
essential for reproduction as well the growth and development of the
- The genitalia are the external reproductive
structures (penis in males, clitoris and vulva in females). They allow for
passage of the sperm from the male into the female.
- Ducts and accessory glands are the
structures connecting the gonads to the genitalia. They are the site of
final preparation of the gametes for fertilization, and in females
also create the environment where fertilization occurs and the resulting
embryo develops until birth.
- The creation of gametes is called gametogenesis.
The process follows the same general sequence in both males and females:
- Gamete production begins with germ cells, which
begin to develop and multiply in the embryo. They are diploid cells,
containing 23 pairs of chromosomes for a total of 46 chromosomes. Germ
cell proliferation occurs through mitosis, giving rise to more germ cells
with the same number of chromosomes as the parent cells.
- The path from germ cell to gamete occurs through
meiosis, in which a germ cell replicates its DNA once but divides
- DNA replication occurs prior to the first replication,
as occurs for mitosis. After replication is complete, each cell possesses
four copies of each of the 23 chromosomes.
- The first meiotic division gives rise to two daughter cells,
each with two copies of each of the 23 chromosomes. These cells are called
primary gametes. Each primary gamete has a total of 46 chromosomes,
the same as most other cells in the body.
- In the second meiotic division, each of these daughter cells
divides again without replicating more DNA. As a result, each secondary
gamete receives only one copy of each of the 23 chromosomes, or half the
normal chromosomal contingent of other cells in the body.
- The cells that arise from meiosis must then undergo
extensive changes before they become actual gametes.
- Besides gametogenesis, the other major function of the
gonads is to produce steroid sex hormones, include androgens (such as
testosterone), estrogens, and progesterone. Hormonal control of the
reproductive system follows the same general pattern in men and women:
- A group of neurons in the hypothalamus acts as the
primary pacemaker of hormonal control of the reproductive system. These
neurons produce bursts of gonadotropin relasing hormone (GnRH) every one
to three hours. GnRH is a small peptide that travels via local blood
circulation to affect cells in the nearby anterior pituitary. The
frequency and magnitude of GnRH release is modulated by the brain in
response to a wide variety of internal and external stimuli.
- GnRH regulates the secretion of two peptide hormones from the
anterior pituitary: follicle stimulating hormone (FSH) and luteinizing
hormone (LH). Both hormones have trophic (stimulatory or
growth-inducing) effects on the gonads, but on different types of cells. As
such, they are collectively called gonadotropins. FSH
acts on the gamete-producing cells to regulate gametogenesis. LH acts on the
endocrine or hormone-producing cells, stimulating release of steroid sex
- There are two levels of feedback control over
production of steroid sex hormones. Gonadal hormones secreted into the
body circulation eventually reach the brain and pituitary and suppress GnRH,
FSH, and LH production. The gonadotropins also directly inhibit
GnRH secretion via a shorter feedback loop.
Male reproductive system
Female reproductive system