IN THIS CHAPTER
Unscrambling the human egg
Getting the gametes together
Responding to pregnancy’s changes
Reviewing reproductive system problems
This chapter is where you find out where babies come from and what happens when they’re born. Like all animals, humans have an instinctive knowledge of mating. However, only we humans seem interested in understanding the processes of mating and reproduction. This chapter gives you information about the anatomy and physiology of reproduction. You’ll have to look elsewhere for information on dating and mating rituals.
Functions of the Reproductive System
The reproductive system is different from all the other body systems discussed so far. The other systems focus entirely on their own survival, but the reproductive system “risks it all” in an effort to contribute genes to future generations. It does nothing to enhance physiological well-being — in fact, it can pose severe threats to the organism’s survival.
The following list gives you an overview of what the reproductive system is responsible for:
· Making gametes: The gametes, also called sex cells, are made within the organs of the female and male reproductive systems. There are two kinds of gametes: The ova (singular, ovum) are the female gametes and the sperm(singular, sperm) are the male gametes. Specialized cells, called germ cells, generate the gametes in a cell-division process called meiosis (see the “Meiosis” section later in the chapter). At the cell level, the processes are essentially identical in female and male bodies. At the tissue, organ, system, and organism levels, the processes are very different. (We discuss the various processes throughout this chapter.)
The terms spermatozoon (singular; literally “seed of an animal”) and spermatozoa (plural) are the correct technical terms, used now almost exclusively in formal scientific writing.
· Moving gametes into place: If the reproductive system is to succeed, one ovum and one sperm must make their way to the same place at the same time under the right conditions for them to fuse. Many of the reproductive system’s tissues and organs chaperone the gametes from the place and time of their production to another place, where they’re most likely to encounter their destiny.
· Gestating and giving birth: Only the female reproductive system has organs for gestating a fetus and giving birth. (See the “Pausing for Pregnancy” section later in the chapter for a detailed discussion of pregnancy.)
· Nurturing the newborn: The female reproductive system has tissues and organs specialized for nourishing the newborn for the first few months of life, until the older baby is capable of digesting other food.
The process of meiosis includes the sequence of cell-level events that result in the formation of sex cells (gametes) from somatic cells (germ cells). (Flip to Chapter 3 for the details of cell division and differentiation.) Meiosis is the only cellular process in the human life cycle that produces haploid cells.
Somatic cells are diploid, meaning that each cell nucleus contains two complete copies of the DNA that came into being in the zygote. Sex cells (gametes) are haploid, meaning that each cell nucleus contains only one copy of the DNA of the mother (somatic) cell. When two gametes fuse to form the zygote, each contributes its DNA to the new zygote, which is, therefore, diploid.
All of our cells divide by mitosis for replacement, growth, development, and repair, as we discuss in Chapter 2. (Chapter 3 also has details about the cycle of cell growth and division.) Only a single designated cell type divides by meiosis, in order to produce gametes — that is, for purposes of sexual reproduction. The process of meiosis is similar in its mechanics to the process of mitosis, but there are several key distinctions between the two.
The most obvious difference is that meiosis has two parts, called meiosis I and meiosis II. Each part proceeds in a sequence of events similar to that of mitosis (prophase, metaphase, anaphase, and telophase). In mitosis, the mother cell is diploid, and both daughter cells are also diploid, each having one complete and identical copy of the mother cell’s genome. In contrast, meiosis results in four haploid daughter cells. What’s more, the four haploid genomes are all different.
The early stages of meiosis (prophase I in Figure 14-1) include a mechanism called crossing-over or recombination for exchanging genes between chromosomes. The result is that the cell that becomes the gamete (one of the four haploid products of meiosis) is carrying chromosomes that are completely unique and not identical to the mother cell’s chromosomes. Like a lot of topics in cell biology, the complexity of these processes is beyond the scope of this book.
Illustration by Kathryn Born, MA
FIGURE 14-1: The process of meiosis.
Note that replication of DNA does occur in meiosis, during the interphase that precedes the onset of meiosis I. After two sequential, dichotomous (in two) divisions, the two complete copies are distributed among the four daughter cells, each having received a single copy of each chromosome.
Meiosis includes a number of mechanisms intended to ensure that each gamete has exactly one complete and correct copy of each gene. Any omission, duplication, or error is very likely to be fatal to the gamete or later to the embryo.
Female gametes: Ova
A mature ovum (see Figure 14-2) is one of the largest cells in the human body, about 120 micrometers in diameter (about 25 times larger than sperm) and visible without magnification. The ovum contains a haploid nucleus, ample cytoplasm, and all the types of organelles usually found in the somatic cell, all within a plasma membrane. The plasma membrane is enclosed within a glycoprotein membrane called the zona pellucida, which protects the zygote and pre-embryo until implantation.
Illustration by Kathryn Born, MA
FIGURE 14-2: The human ovum.
Oogenesis (the development of ova) in humans begins in embryonic and fetal development with specialized somatic cells called oogonia. Millions of these cells head down the path of meiosis, producing cells called primary oocytes.However, they are suspended in meiosis at the prophase I point until the female reaches puberty. By birth, the human female has only about 700,000 primary oocytes left.
After the onset of puberty, the primary oocyte resumes meiosis I, producing two cells, called a secondary oocyte and the first polar body. However, cytokinesis happens unevenly so most of the primary oocyte’s cytoplasm moves to the secondary oocyte. The first polar body completes meiosis II, and its daughter cells degenerate. The secondary oocyte continues with meiosis II but then stops again, this time during metaphase II.
The cells released from the ovary at ovulation are secondary oocytes. If the secondary oocyte is not fertilized, it degenerates without completing meiosis II.
When (or if) a sperm initiates fertilization, the secondary oocyte immediately resumes meiosis II, producing the ovum (plus a second polar body, which degenerates). Following fertilization, the ovum contains the sperm nucleus, and after approximately 12 hours, the two haploid nuclei fuse, producing the zygote.
Male gametes: Sperm
A mature sperm has three parts: a head that measures about 5 x 3 micrometers, containing a haploid nucleus; a short middle section; and a long flagellum. The sperm is adapted for traveling light — it has very little cytoplasm (see Figure 14-3). The head is covered by a structure called the acrosome that contains enzymes that break down the ovum’s membrane to allow entry. The middle contains mitochondria and little else. Mitochondria produce the energy that fuels the sperm’s highly active flagellum, which propels the sperm through the female reproductive tract.
© John Wiley & Sons, Inc.
FIGURE 14-3: The human sperm.
The process of sperm development (spermatogenesis) from meiosis to maturation takes place inside the testes. Specialized cells called spermatogonia divide by mitosis to produce another generation of spermatogonia. Mature spermatogonia, called primary spermatocytes, divide by meiosis, producing four haploid gametes called spermatids.
Similar to the case with females, males are born with spermatogonia in their seminiferous tubules, which remain dormant until puberty. During puberty, hormonal mechanisms pull the spermatogonia out of dormancy.
In contrast to oogenesis, which is cyclic, spermatogenesis is continuous beginning at puberty and continuing lifelong in most men. In contrast to the one-per-month gametogenesis in females, males produce astronomical numbers of sperm. Each ejaculation produces about 1 teaspoon of semen, which contains about 400 million sperm in a matrix of seminal fluid. Mature sperm can live in the epididymis and vas deferens for up to six weeks.
An important difference between males and females is that in females, all chromosome pairs are made up of two identical-looking strands, whereby in males, the strands are different from each other. This difference is easily visible under a high-power microscope: One of the pair is “normal” length (about the same length as all the other chromosomes) and the other is markedly shorter than all other chromosomes. The first is called the X chromosome, and females have one set of them in all their somatic cells. The second is called the Y chromosome, and males have a mismatched pair (one X and one Y chromosome) in all their somatic cells. After meiosis in the female, all ova have one X chromosome. After meiosis in the male, each sperm has either an X or a Y. The fusion of an ovum with an X sperm produces a female (XX) zygote. The fusion of an ovum with a Y sperm produces a male (XY) zygote.
ERRORS IN SEX CHROMOSOME DISTRIBUTION
Occasionally, cell division processes go awry. For example, two chromosomes get pulled to one side of the cell, leaving the other without a copy at all. Numerous genetic disorders are caused by just such an error, Down syndrome being the most notable (people affected by Down syndrome have three copies of chromosome 21). The sex chromosomes (X and Y) are not exempt from this potential error, leading to offspring with more or fewer than two copies.
In Klinefelter syndrome, men have an extra X chromosome in their cells (XXY). Affected boys develop normally until they reach puberty. Then, because the testes are underdeveloped, very little testosterone is produced. Males don’t develop the secondary sex characteristics associated with maleness (increased muscle mass, growth of body hair), may develop breast tissue, and are usually infertile. The earlier the condition is diagnosed, and the earlier testosterone replacement can begin, the greater the chance that the boy will develop normally and may even be able to father children with assistive reproductive procedures.
It is also possible for an offspring to develop with only a single copy of the X chromosome. Because males only receive one in the first place, this seemingly wouldn’t cause too much trouble. However, girls born with Turner syndrome need regular medical care throughout their entire lives. In addition to the lack of sexual development, there are numerous physical symptoms (short stature, broad chest, and so on), and learning disabilities are common. Hormone replacement therapy is a necessary part of treatment, but the effects are widespread across all organ systems.
The Female Reproductive System
The female body is specialized for reproduction to a much greater extent than the male body. The “Reproductive System (Female and Male)” color plate in the center of the book shows the female reproductive system in detail. Following is a brief discussion of the organs of the female reproductive system.
Organs of the female reproductive system
The organs of the female reproductive system are concentrated in the pelvic cavity. Many of the female reproductive organs are attached to the broad ligament, a sheet of tissue that supports the organs and connects the sides of the uterus to the walls and floor of the pelvis.
The ovaries are two almond-shaped structures approximately 2 inches (5 centimeters) wide, one on each side of the pelvic cavity. They house groups of cells called follicles.
The ovaries are the primary sex organs because they’re the site of oogenesis, the process of oocyte maturation. The ovaries also have a major role in endocrine signaling, especially the production and control of hormones related to sex and reproduction, namely estrogen and progesterone.
Beginning at the female’s puberty, the process of ovulation begins. The primary oocytes that have been dormant in her ovaries since early in her fetal development are hormonally activated, and secondary oocytes are released at a rate of approximately one per month from menarche (the first menstrual period of her life) to menopause (the last) — that is, from her early teen years to her late 40s or early 50s. The human female ovulates about 400 times during her lifetime.
The uterus or womb nourishes and shelters the developing fetus during gestation. It’s a muscular organ about the size and shape of an upside-down pear. The walls of the uterus are thick and capable of stretching as a fetus grows.
The lining of the uterus, called the endometrium, is built and broken down in the menstrual cycle, which we discuss in the section “Cycling approximately monthly” later in the chapter. A portion of the endometrium (deciduas basalis)becomes part of the placenta during pregnancy.
The cervix is a cylindrical muscular structure about 1 inch (2.5 centimeters) long that rests at the bottom of the uterus like a thimble. It controls the movement of biological fluids and other material (not to mention, occasionally, a baby) into and out of the uterus. Normally, the cervix is open ever so slightly to allow sperm to pass into the uterus. During childbirth, the cervix opens wide to allow the fetus to move out of the uterus.
The uterine tubes run from the ovary to the uterus. They are not literally connected to the ovaries; they just kind of hang over them. At the ovary end the uterine tube expands into a funnel shape called the infundibulum. This branches into fingerlike structures called fimbriae, which guide the egg into the uterine tube, which transports it to the uterus. The process of fertilization usually occurs in the uterine tube.
The vagina is the part of the female body that receives the male penis during sexual intercourse and serves as a passageway for sperm to enter into the uterus and uterine tubes. The vagina is about 3 to 4 inches (8 to 10 centimeters) in length. The cervix marks the top of the vagina.
During childbirth, the vagina must accommodate the passage of a fetus weighing on average about 7 pounds (3 kilograms), so the vagina’s walls are made of stretchy tissues — some fibrous, some muscular, and some erectile. In their normal state, the vagina’s walls have many folds, much like the stomach’s lining. When the vagina needs to stretch, the folds flatten out, providing more volume.
In females, the external genitalia comprise the labia majora, labia minora, and the clitoris. Together, these organs are called the vulva. The term labia (singular, labium) means “lips.” The labia of the vulva are loose flaps of flesh, just like the lips of the mouth (called labia mandibulare and labia maxillare, by the way). The labia protect the vagina’s opening and cover the pelvis’s bony structures.
Here are some details about the three parts of the vulva:
· Labia majora: These large folds of skin — one fold on each side — cover the smaller labia minora. The labia majora extend from the mons pubis (pubic mound) back toward the anus. The mons pubis contains fat deposits that cover the pubic bone. Following puberty, pubic hair covers the mons pubis and the labia majora.
· Labia minora: These hairless folds of skin lie underneath the labia majora and cover the opening of the vagina. The labia minora are attached near the vaginal orifice (opening) and extend upward, forming the foreskin that covers the clitoris.
· Clitoris: This part of the vulva located above the vagina’s opening and above the urethra has a shaft and glans tip, just as a penis does, and it’s extremely sensitive to sexual stimulation. The clitoris contains erectile tissues that fill with blood during sexual stimulation. Because the tissue of the labia minora cap the clitoris, the swelling and reddening is also obvious in the labia minora. Stimulation of the clitoris can lead to orgasm in the female. Although females don’t ejaculate, females do experience a building and release of muscular tension. Female orgasm causes the muscle tissue that lines the vagina and uterus to contract, which helps to pull the sperm up through the reproductive tract.
All humans have mammary glands, but only females produce a substance we call milk for the nutrition of relatively helpless infants with high calorie requirements. Besides nutrition, breast milk boosts the infant’s immune system.
The breast contains about two dozen lobules that are filled with alveoli that make and store milk. The milk is released into lactiferous ducts, which merge at the nipple (see Figure 14-4). During puberty, the lobules and ducts develop, and adipose tissue is deposited under the skin to protect the lobules and ducts and give shape to the breast. During pregnancy, hormones increase the number of milk-producing cells and increase the size of the lobules and ducts.
Illustration by Kathryn Born, MA
FIGURE 14-4: The human breast.
After the infant is born, the mother’s pituitary gland secretes the hormone prolactin, which causes the milk-producing cells to create milk, and lactation begins. The infant suckles the milk out of the ducts through the nipple. Lactation continues as long as a child nurses regularly.
The hormone oxytocin is strongly involved in milk release (let-down reflex). Stimulation of the nipple prompts the secretion of oxytocin from the mother’s pituitary gland. Oxytocin expels milk from the lobules by causing them to contract, just as it stimulates uterine contractions to expel the fetus. This hormone has also been strongly correlated with neuro-emotional phenomena, such as family bonding.
Cycling approximately monthly
The menstrual cycle (monthly cycle) consists of both the ovarian cycle and the uterine cycle, both of which are approximately 28 days in duration (see Figure 14-5). These cycles run concurrently to prepare the ovum and the uterus, respectively, for pregnancy.
Illustration by Kathryn Born, MA
FIGURE 14-5: The menstrual cycle.
By convention, the first day of menstrual bleeding is counted as Day 1 of the menstrual cycle. Menstrual bleeding begins at a point in the cycle when the levels of estrogen and progesterone are at their lowest. However, the entire menstrual cycle is directed by several hormones, not just estrogen and progesterone.
Looking at the ovarian cycle
The 28-day ovarian cycle is the most important part of the menstrual cycle because it’s responsible for producing the hormones that then control the uterine cycle. (See the next section, “Clocking the uterine cycle.”) From Day 1 to Day 13, triggered by low estrogen level, follicle-stimulating hormone (FSH) stimulates the development of a follicle and luteinizing hormone (LH) stimulates the maturation of an oocyte in one of the ovaries. When the follicle is developed enough, it begins to secrete estrogen. When the level of estrogen reaches the appropriate level, a negative feedback mechanism involving the hypothalamus briefly slows the secretion of FSH and LH. When the follicle is fully mature and the oocyte is ready to be released, FSH and LH secretion spikes. This occurs on Day 14 and triggers ovulation (release of the oocyte). An oocyte lives for only 12 to 24 hours after ovulation (if unfertilized).
At the time of ovulation, the anterior pituitary gland, which has been secreting FSH and LH simultaneously, secretes a surge of LH that causes the follicle from which the oocyte was released to become a corpus luteum (yellow body). The corpus luteum secretes the hormone progesterone, which triggers the hypothalamus. When the corpus luteum has secreted a sufficient amount of progesterone, the hypothalamus stops the anterior pituitary gland from secreting any more LH. At that point, the corpus luteum begins to shrink (about Day 17). When the corpus luteum is gone (about Day 26), the levels of estrogen and progesterone are at the lowest levels of the cycle (sometimes causing symptoms of premenstrual syndrome), and menstruation starts (about Day 29, or Day 1 of the new cycle).
Like any cycle, the whole process starts over. When the level of estrogen is low during menstruation, the hypothalamus detects the low level and secretes gonadotropin-releasing hormone (GnRH), which prompts the anterior pituitary gland to release its gonadotropic hormone — FSH — so that another follicle is stimulated to develop a new oocyte that secretes estrogen. Now, you’re back to the first paragraph of this section.
Clocking the uterine cycle
The 28-day uterine cycle, which aims to prepare the uterus for a possible pregnancy, overlaps with the ovarian cycle.
· Days 1 to 5: The first 5 days of the uterine cycle is when the level of estrogen and progesterone are lowest — the period of menstruation. The low level of sex hormones fails to prevent the tissues lining the uterus (the endometrium) from disintegrating and shedding. As the hormone levels drop, blood vessels spasm, cells undergo autolysis (self-destruction), tissues tear apart from the uterus wall, and blood vessels rupture, causing the bleeding that occurs during a period. The blood and tissue (menstrual flow) passes out of the uterus through the cervix and then out of the body through the vagina.
· Days 6 to 14: During this proliferative phase, the developed follicle secretes high levels of estrogen, which makes the endometrium regenerate fresh tissue. The tissues lining the uterus and the glands in the uterine wall grow and develop an increased supply of blood. All these changes are preparation for nourishing an embryo and supporting a pregnancy, should the oocyte, which is released on day 14, become fertilized and implant in the wall of the uterus. (See the section “Pausing for Pregnancy” later in this chapter.)
· Days 15 to 28: During this secretory phase, the corpus luteum secretes an increasing level of progesterone, which further thickens the endometrium, and the glands of the uterus secrete a thick mucus. If the egg becomes fertilized, the thickened endometrium and mucus help to “trap” the fertilized egg so it implants properly in the uterus. If the egg doesn’t become fertilized within a day or two, the corpus luteum begins to shrink because it won’t be needed for a pregnancy. As the corpus luteum shrinks, the progesterone and estrogen levels decline, which causes the endometrium to “shred and shed” just before menstruation.
Early on in a pregnancy, the corpus luteum serves as a source of estrogen and progesterone until the placenta develops and can secrete estrogen and progesterone on its own.
Winding down the cycle
Physiologically, menopause essentially reverses the hormonal pathway of adolescence. When a woman enters menopause, her ability to reproduce ends — ovulation stops, and she no longer can become pregnant. She may also experience hot flashes and sweat baths if faulty signals from the parasympathetic nervous system disrupt the body’s ability to accurately monitor its temperature. Other body processes slow down, including cellular metabolism and the replacement of the structural proteins in the skin, which leads to wrinkles. A woman’s bones may also weaken when the breakdown of bone tissue occurs faster than the buildup of bone tissue during bone remodeling.
Menopause is one of the unique aspects of human physiology. Not that reproductive cycling declines and stops as the female ages. That happens in many mammal, bird, and reptile species (although relatively few individuals in any species live long enough to experience the decline of their reproductive capabilities).
The unique part is that human females often live a substantial proportion of their life span beyond their reproductive capacity. (A woman in her 80s has lived around 40 percent of her life after menopause.) Much research into this phenomenon is concentrated at the interface of biology and culture. One theory holds that an adult female with no offspring of her own to feed tends instead to feed her grandchildren or other children in her community. Children with grannies eat better, the theory goes, improving their chances of surviving to reproductive age and pushing her genes into one more generation.
The Male Reproductive System
The male reproductive system produces sperm and moves them into the female reproductive system. On a rare occasion (relative to the astronomical number of sperm that the average human male produces), a sperm fertilizes an egg. All the billions and billions of other sperm a man produces in his lifetime have a limited life span — about six weeks if they stay in the male’s body, or up to five days if in the female’s body.
The female gamete is released from the ovary as a secondary oocyte. Only when fertilization is initiated does the ovum (egg) come into being.
If reproduction is defined as ending with the creation of a new organism, that’s the end of the male’s reproductive function. If reproduction is defined as including the nurturance of the new organism until it is itself ready for reproduction, the anatomy and physiology of the male may well be substantially devoted to the task for decades, along with those of the female. For the purposes of this chapter, we use the more limited definition.
Some people are confused about the meaning of “evolutionary success” for the individual (you, for example). Truly, it’s not how many zygotes arise from your gametes, or even how many offspring are born from these zygotes, but how many of these offspring survive to reproduce. Simply, evolutionary success is rated not by the number of your children (and certainly not by your level of sexual activity or number of opposite-sex partners) but by the number of your grandchildren.
The organs of the male reproductive system
The organs of the male reproductive system produce gametes, called sperm, and transfer them to the female reproductive system. (Refer to the “Reproductive System (Female and Male)” color plate in the center of the book for a look at the male reproductive system.) In contrast to some other organ systems, and especially to the reproductive system of females, the male reproductive organs are located in an exposed location on the periphery of his body.
Testes and scrotum
The testes (singular, testis) are paired organs that produce sperm and hormones. Like the ovaries of the female reproductive system, the testes are the site of gamete production and therefore the primary sex organs. Refer to Figure 14-6for an illustration of the testicular structures.
Illustration by Kathryn Born, MA
FIGURE 14-6: The testicular structure.
The testes contain fibrous tissue that forms long, coiled compartments called seminiferous tubules. These are the site of spermatogenesis, the process of sperm development from meiosis. The walls of the seminiferous tubules are lined with thousands of spermatogonia (immature sperm). The seminiferous tubules also contain Sertoli cells that nourish the developing sperm and regulate how many of the spermatogonia are developing at any one time.
The seminiferous tubules transport the new sperm (spermatids) to another long, cordlike structure that lies atop each testis. This is the epididymis and maturation of the sperm occurs here. The epididymis is continuous with (that is, “becomes”) the vas deferens, a tube that connects the epididymis of each testis to the penis. Sperm bide their time here until they’re released via ejaculation.
The testes are held within the scrotum beneath and outside of the abdomen. The scrotum contains smooth muscle that contracts when the scrotal skin senses cold temperatures and pulls the scrotum (and thereby the testicles) closer to the body to keep the sperm at the right temperature. The scrotum’s inside muscle layers are an outpouching of the pelvic cavity. The scrotum’s outside skin is continuous with the skin of the perineum and groin.
Several other structures secrete the substances that make up the ejaculatory fluid that provides a matrix for the propulsion of sperm into the female reproductive tract. Among these are the prostate and the seminal vesicles. The prostate also contains some smooth muscles that help expel semen during ejaculation.
The penis consists of a shaft and the glans penis (tip). The tube-shaped urethra runs through the shaft of the penis, and the glans penis contains the urethral orifice. The semen is ejaculated through the urethra and urethral orifice. Foreskin (also called prepuce) covers the glans penis. The foreskin of newborn males is often removed in a surgical procedure called circumcision.
During sexual arousal, erectile tissue in the shaft became engorged with blood, enabling the penis to be inserted into the female vagina, delivering the sperm to the vicinity of the secondary oocyte (if one is available).
The urethra and urethral orifice also function in the urinary system as the tube through which urine leaves the body. However, semen doesn’t contain urine. At the point of ejaculation, a sphincter closes off the bladder to keep urine, which is acidic, from mixing with the sperm, which live in a basic environment.
Seminal fluid and ejaculation
The seminal vesicles, glands located at the juncture of the bladder and vas deferens, have ducts that allow the fluid they produce to sweep the sperm from the vas deferens into the urethra.
Next, the prostate gland adds its fluid, which contains mainly citric acid and a variety of enzymes that keep semen liquefied. The prostate gland surrounds the urethra just below where it exits from the urinary bladder.
These two glands — the seminal vesicles and the prostate gland — secrete fluids that have several functions:
· They’re slightly basic with a pH of 7.5, just the way sperm like their environment to be.
· They nourish the sperm by providing the sugar fructose so that the sperm’s mitochondria can make enough energy to move its tail and travel all the way to the egg.
· They contain prostaglandins, which are chemicals that make the uterus reverse its downward contractions. When the uterus contracts, the sperm are pulled farther upward into the female’s reproductive tract.
As the glands add the secretions, forming the semen, pressure builds up on the structures of the male reproductive tract. When the pressure has reached its peak, the semen is expelled out of the urethra through the penis. Peristaltic waves (like those that occur in the digestive tract; see Chapter 11) and rhythmic contractions move the sperm through the vas deferens and urethra. The term for this discharge is ejaculation — part of orgasm in males, as is the contraction and relaxation of skeletal muscles at the base of the penis. As the muscles contract rhythmically, the semen comes out in spurts.
The bulbourethral glands, also called Cowper’s glands, sit within the floor of the pelvis near the base of the penis on either side of the urethra. These two small glands have ducts leading directly to the urethra and secrete a mucuslike fluid in response to sexual stimulation. This clears the urethra of any acidity as well as provides lubrication for intercourse. Most of the lubrication, however, comes from the vestibular glands of the female, located near the vaginal opening.
Sperm develop motility (the ability to move) in the epididymis, but they don’t have the capacity to move until after ejaculation. That is, they don’t actually “swim” until they have been released into the female reproductive tract. Prior to that, they’re moved along by the ciliated cells that line the tubes.
Pausing for Pregnancy
Pregnancy is established in two stages: fertilization of the secondary oocyte and implantation of the blastocyst in the uterus. Development of the embryo after implantation is the subject of Chapter 15. The female body makes many and various adaptations to pregnancy and delivery, which we examine in the following sections.
Steps to fertilization
Ovulation sends a secondary oocyte from the follicle of the ovary into the uterine tube. Then, within an appropriate time, heterosexual intercourse results in the ejaculation of semen into the vagina. Some few million sperm make their way through the cervix, up through the uterus, and into the uterine tube to the vicinity of the waiting secondary oocyte.
To achieve fertilization, one sperm must penetrate the secondary oocyte’s membrane, and its nucleus must fuse with that of the ovum. At this point, the secondary oocyte is fertilized, the ovum is developed, and the zygote comes into being (see Chapter 15).
The probability of any act of intercourse resulting in fertilization is actually quite low because many complicating factors exist. The timing of intercourse relative to ovulation is crucial. The released secondary oocyte is viable for only a matter of hours; sperm live a little longer in the female reproductive tract (one to two days on average). The environment within the female reproductive tract may be more or less hospitable to the sperm, depending on the female’s hormone levels and other physiological processes. Even when a single sperm has made contact with the secondary oocyte, fertilization is not assured.
Following fertilization, the zygote divides immediately. Several more cell division cycles take place as the pre-embryo moves down the uterine tube. Experts believe that many pre-embryos die at this stage, sometimes because of genetic or developmental abnormalities. Only if the pre-embryo arrives at the uterus and properly embeds itself into the endometrium is pregnancy established.
A successfully implanted pre-embryo, now called a blastocyst, begins immediately to take over its mother’s body. As the outer layer of the blastocyst begins to form the placenta, a hormone called human chorionic gonadotropin(hCG) is released, which maintains the corpus luteum, elevating levels of progesterone and estrogen and inhibiting menstruation.
The presence of hCG can be detected chemically in the urine of a pregnant woman within 10 to 14 days after fertilization. It may be detected before that symptomatically by the mother as a sensation of being nauseated in every cell of her body.
Adapting to pregnancy
The maternal body responds to pregnancy with many anatomical and physiological changes to accommodate the growth and development of the fetus. Most structures and processes revert to the nonpregnant form (more or less) after the end of the pregnancy. See Chapter 15 for details about how the fetus grows in the uterus.
During pregnancy, the uterus grows to about five times its nonpregnant size and weight to accommodate not only the fetus but also the placenta, the umbilical cord, about a quart of amniotic fluid, and the fetal membranes. The size of the uterus usually reaches its peak at about 38 weeks gestation. During the last few weeks of pregnancy, the uterus has expanded to fill the abdominal cavity all the way up to the ribs. The size of the expanded uterus and the pressure of the full-grown fetus may make things difficult for the mother.
The placenta acts as a temporary endocrine gland during pregnancy, producing large amounts of estrogen and progesterone by 10 to 12 weeks. It serves to maintain the growth of the uterus, helps to control uterine activity, and is responsible for many of the changes in the maternal body.
Near the end of pregnancy, the cervix softens. Enlarged and active mucus glands in the cervix produce the operculum, a mucus “plug” that protects the fetus and fetal membranes from infection. The mucus plug is expelled at the end of the pregnancy. Additional changes and softening of the cervix occur at the onset of labor.
Hormonal mechanisms prevent follicle development and ovulation in the ovaries.
The breasts usually increase in size as pregnancy progresses and may feel inflamed or tender. The areolas of the nipples enlarge and darken. The areola’s sebaceous glands enlarge and tend to protrude. By the 16th week (second trimester), the breasts begin to produce colostrum, the precursor of breast milk.
Other organ systems
Pregnancy affects all organ systems as they support the growth and development of the fetus and maintain homeostasis in the female. Here are a few important physiological consequences of pregnancy:
· The other abdominal organs are displaced to the sides as the uterus grows.
· Decreased tone and mobility of smooth muscles slows peristalsis and enhances the absorption of nutrients. An increase in water uptake from the large intestines increases the risk for constipation. Relaxation of the cardiac sphincter may increase regurgitation and heartburn. Nausea and other gastric discomforts are common.
· Increases occur in blood volume, cardiac output, body core temperature, respiration rate, urine volume, and output from sweat glands.
· Immunity is partially suppressed.
· Spinal curvature is realigned to counterbalance the growing uterus. Slight relaxation and increased mobility of the pelvic joints prepares the pelvis for the passage of the infant. This can compromise the woman’s lower-body strength starting in the second trimester.
Labor and delivery
Labor is initiated by complex hormonal signaling between the maternal and fetal bodies. In the ideal labor and delivery process, called parturition, powerful contractions of the uterus push the fully mature fetus (infant) past the cervix and down the birth canal without undue trauma to either the mother or the infant. In this positive feedback mechanism (see Chapter 2), more stretch in the uterus and cervix triggers the release of hormones (particularly oxytocin), which cause stronger contraction and, thus, more stretch. Parturition occurs in three stages.
Stage 1 labor progresses in three phases: early, active, and transition. Contractions intensify and the amniotic membrane ruptures (water breaks) at any point in this stage.
Early labor beings with irregular, mild contractions lasting around 30 seconds with 5 to 30 minutes between them. During the 8 to 12 hours of early labor, the cervix will become thinner, or efface (see Figure 14-7). It will also begin to dilate.
Illustration by Kathryn Born, MA
FIGURE 14-7: Effacement of the cervix in early labor.
After the cervix dilates to about 3 cm, active labor begins. Here, the contractions become more painful, squeezing inward from all directions. The contractions continually become stronger, last longer (45 to 60 seconds), and occur more frequently (every three to five minutes). Active labor generally lasts around three to five hours, until the cervix reaches about 8 cm.
During transition, the contractions shift from squeezing in to pushing down. This phase lasts only 30 minutes to two hours. The intense contractions last 60 to 90 seconds with only 30 seconds to 2 minutes between. Many women (who don’t opt for an epidural to block all this pain) state that transition is the most painful part of the entire parturition process. On completion of this phase, the cervix is dilated to 10 cm and stage 2 begins.
As the contractions continue at a now regular pace (60 to 90 seconds every three to five minutes), there is a natural urge to push. As the mother pushes with each contraction, the baby makes specific motions to ease its movement through the birth canal (see Figure 14-8). It flexes its head out, and then turns to aid passage of the shoulders.
Illustration by Kathryn Born, MA
FIGURE 14-8: An overview of delivery.
This stage lasts about 30 minutes to two hours. Just after delivery, the infant’s umbilical cord is cut and tied off. The infant is now totally separated from the mother and will soon have a stylish belly-button.
After the baby is delivered, uterine contractions continue so that the placenta separates from the uterus wall. About 15 minutes after the baby is born, the placenta passes through the birth canal. Uterine contractions continue, during which the uterus contracts, returning eventually to near prepregnant size.
Pathophysiology of the Reproductive System
Reproduction is a dangerous business for all animals. The investment of energy is huge, the risks are just as great, and the rewards are distant. It is an intricate interaction among many structures and processes besides those explicitly identified with the reproductive system. Structural defects, hormonal problems, genetic abnormalities, and cancers can all cause problems in reproduction. Here are some of the problems that can affect the reproductive system.
Infertility is the inability to fertilize or be fertilized. Infertility may be due to the failure to generate viable gametes, blockages in the “travel routes” of the gametes or the pre-embryo, or damage to or disease in the endocrine glands that control reproduction. Certain bacterial or viral infections, such as mumps, can result in orchitis (inflammation of the testes) that can affect fertility. Fertility decreases with age, ending abruptly at menopause in females and declining more gradually in males. Turn to Chapter 15 for more information on aging.
Sexually transmitted infections
Some microbial diseases are transmitted via sexual contact. These sexually transmitted infections (STIs) are endemic in human populations (that is, they exist in all human populations to one extent or another and have since forever) because bacteria and viruses can easily be spread from one person to another via the organs and secretions of the reproductive system.
The terms STI and STD (sexually transmitted disease) are often used interchangeably, but STI is becoming more prevalent. The reason for the shift is that STD implies that the presence of the pathogen led to characteristic symptoms. This, however, is not always the case. The microbe can be passed on without any indication of disease (hence, STI).
Sexually transmitted infections are similar in most ways to other microbial infections. All the major groups of microbes (bacteria, fungi, protists, and viruses) have evolved some ability to propagate themselves in the very hospitable environment of the human reproductive tract. They cause problems in all the same ways: by inducing inflammation, over-activating the immune response, and destroying cells.
In the very special case of HIV, the infectious organism (an exotic creature called a retrovirus) destroys structures of the immune system, which leaves the body vulnerable to attack by other microbes.
The infectious bacterium Chlamydia trachomatis is transmitted by other means as well as sexually; it infects the human eye, joints, and lymph nodes, and also takes up residence in arteries.
Up to 80 percent of menstruating women experience both physical and mental changes just before the onset of menstrual blood flow. The type and severity of symptoms varies greatly from one woman to another, but the symptoms tend to remain stable through her reproductive life. Premenstrual syndrome (PMS) is a term used to describe a medical syndrome of mood swings, mild edema (fluid retention in the tissues), irritability, fatigue, food cravings, and uterine spasms (cramps) that affect an estimated 20 percent to 40 percent of women. Another more severe form, called premenstrual dysphoric disorder (PMDD), affects 2 percent to 10 percent of menstruating women, has much in common with mood disorders, and can result in severe disruption of daily activities. Physiological, psychological, environmental, and social factors all seem to play a part in the development of these disorders.
The endometrium is the lining of the uterus, which is shed during menstruation. In endometriosis, endometrial tissue grows in or on organs of the body other than the uterus — usually organs in the pelvic cavity, such as the bladder, ovary, or large intestine. Because the ovaries are not directly attached to the uterine tube, the endometrial tissue migrates and “falls out” into the pelvic cavity. During the uterine cycle, the tissue, regardless of its location, responds by building up and then disintegrating, sometimes causing extreme pain.
During fetal development, the testes are located inside the pelvic cavity, but around the time of birth, the testes descend into the scrotum. Failure of testes to descend is called cryptorchidism. Unless corrected by surgery, cryptorchidism results in sterility.
Problems with the pituitary gland, such as injury or tumors, can cause hypogonadism, a decline in the function of ovaries or testes. The pituitary gland secretes follicle-stimulating hormone (FSH), which normally spurs on maturation of the oocyte or spermatocyte and the subsequent release of estrogen or testosterone. Symptoms in women include amenorrhea (absence of menstruation) and infertility. In men, the symptoms of hypogonadism are impotence and infertility.
Erectile dysfunction (ED), also called impotence, is a condition in which penile erection doesn’t follow sexual stimulation. ED has a variety of possible causes, including damaged blood vessels, sometimes because of diseases such as diabetes; psychological factors such as stress and fear; and nerve damage. Some degree of ED is considered a normal part of aging.
Pathophysiology of pregnancy
Even a healthy, young-adult woman carrying a pregnancy under ideal social and economic conditions is at risk for pathophysiology in many organ systems. Carrying a pregnancy and giving birth are major contributors to disability, disease, and death in females. And her existing dependent children experience consequences as well.
Childbirth has other risks — the wounds incurred expose a woman to some kinds of infections, and significant blood loss is quite common. Some other pregnancy-related disorders are
· Ectopic pregnancy: An ectopic pregnancy is an abnormal pregnancy where the pre-embryo implants outside the uterus, most commonly in the uterine tubes. An ectopic pregnancy is often caused by a condition that blocks or slows the movement of a pre-embryo through the uterine tube to the uterus. This may be caused by a physical blockage, hormonal factors, and by other factors, such as smoking. The fetus can’t survive and often stops developing altogether. Ectopic pregnancy is a life-threatening condition.
· Gestational diabetes: Gestational diabetes is hyperglycemia (excess glucose in the blood) that develops during pregnancy and that affects both mother and fetus. Diabetes can complicate delivery and can increase risk for diabetes mellitus after the pregnancy.
· Incompetent cervix: In this condition, the cervix is unable to support the pregnancy. The cervix may have been traumatized during an earlier birth. A woman carrying multiples is at increased risk. The cervix dilates prematurely (before the onset of labor), a serious risk for pregnancy loss. To alleviate the problem, in a procedure called cerclage, the cervix can be stitched to give it extra support.
· Pre-eclampsia and eclampsia: Just as urine is checked for glucose to help stave off gestational diabetes, the urine is also checked for protein, diagnostic of risk for pre-eclampsia. Pre-eclampsia (high blood pressure during pregnancy) can easily escalate into eclampsia, characterized by seizures, and possibly coma or even death.
· Placenta previa: In this condition, the placenta covers the cervix, partially or completely, blocking delivery of the fetus or causing heavy bleeding. Placenta previa in the second or third trimester may result in blood loss from the placenta as the growing fetus presses on it. The bleeding puts the mother at risk for preterm labor and the fetus at risk for premature birth. Pregnancies complicated by placenta previa are treated by cesarean delivery.
· Placental abruption: In this condition, part of the placenta peels away from the uterine wall before the fetus is ready to be delivered. Such an event can deprive the fetus of oxygen and nutrients and, depending on the extent of the tear, can bring on preterm labor and cause life-threatening hemorrhaging in the mother. Maternal hypertension, physical trauma to the mother and fetus (such as a car accident), and a short umbilical cord are among the most common causes of placental abruption.
· Fetal distress: Labor and delivery are stressful for the fetus. Fetal distress can sometimes complicate the delivery.
Pregnancy loss, also called spontaneous abortion and miscarriage, is the death of an embryo or fetus without apparent cause within the first 20 weeks of gestation. From 10 percent to 25 percent of all clinically recognized pregnancies end in miscarriage. Many more pregnancies end shortly after implantation, often without the woman realizing that she was pregnant. The bleeding that results may begin around the expected time of her menstrual period.
Miscarriage has many causes. Clinicians speculate that many may be caused by abnormalities in the embryo and not because of any disorder of the woman’s reproductive system. Many women experience miscarriage and go on to have a normal pregnancy with a normal outcome (a cute baby). Repeated miscarriage, however, may indicate some form of disorder in either the male or female parent.