9 The Reproductive Systems
Learn how reproductive anatomy supports gamete production and transport, how hormones regulate reproduction, and how fertilization can begin early development.
Reproductive functions
Reproductive systems produce and sex hormones, and provide the structures through which fertilization can occur. The female reproductive system also provides structures through which pregnancy and birth can occur. Anatomy and reproductive function vary among individuals.
Male reproductive anatomy
The are the male gonads. Within their , germ cells develop into sperm. The also produce testosterone, which supports sperm production and reproductive function. The scrotum is a skin-and-muscle sac outside the pelvic cavity; it holds the and helps maintain a temperature suitable for sperm production.
Sperm travel from the to the , where they mature and are stored. During ejaculation, sperm move through the ductus deferens, also called the vas deferens, and ejaculatory ducts into the urethra. The seminal vesicles, prostate gland, and bulbourethral glands add fluids that, together with sperm, form .
The penis contains erectile tissue and the urethra. The urethra carries out of the body during ejaculation and urine at other times. A sperm cell has a DNA-containing head, an energy-producing midpiece, and a tail that helps it move. The head's acrosome contains enzymes involved in fertilization. Sperm are haploid: each carries chromosomes, half the usual number in a human body cell.
Female reproductive anatomy
The ovaries are the female gonads. They contain developing oocytes and produce hormones, including estrogen and progesterone. An is an immature female gamete; the term egg is often used informally.
In a typical cycle, an is released from an ovary during and enters a , also called a fallopian tube. Cilia and smooth-muscle contractions help move it toward the uterus. Fertilization, when it occurs, usually takes place in a .
The uterus is a muscular organ. Its inner lining, the , changes during the menstrual cycle and can support implantation and early development. The cervix is the lower part of the uterus and opens into the vagina, a muscular canal that carries menstrual flow and serves as the birth canal. The external genital structures are collectively called the vulva and include the labia and clitoris. Mammary glands in the breasts can produce milk after birth.
Gamete formation and hormonal control
reduces chromosome number by half, producing haploid . In spermatogenesis, precursor cells divide and develop into sperm in the . In , an develops within an ovarian follicle; typically, one is ovulated during a cycle.
begins before birth. Unlike sperm production, which continues from puberty through much of adult life, development follows a limited, cyclical pattern. The cell released at is generally a secondary ; it completes only if fertilization occurs. A mature female gamete, like a sperm, has chromosomes.
The hypothalamus and pituitary gland help coordinate reproductive function by releasing hormones that act on the gonads. In males, supports sperm development, while stimulates testosterone production. In females, FSH supports follicle development, and a rise in LH helps trigger . Ovarian estrogen and progesterone help regulate the menstrual cycle and prepare the uterine lining.
The menstrual cycle
The menstrual cycle involves coordinated changes in the ovaries and uterus. Cycle length varies, so understanding the sequence is more useful than assuming every cycle lasts the same number of days.
Menstruation: If pregnancy has not begun, the functional layer of the is shed as menstrual flow.
Follicular phase: FSH supports the growth of ovarian follicles. Rising estrogen helps rebuild the .
: A surge in LH triggers the release of an from a mature follicle.
Luteal phase: The emptied follicle becomes the corpus luteum, which produces progesterone and helps maintain the uterine lining. If implantation does not occur, hormone levels fall and the next menstruation begins.
Fertilization and early development
During ejaculation, may be deposited in the vagina. Sperm can move through the cervix and uterus into a . If a sperm penetrates and fuses with the secondary , fertilization begins; the then completes , and the genetic material combines to form a with chromosomes.
The divides as it travels toward the uterus. If the resulting early embryo implants in the , pregnancy is established and development continues. If fertilization or implantation does not occur, the cycle proceeds to menstruation.