The contrast between sexual and asexual reproduction is examined in Reproduction, an essay that also provides examples of plant reproduction through pollination. The present essay is concerned primarily with human sexual reproduction and secondarily with animal sexual reproduction. Some technical aspects of reproduction at the cellular level require consultation of processes explained in Genetics; here we confine our technical discussion to reproduction at the level of organs, fluids, and other bodily components. Reproduction is facilitated by the reproductive system, a group of organized structures that can be subdivided into male and female reproductive systems. During puberty, which typically occurs between the ages of 10 and 14 years, the reproductive systems of both sexes mature. This phase is marked in part by the release of eggs (female sex cells) in the female ovary and the formation of sperm (male sex cells) in the male testes. Reproduction can take place only when a sperm unites with an egg, a process called fertilization.
The testes are the pair of male reproductive glands located in the scrotum, a skin-covered sac that hangs from the groin. Each testis produces sperm cells, while the testes as a whole secrete testosterone. Testosterone is a hormone—a type of molecule that sends signals to spots remote from its point of origin to induce specific effects on the activities of other cells. Testosterone is associated with masculinity, though females secrete it in much smaller quantities as well. In males, testosterone secretion is critical to the development of secondary sexual characteristics—those unique traits that mark a person as a male or female, though they do not occur in the sexual organs themselves. A deepened voice is an example of a male secondary sex characteristic evident at puberty.
Sperm cells produced in the testes move to the epididymis, a coiled tube at the base of the penis where they are stored and matured. During ejaculation, or the ejection of sperm from the penis during orgasm, sperm travel from the epididymis through a long tube called the vas deferens to the urethra. This single tube, which extends from the bladder to the tip of the penis, is also the means by which urine passes out of the body. Liquid secretions from various glands combine with sperm (itself a gooey substance that is barely liquid) to form the semen, or seminal fluid. Ejaculated semen may contain as many as 400 million sperm.
The female system is much more complicated than the male version and has a role in all stages of reproduction. Whereas the male system primarily delivers semen to the vagina, the female system plays a critical part from fertilization until long after the birth of offspring. It produces ova, or eggs, receives sperm from the penis, houses and provides nutrients to the developing zygote (fertilized egg) and later the embryo and fetus, gives birth to offspring, and feeds those offspring after birth.
The visible part of the female reproductive system, which, of course, is not even half of the entire picture, includes the opening of the vagina and the external genital organs, or vulva. The vagina, a muscular tube extending from the uterus to the outside of the body, is the receptacle for sperm ejaculated during sexual intercourse and also forms part of the birth canal that will be used later, when the offspring comes to term. The external genital organs, known collectively as the vulva, include the labia, folds of skin on both sides of the openings to the vagina and urethra; the clitoris, a small, sensitive organ that is comparable to the male penis inasmuch as it swells when stimulated; and the mons pubis, a mound of fatty tissue above the clitoris.
Eggs are produced in the ovaries, oval-shaped organs in the groin that also generate sex hormones. At birth, a female's ovaries contain hundreds of thousands of undeveloped eggs, each surrounded by a group of cells to form a follicle, or sac; however, only about 360-480 follicles reach full maturity. During puberty the action of hormones causes several follicles to develop each month. Normally, just one follicle fully matures, rupturing and releasing an ovum through the ovary wall in a process called ovulation. The mature egg enters one of the paired fallopian tubes, where it may be fertilized by a sperm and move on to the uterus to develop into a fetus. The lining of the uterus, called the endometrium, prepares for pregnancy each month by thickening, but if fertilization does not take place, the endometrium is shed during menstruation.
During sexual intercourse, a man releases approximately 300 million sperm into a woman's vagina, but only one of the sperm can fertilize the ovum. The successful sperm cell must enter the uterus, swim up the fallopian tube (a trumpet-shaped passageway between the ovary and the uterus) to meet the ovum, and then pass through a thick coating, known as the zona pellucida, that surrounds the egg. The head of the sperm cell contains enzymes (a type of protein that speeds up chemical reactions—see Enzymes) that break through the zona pellucida and allow the sperm to penetrate the egg. Once the head of the sperm is inside the egg, the tail falls off, and the outside of the egg thickens to prevent another sperm from entering. Many variables affect whether fertilization occurs after intercourse among humans. One factor is a woman's ovulatory, or menstrual, cycle. Human eggs can be fertilized for only a few days after ovulation, which typically occurs only once every 28 days. (To learn about what happens after fertilization, see Pregnancy and Birth.)
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