The Reproductive System - human - male and female
The organs and functions of the reproductive system make possible the creation of new life. Physician specialists who treat health conditions of the male reproductive system are urologists. Physician specialists who treat health conditions of the female reproductive system are gynecologists. Health-care providers who provide care during PREGNANCY and CHILDBIRTH are obstetricians (physician specialists) and midwives (usually registered nurses). Nurse practitioners (registered nurses with advanced specialized training and credentials) often function as women’s health-care specialists, providing routine wellness care and treatment for minor reproductive and SEXUAL HEALTH conditions.
This section, “The Reproductive System,” presents a discussion of the organs and structures of the male and female reproductive systems, an overview of reproductive and sexual health and disorders, and entries about the health conditions that involve the male and female reproductive systems. “The Urinary System” contains entries about male organs and structures that share urinary and reproductive functions. “Genetics and Molecular Medicine” contains entries about the genetics of reproduction. “The Endocrine System” contains entries about the sex hormones.
Functions of the Reproductive System
Human reproduction requires participation from both male and female, each of which contributes one half the genetic material necessary for human existence. The cells that carry this material are the gametes—the OVA, or eggs, from the female half and the spermatozoa, or SPERM, from the male half. Determining gender are the sex chromosomes, one from each gamete to form the pair that designates the gender of the new life. Though gender distinction is apparent at birth, functional characteristics of gender do not emerge until late childhood when hormonal shifts initiate the changes of PUBERTY. At puberty the levels of androgens and estrogens increase in the body, initiating the emergence of the secondary sexual characteristics that mark reproductive maturity.
From a common beginning: embryonic gender differentiation
The glands that both represent and sustain gender are the gonads—the OVARIES in the woman and TESTICLES, or testes, in the man. In the embryo these structures arise from the same base cells in the mesoderm (the middle layer of germ cells in the embryo) called the gonadal ridge. Until seven weeks, the embryo is androgynous (has no gender characteristics; male and female appear the same) with rudimentary structures—the genital tubercle, labioscrotal swellings, and urogenital groove and folds—that will evolve into genderappropriate organs as the embryo develops.
At seven weeks the gonadal ridge organizes into a two-layer structure. If the genetic composition of the embryo is male, the gonadal ridge begins to produce TESTOSTERONE and a hormone unique to embryonic development, müllerianinhibiting hormone. In response to testosterone the inner layer, the medulla, begins to take shape as the testes and the outer layer, the cortex, degenerates. The genital tubercle becomes the glans PENIS, the urogenital groove and folds enlarge to form the rest of the penis, and the labioscrotal swellings fuse to form the SCROTUM.
If the embryo’s genetic composition is female there is no secretion of testosterone and müllerian-inhibiting hormone. So instead the outer layer, the cortex, begins to develop into the ovaries and the medulla deteriorates. The genital tubercle becomes the CLITORIS, the urogenital groove and folds become the VAGINA and labia minora, and the labioscrotal swellings form the labia majora. Primordial germ cells from the yolk sac migrate into the evolving testes or ovaries to become gametes (ova or sperm) as development continues. By 12 weeks gender differentiation is complete, though gender does not become detectable with ULTRASOUND imaging until the 20th week or later.
THE MYTHOLOGY OF THE HYMEN
The hymen, a narrow ring of membranous tissue that extends across the opening of the VAGINA, derives from the Greek god of the wedding feast, Hymen (also Hymenaeus). Hymen was the progeny of Dionysus, the god of FERTILITY, and Aphrodite, the goddess of love. Though the belief persists today that an intact hymen is evidence of a woman’s virginity, in truth numerous activities (tampon use, horseback riding, bicycling, and gymnastics to name a few) can rupture the hymen. As well, the structure of the hymen varies widely among women and may be so insignificant as to not impede the penetration of the erect PENIS.
Transition to fertility: puberty
Despite genital differences, boys and girls are fairly much alike physiologically for a dozen years or so after birth. Then hormonal signals trigger the onset of puberty, the transition from childhood to sexual and reproductive maturity. Though researchers do not know what activates the hormonal signals, the consequences are very familiar: the emergence of SECONDARY SEXUAL CHARACTERISTICS. The ovaries and testicles again become active, initiating the anatomic and physiologic changes that transform boys to men and girls to women. With sexual and reproductive maturity complete, SEXUAL INTERCOURSE and pregnancy become possible.
New life: conception, pregnancy, and childbirth
On the surface of it, reproduction is an astonishingly simple premise, and its organs uniquely suited to its purpose. During sexual intercourse the erect penis fits precisely within the vagina, reaching to the CERVIX. EJACULATION deposits millions of sperm in the upper vagina, a short swim from the cervical os (opening through the cervix into the UTERUS). When conditions and timing are right, the sperm make their way through the cervix and uterus and into one of the FALLOPIAN TUBES, encounter the ovum (egg), and one of the millions penetrates the ovum to fertilize it. The resulting ZYGOTE travels down the fallopian tube, tumbles into the uterus, and implants itself into the dense, spongy endometrium: CONCEPTION.
The woman’s body nourishes and shelters the developing FETUS, expanding and changing to accommodate its needs. The uterus stretches up to 10 times its normal size, pushing the abdominal wall outward. Again it is hormones that facilitate and support these processes, and hormones that bring the pregnancy to its conclusion: childbirth.
In the process of it, of course, there is nothing simple about any dimension of reproduction. Reproduction represents one of the most intricately choreographed experiences the human body can accommodate. Hundreds of hormones direct countless interactions, each of which spurs other events. Numerous factors, internal and external, influence reproduction to make it possible or not possible. Among the most significant advances in reproductive medicine are technologies to assist the process at various points along the reproductive continuum, from fertilization through childbirth.
Completing the cycle: menopause
Men remain fertile nearly the rest of their lives after puberty, though sperm production and quality tend to diminish in later life as testosterone levels decrease. FERTILITY ends for women with MENOPAUSE, the cessation of OVULATION and menstrual cycles. The ovaries contain a finite number of ova, present at birth. From the onset of MENSTRUATION to midlife, each monthly cycle causes a half dozen to a dozen ovarian follicles to ripen. Usually only one ovum (egg) reaches full maturity and leaves the ovary; the others atrophy (shrink) and the ovary reabsorbs them. By midlife the ovaries have made it pretty much through their supply of ova, far fewer follicles activate with each MENSTRUAL CYCLE, and ovarian function begins to shut down. Over a period of five to eight years, menstrual cycles become irregular and eventually infrequent until they stop altogether.
Health and Disorders of the Reproductive System
The health of the reproductive system, male or female, experiences internal and external influences. Internally the reproductive system relies significantly on an intricate hormonal balance as hormones direct nearly all sexual and reproductive functions. Disorders are often endocrine in origin. The organs of reproduction are particularly vulnerable to cancers that thrive on hormones, such as BREAST CANCER, OVARIAN CANCER, ENDOMETRIAL CANCER, PROSTATE CANCER, and TESTICULAR CANCER. Externally the reproductive system is vulnerable to injury and illness, to great extent through sexual activity. SEXUALLY TRANSMITTED DISEASES (STDS) can cause serious and sometimes lifethreatening health conditions. STDs are a leading cause of INFERTILITY in women and can cause illness in newborns who are exposed during birth. Other factors that can affect reproductive health include exposures to chemicals such as pesticides or to radiation, which can damage the DNA of sperm or ova.
External factors are particularly important during pregnancy, when certain exposures at vulnerable points of fetal development can cause permanent injury. INFECTION with common viruses such as RUBELLA (German MEASLES) or measles can cause devastating BIRTH DEFECTS, as can taking certain drugs and medications. Nearly any substance the woman takes into her body may cross the PLACENTA to enter the fetus’s BLOOD circulation. FETAL ALCOHOL SYNDROME, a constellation of physical birth defects and developmental abnormalities that occurs as a result of fetal exposure to ALCOHOL, is entirely preventable.
