Reproduction

Reproduction in humans and others mammals is the process by which sperm and egg cells are produced, brought together, join and develop into a new individual. Reproduction is the key to the continued existence of our species.

Mammals reproduce by uniting sperm and egg internally in the female body by a process called internal fertilization.

Many mammals reproduce only during particular seasons of the year; not so with humans. Men produce sperm more or less continuously and women ovulate (produce an egg) about once a month.

The male reproductive tract consists of the paired gonads (organs that produce sex cells), the testes, where sperm are produced, plus accessory structures that store the sperm, produce secretions to activate them and finally conduct them to the inside of the female reproductive tract.

A diagram of the human male reproductive tract is provided for you.

The testes produce both sperm and male sex hormones. The testes are located in the scrotum, a sac or pouch that hangs outside the main body. This external location allows the testicles to have a temperature about 4°C cooler than the core body temperature which seems to be optimal for sperm development.

Seminiferous tubules, the coiled, hollow tubes in which sperm are produced nearly fill each of the testis. In spaces in between the tubules are the Sertoli cells, which synthesize testosterone.

Just inside the wall of each seminiferous tubule lie the spermatogonia and Sertoli cells. Both are diploid cells.

When spermatogonia divide they can either undergo mitosis and produce more spermatogonia or they can undergo meiosis and produce spermatocytes which differentiate into sperm cells. Sperm cells, remember, are haploid cells.

Sertoli cells regulate the process of spermatogenesis and nourish the developing sperm cells.

A human sperm cell is unlike any other cell in the body. Most of its cytoplasm is lost. It consists largely of a nucleus, mitochondria and a flagellum. At the tip of the sperm cell is a specialized lysosome called an acrosome. The acrosome contains specialized enzymes necessary for penetration of the egg cell.

Sperm production does not begin until puberty. At that time, the hypothalamus releases a hormone, gonadotropin-releasing hormone, that stimulates the anterior pituitary gland to produce luteinizing hormone and follicle-stimulating hormone.

Spermatogenesis begins as a result of the interplay of luteinizing hormone, follicle-stimulating hormone and testosterone (the latter secreted by the testes).

Luteinizing hormone stimulates the interstitial cells of the testes to produce testosterone. The combination of testosterone and follicle-stimulating hormone stimulates the Sertoli cells and spermatogonia, causing spermatogenesis.

Let me go over this sequence again, this time inserting feedback control notions. Gonadotropin-releasing hormone from the hypothalamus stimulates the anterior pituitary to release luteinizing hormone and follicle-stimulating hormone. Luteinizing hormone stimulates the interstitial cells of the testes to produce testosterone. Follicle-stimulating hormone stimulates the Sertoli cells and the spermatogonia to undergo spermatogenesis. Testosterone and chemicals produced during spermatogenesis inhibit further release of luteinizing hormone and follicle-stimulating hormone, forming a negative feedback loop that keeps the rate of spermatogenesis and the level of testosterone in the blood nearly constant.

Testosterone also stimulates the development of secondary sexual characteristics such as the growth of facial hair in males. It maintains sexual drive and is absolutely required for successful sexual intercourse or copulation by the male.

Interestingly, sperm are not involved in these functions. If one could suppress the production of follicle-stimulating hormone (which would block spermatogenesis) BUT allow the release of luteinizing hormone (which would allow the production of testosterone), a man would be infertile but not impotent. Birth control where the male is the controlled individual would be the result.

Sperm is not semen. The seminiferous tubules merge near the top of the testicle to form an structure called the epididymus. This becomes the vas deferens which leaves the scrotum and enters the abdominal cavity. Most of the hundreds of millions of sperm produced each day are stored in the vas deferens and epididymus.

The vas deferens joins the urethra which moves to the tip of the penis.

The fluid ejaculated from the penis is semen, a mixture of sperm cells, and secretions from the seminal vesicles, the prostate gland and the bulbourethral glands. These secretions activate the sperm into swimming structures and neutralize the acidic fluids of the vagina that normally inhibit bacterial growth.

The female reproductive tract is almost entirely within the abdominal cavity and consists of paired gonads, the ovaries and accessory structures that accept sperm, conduct the sperm to the egg and nourish the developing embryo.

A diagram of the human female reproductive tract is provided for you.

The human female produces precursor egg cells or oogonia while still a fetus within mother. These oogonia divide by mitosis to become primary oocytes. No oogonia remain after about the third month of fetal development and no new ones form during the rest of the female's life.

Still during the fetal development, all oocytes begin meiosis but the process stalls during meiosis I. At birth, a female has about 2 million primary oocytes. Many die each day until at puberty only about 400,000 remain. Only a few oocytes resume meiosis each month during a woman's reproductive years. There is no shortage of oocytes.

Surrounding each oocyte is a layer of much smaller cells that both nourish the developing oocyte and secrete female sex hormones. Together the oocyte and these accessory cells are referred to as a follicle.

Approximately once a month during a woman's reproductive years, she undergoes a menstrual cycle. During this cycle, pituitary hormones stimulate the development of a dozen or more follicles. Usually only one matures.

During this maturation, the primary oocyte completes its first meiotic division to become a secondary oocyte. At the same time, the accessory cells of the follicle multiple and begin to secrete estrogen.

All of this leads to a size increase in the follicle and it eventually erupts through the surface of the ovary and releases the secondary oocyte. Some of the accessory cells are also released with the secondary oocyte but most remain associated with the ovary to form the corpus luteum, a structure that now secretes not only estrogen but also progesterone. If fertilization occurs, the corpus luteum breaks down a few days later.

Each ovary is adjacent to but not touching an oviduct or fallopian tube. The open end of the oviduct is fringed with cilia called fimbriae that nearly surround the ovary and beat such that any released egg is swept into the oviduct.

The second meiotic division occurs in the oviduct and the secondary oocyte becomes an ovum. Fertilization, where sperm meets egg usually occurs within the oviduct. A fertilized egg is called a zygote.

The zygote moves down the oviduct and into the uterus, a pear-shaped organ.

The wall of the uterus has two layers: an inner lining, the endometrium, is richly supply with blood vessels and will lead to the formation of the placenta, a structure to deliver oxygen and nutrients to the developing fetus and move wastes and carbon dioxide from it.

A second layer, an outer myometrium, is a muscular region which will contract at the proper time to expel the fetus to the outside world.

Developing follicles secrete estrogen which stimulates the endometrium to grow an extensive network of blood vessels. After the release of the egg cell from the ovary, estrogen and progesterone produced by the corpus luteum promote continued growth of the endometrium's network of blood vessels.

Thus, if an egg is fertilized, it finds a rich blood bed in which to embed itself and derive sustenance for 9 months. If the egg is NOT fertilized, the corpus luteum disintegrates and estrogen and progesterone levels fall. The overgrown endometrium disintegrates as well. The uterus contracts and sloughs off the excess endometrial tissue. The resulting flow of tissue and blood is called menstruation.

The outer end of the uterus is nearly closed off by a ring of connective tissue called the cervix. The cervix holds the developing baby in the uterus, expanding only during labor just prior to child birth. Beyond the cervix is the vagina which opens to the outside of the body. The vagina is both the receptacle for the penis during intercourse and the birth canal.

Hormonal events for females are complex. Let's go over it again with a scrollable diagram and some text.

Sperm are quite fragile. Internal fertilization or copulation allows the deposition of sperm to occur in the moist environment of the female reproductive tract. To accomplish this, the penis is inserted into the vagina and sperm are released during ejaculation. The sperm swim upward from the vagina, through the opening in the cervix into the uterus and on into the oviducts. If the female has ovulated in the past day or so, and a sperm finds the egg, fertilization can occur. Only one sperm can fertilize an egg.

Copulation for the male begins with erection of the penis. Prior to erection, the penis is flaccid because arterioles supplying it are restricted and allow rather little blood flow into the appendage.

Through both psychological and physiological stimulation, the arterioles dilate and blood flows into spaces within the penis. As these spaces fill, they squeeze off the veins that drain the penis. Pressure builds up, causing an erection.

After the penis is inserted into the vagina, movements stimulate touch receptors on the penis and trigger ejaculation. Ejaculation occurs when muscles encircling the vas deferens, epididymus and urethra contract and force semen out of the urethra. On average, 3 or 4 ml of semen containing perhaps 300 to 400 million sperm are ejaculated. Male orgasm causes both ejaculation and a feeling of intense pleasure and release.

Similar changes occur in the female. Sexual excitement causes increased blood flow to the vagina and external parts of the reproductive tract including the labia and clitoris. The clitoris is derived from the same embryological tissue as the penis and becomes erect. Stimulation by the male penis often, but not always, results in female orgasm, a series of rhythmic contractions of the uterus and vagina accompanied by feelings of pleasure and release. Female orgasm is NOT necessary for fertilization to occur.

Neither sperm nor egg are able to live very long. An egg lives for perhaps a day; sperm for perhaps two days. In the oviduct, the egg is surrounded by residual follicle cells, the so-called corona radiata. These cells form a barrier to sperm. A second barrier, a jelly-like region called the zona pellucida is also present. The human egg releases attractants for sperm.

In the oviduct, hundreds of sperm reach the egg. Actually, it is necessary for a lot of sperm to reach the egg because they release enzymes that allow penetration of a single sperm to effect fertilization. If insufficient sperm are present, not enough enzyme is released and fertilization does not occur. Human males with less than 20 million sperm per ml of semen (about 1/5 the normal amount) usually cannot fertilize a woman because too few sperm reach the egg.

If fertilized, the zygote begins dividing during its passage down to oviduct to the uterus. This passage takes perhaps four days. About a week after fertilization, the zygote has developed into a ball of cells referred to as a blastocyst. A thickened inner region of this blastocyst will become the embryo proper; the outer region sticks to the uterus wall and "burrows" into the endometrium-- a process called implantation. A placenta then develops which nourishes the developing embryo for about 9 months.

How does one limit fertility? The most effective for sexually active individuals is sterilization. In males this involves slicing into the scrotum, cutting both vas deferens and tying off all ends. Sperm are still produced and ejaculation still occurs but sperm cannot reach the penis and enter the female reproductive tract.

In females, one must slice into the abdomen and cut the oviducts, tying off both ends of each duct. Ovulation still occurs but eggs are unable to reach the uterus and sperm cannot reach the eggs.

Although sometimes reversible, sterilization should be thought of as permanent.

Other means of birth control:

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