Prof. Atsma © 2005

The following is a narrative summary of the topic. Click here for the "Classroom Notes" that you can print out and bring to class to save yourself a lot of note-taking.

So far, all of the systems we have covered related to the survival of the organism. The last systems to be covered deals with survival of the species. In both the male and female reproductive system, a distinction is made between the essential organs (ovaries and testes), and the accessory organs/structures. Although I have found this terminology somewhat humorous, I suppose it has some basis in logic (especially since the advent of "test-tube babies"). The gonads or essential organs produce the gametes ("eggs" and sperm). The accessory organs/structures help the gametes get together, or promote the survival of the embryo and fetus.

In both males and females, the gametes are produced by meiosis. This form of cell division essentially copies the DNA once, but divides twice, thus reducing the chromosome number in half. Since sperm are supposed to be small, lightweight cells, meiosis in the male produces four sperm cells which are much smaller than the original stem cell. However, since the oocyte must be the supplier of all of the cytoplasm and organelles for the zygote after fertilization, both meiotic divisions are unequal for the future "egg." As such one large, viable oocyte (the proper name for the egg cell) is produced along with three small polar bodies.

The testes and sperm
In humans, the testes are both an exocrine glands which produces sperm, and an endocrine glands which secrete testosterone into blood supply. All other structures in the male reproductive system are there simply to help nourish, protect, and get the sperm started on its journey toward the oocyte. The microscopic seminiferous tubules are where the precursors to sperm cells divide and begin maturation. The stem cells or spermatogonia divide mitotically, so that one cell may stay behind to be the source of future cells. The other cell differentiates into a primary spermatocyte as it enters meiosis. Once the primary and secondary spermatocytes have completed meiosis, they become spermatids which gradually transform into "immature" spermatozoa.

A sperm will have a nucleus-containing head, on which sits a pointed hat called the acrosome which holds enzymes needed for ovum penetration. Below the head is the neck which has numerous mitochondria needed to produce energy for the attached flagellum (used to propel the sperm through fluids).

Accessory structures
The duct system of the testes eventually carries the immature sperm to a structure called the epididymis where sperm are stored. Although it looks sac-like on the exterior, the epididymis is a long tubular structure on the inside. This is another case of structure meeting function as the epididymis would be a faulty storage warehouse if the sperm were just piled into a sac. The sperm require at least several days to finish the internal changes needed to mature, and a sac-like structure would allow old (perhaps very old) sperm to mix with immature sperm. As a long, coiled tube, it follows the first in, first out principle of inventory control and ensures all sperm will have several days to mature before they reach the front of the line. Final maturation of the sperm occurs in the relatively long period it takes for the sperm to migrate and accumulate at the distal end of the epididymis.

The ductus (vas) deferens (the tube cut in a vasectomy) is the duct which peristaltically propels sperm toward the glands that will activate and equip the sperm cells for their quest. This long tube starts in the scrotum, runs up and around the pubic bone, loops up, over and around the back of the urinary bladder, and eventually merges with a duct from the seminal vesicle.

The seminal vesicles produce a fructose-containing fluid which the mitochondria of the sperm will use as an energy source to produce ATP for the flagellum. The vas deferens becomes the ejaculatory duct after merging with the seminal vesicles’ duct. The ejaculatory duct runs through the prostate gland and opens into the urethra.

The prostate gland surrounds the first or deepest portion of the urethra, and contributes a fluid which helps to protect sperm while they travel through the urethra and vagina. It is speculated that citric acid in the prostatic fluid is responsible for sperm activation. The prostate is a spongy gland which contracts to force its fluid through numerous pinhole openings in the prostatic urethra.

The small bulbourethral (Cowpers) glands secrete a small amount of an alkaline lubricating fluid. Although this "pre-ejaculatory fluid" is said to play a role in lubrication, its more important function seems to be to neutralize the acidic environment of the urethra.

The penis contains blood sinuses which are responsible for erection when filled under high arterial pressure. The corpus cavernosum and corpus spongiosum are the two areas of erectile tissue. Dilation of arteries leading in, and simultaneous closure of veins exiting the erectile tissue allow blood pressure to build.

Testosterone
All of the above accessory structures require the stimulation of the male hormone testosterone to reach adult size and viability. The endocrine cells are located in between the sperm-producing tubules of the testis and are called "interstitial cells." In addition to growth of the accessory structures, testosterone stimulates other anatomical changes associated with "maleness," such as facial hair and the stimulation of muscle growth.

Ovaries and ova
In contrast with the male's production of millions of sperm daily, the human female usually produces one oocyte (more commonly called the ovum or egg) per cycle. The ova and surrounding cells (primordial follicle) develop partially and the ova begin meiosis during fetal development. Although the follicles do develop somewhat during childhood, they remain dormant until properly stimulated by hormones released beginning with puberty.

Accessory structures
The complexity of the female system springs mainly from the post-fertilization requirements of reproduction. This would even include the mammary glands which provide nourishment for highly dependent young. Fertilization usually occurs in the fallopian (uterine) tube which sucks up the ovum as it erupts from the ovary. If fertilized, the oocyte becomes a zygote and starts dividing into an embryo.

The embryo is then carried to the uterus where it attaches ("implantation") to the inner wall. The uterus has an inner lining or endometrium which has the unusual feature of being a vascularized epithelial tissue at certain times during the female cycle.

The other accessory structures such as the vagina and the vestibular glands (for lubrication) allow for successful copulation. Both the vagina and the uterus have tremendous potential to stretch - an important feature considering the size of the fetus which must grow and pass through.

Female hormones and the female cycle
Four hormones (two ovarian, two from the pituitary) are critical to ovulation and preparation of the uterus for implantation on a regular basis. Follicles develop under the stimulation of the pituitary hormones, especially FSH (follicle stimulating hormone).

The fastest follicle to develop secretes sufficient amounts of estrogen to inhibit FSH release, but stimulates greater release of LH (a positive feedback loop). When LH levels rise enough, the follicle goes through a short, explosive growth period which results in violent eruption of the ovum from the follicle. The damaged follicle is repaired and becomes a corpus luteum under stimulation from LH (Luteinizing hormone), and secretes both estrogen and progesterone. Estrogen (and progesterone) have meanwhile stimulated the uterine wall to become a rich vascular bed of tissue for potential implantation of an embryo. Estrogen and progesterone inhibit release of the pituitary hormones which in turn normally causes the corpus luteum to degenerate, with the subsequent decline in estrogen/progesterone levels.

Since estrogen/progesterone were stimulating growth and nourishment of the uterine lining, a drop in these hormones reduces blood flow to the uterine lining, eventually causing its demise and elimination. When the highly vascularized uterine lining is lost at the end of a menstrual cycle, millions of red blood cells are also lost.

However, if fertilization/implantation occurs, a developing embryo will take over for the pituitary and secrete its own hormone (HCG) to keep the corpus luteum going. Eventually, the embryo will develop a placenta, which is not only a nutrient/waste filter between the maternal and fetal circulation, but also eventually a producer of its own progesterone and estrogen.

Pregnancy
The reason why fertilization must occur in the first third of the uterine tube is that several days of cell division are needed for the embryo to develop two different populations of cells: the Inner cell mass (the future embryo/fetus) and the outer trophoblast cells (which eventually form the placenta). The trophoblast cells are the ones capable of attaching to, and eating into the uterine lining. The growth of the trophoblast cells eventually forms a pattern of finger-like villi which will hold growing embryonic/fetal blood vessels. These placental villi immerse the fetal blood vessels in maternal blood sinuses, but keep them enclosed in a membrane such that there is no actual mixing of maternal and fetal blood.

We are going to skip a bit here, since embryonic/fetal development goes beyond the scope of this course. But a couple of important points should be mentioned. The foundation of most of the organs is laid down during the first few weeks of embryonic development. Although most of these organs are quite immature, the rest of fetal development mainly consists of growth in size of the fetus and its organs. That is why proper nutrition and avoidance of toxins and other hazardous materials is important for all mothers-to-be right from the very beginning (even the week or two before you know you are pregnant. Another thing, I have not been writing "embryo/fetus" because the terms are synonymous. There seems to be a surprising amount of disagreement among "authorities" about what constitutes and embryo or a fetus, as well as the total length of gestation. Let us say that embryonic development is the first eight weeks, and it is a fetus from 9 weeks until birth. Most reproductive authorities declare a 40-week gestation period. However, those that feel it is silly to measure gestation from the last menstrual period prefer 38 weeks. Much ado about nothing since the fetus arrives when it wants to (or rather when its placenta wants it to).

Ok, now let us skip to the late third trimester. During this time the uterus has been up-regulating oxytocin receptors. When labor is imminent, the placenta begins releasing prostaglandins into the uterine tissue. This "jump-starts" contraction, which will lead to the oxytocin positive feedback loop discussed during the endocrine system. This is the main reason why aspirin and related medications carry warnings against use by women in their third trimester -  they are prostaglandin inhibitors.

Consequences for Reproductive Success of the Human Species

One of the marvelous things about the human reproductive system is the way every detail seems centered on increasing the odds of producing a pregnancy. This is not surprising since evolution works by encouraging survival of the most (reproductively) fit (to update Darwin a little). Being strong and sterile makes you extinct as we say with a smile in general biology class.

High estrogen levels do not just thicken the uterine lining, and stimulate mild uterine contraction. The peak in estrogen right before ovulation give any sperm in the vagina, and even gives them a little boost into the uterus with the mild contractions (working like a suction bulb).

Conversely, progesterone quiets uterine contraction so as not to expel an embryo who may be trying to implant. Progesterone also seems to gradually increase mucus production in the female reproductive tract. At first, the relatively thin mucus actually acts like a bridge for sperm trying to swim upstream. It later thickens and acts like a plug so that no more sperm (or perhaps no pathogens either) can easily access the uterus.

Estrogen, progesterone, and oxytocin also have receptors in the brain, and contribute to many aspects of reproduction-related behaviors. Estrogen increases the female sex drive, and its high levels the day before ovulation would tend to improve the odds of having sperm swimming up her reproductive tract just as the oocyte enters the fallopian tube. Progesterone seems to enhance bonding with her mate - a useful move in case pregnancy is soon to follow. It has been speculated that the drop in progesterone near the end of the cycle may be a contributing factor in PMS. Interestingly, even this may be of survival value to the species - if a mate has failed to impregnate a female, that drop in progesterone may make it more likely she will attempt to dump him and find someone else who will. Finally, oxytocin, which plays a role in both male and female orgasm in addition to stimulating uterine contraction and milk letdown, contributes to the feelings of bonding between couples. It is also almost certainly behind much of the strong mother-child bonding that occurs following childbirth.

This is not to say that the hormones in the preceding paragraph call all the shots. There are certainly instincts, psychological factors, cultural influences, and learned behaviors that contribute heavily to reproduction-related behaviors. For example, all women have progesterone levels drop at the end of their cycles, but far from all become irrationally hostile toward husbands and boyfriends. Arguably, that is far more likely to be attributed to a psychological or learned behavior aspect. Women who are psychologically predisposed to, or have learned to use verbal aggression as a way of satisfying their needs, may slip into that negative behavior pattern. Conversely, those who's personality types or learned behaviors tend toward patience, caring, and reasoning their way through conflicts, may react to that same drop in progesterone with far more positive coping mechanisms. For a more positive example of hormones not being the sole determinant of reproduction-related behavior, women who adopt children (and all men) do not experience the oxytocin-related bonding with a newborn. But strong maternal and paternal instincts usually develop anyway. 

One last interesting tidbit.... there are typically many sperm that reach the oocyte at the same time. Having a second sperm enter the oocyte would be a disaster. Seeing how Down's syndrome disrupts proper development, and that is due to just one extra chromosome, imagine how bad it would be for a whole extra set of 23 chromosomes to get into the oocyte. Once one sperm contacts the oocyte membrane, there are clever blocks to polyspermy, or fertilization by more than one sperm. As soon as the first sperm reaches the oocyte membrane, the oocyte depolarizes, temporarily inhibiting any sperm from attaching to the membrane (the "fast block"). The slow block to polyspermy follows immediately as solute is dumped out of the oocyte into the space between its membrane and the zona pellucida (the "jelly coat" around the oocyte). Now that the fluid is hypertonic, water is drawn in, lifting the zona pellucida away to form a hard ring around the cell.

.... Hey, guess what? With the completion of this section, you have finished the entire (although certainly abridged) story of the human body. Thanks for reading!

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