Sex Determination

The reproductive systems of males and females appear similar for the first 40 days (in humans) after conception. During this time, the cells that give rise to ova or sperm migrate from the yolk sac to the embroyonic gonands, which have the potential to become either ovaries in females or testes (singular, testis) in males.

For this reason, the embryonic gonads are said to be " indifferent." If the embryo is a male, it will have a Y chromosome, this gene and the protein it encods are absent, and the gonads become ovaries. Evidence suggests that the sex-determining gene may be one known as SRY ( sex-determining region of the Y chromosome), which appears to have been highly conserved during the evolution of different vertebrate groups.

Structure and Function of the Male Reproductive System

If testes are formed in the human embryo, they develop seminiferous tubules beginning at around 43-50 days after conception. The seminiferous tubules are the sites of sperm production. At about 9-10 weeks, the Leydig cells, located between the seminiferous tubles, begin to secrete testosterone ( the major male sex hormone, or androgen).

Testosterone secretion during embryonic development converts indifferent structures into the male external genitalia, the penis and the scrotum, a sac that contains the testes. In the absence of testosterone, these structures develop into the female external genitalia. Testosterone also helps to promote the production of sperm.

However, the secretion of testosterone by the testes declines to very low levels by about 21 weeks of gestation and remains low until the onset of puberty. Therefor, although sperm production begins during embryonic life, it is arrested until puberty. At that time, the increased secretion of testosterone by the testes renews sperm production and promotes the development of male secondary sexual characteristics, which include the growth of facial hair, lowering of the voice, and larger muscle mass.

Another important role of testosterone is to support the development of the accessory sex organs in the male.

Note: Accessory sex organs are reproductive structures other than the gonads that assist in the maturation and transport of the gametes.

In an adult, each testis is composed primarily of the highly convoluted seminiferous tubules. Although the testes are actually formed within the abdominal cavity, shortly before birth they are pulled through an opening, the inguinal canal, into the scrotum, which suspends them outside the abdominal cavity. In this position, the testes are maintained at around 34 degrees centigrade, slightly lower than the core body temperature (37 degrees C.) This lower temperature is required for normal sperm development in humans.

Production of Sperm

Sperm are haployed ( with 23 chromosomes each, in humans) and are produced by the division of diploid parent cells (with 46 chromosomes) located at the periphery of the seminiferous tubules. Each parent cell duplicates by mitosis, and one of the two daughter cells then undergoes meiosis to form sperm; the other remains as a parent cell. In that way, the male never runs out of parent cells to produce sperm. Adult males produce an anerage of 100-200 million sperm each day, and can continue to do so throughout most of the rest of their lives.

The diploid daughter cell that begins meiosis is called a primary spermatocyte. It has two chromatids per chromosome. After the first meiotic division, two haploid secondary spermatocytes are produced that have only one chromatid per chromosome; however, the homologous pairs of chromatids have separated. Each of these cells then undergoes the second meiotic division to produce two haploid cells, the spermatids. Therefore, a total of four hapliod spermatids are produced by each primary spermatocyte. All of these cells constitute the germinal epithelium of the seminiferous tubules, because they "germinate" the gamates.

In addition to the germinal epithelium, the walls of the seminiferous tubules contain nongerminal cells known as Sertoli cells. The Sertoli cells nurse the developing sperm and secret products required for spermatogenesis (sperm production). They also help convert the spermatids into spermatozoa by engulfing their extra cytoplasm.

Spermatozoa are relatively simple cells, consisting of a head, body, and tail. The head encloses a compact nucleus and is capped by a vesicle called an acrosome, which is derived from the Golgi complex. The acrosome contains enzymes that aid in the penetrattion of the protective layers surrounding the egg. The body and tail provide a propulsive mechanism: within the tail is a flagellum, while inside the body are a centriole, which acts as a basal body for the flagellum, and mitochondria, which gnerate the energy needed for flagella movement.

Male Accessory Sex Organs

After the sperm are produced within the seminiferous tubules, they are delivered into a long, coiled tube called the epididymis. The sperm are not motile when they arrive in the epididmis, and they must remain there for at least 18 hours before their motility develops. From the epididymis, the sperm enter another long tube, the vas deferens, which passes through into the abdominal cavity via the inguinal canal.

The vas defernce from each testis joins with one of the ducts from a pair of glands called the seminal vesicles, which produce a fructose-rich fluid. After this point, the vas deferens continues as the ejaculatoryduct and enters the prostate gland at the base of the urinary bladder.

In humans, the prostate gland is about the size of a golf ball and is spongy in texture. It contributes about 60% of the bulk of the semen, the fluid that contains the products of the testes, seminal vesicles, and prostate gland. Within the prostate gland, the ejaculatory duct merges with the urethra from the urinary blader. The urethra carries the semen out of the body through the tip of the penis. A pair of pea-sized bulbourethral glands secrete a fluid that lines the urethra and lubricates the tip of the penis prior to coitus (sexual intercourse).

In addition to the urethra, there are two columns of erectile tissue, the corpora cavernosa, along the dorsal side of the penis and one column, the corpus spongiosum, along the ventral side. Penile erection is produced by neurons in the parasympathetic division of the autonomic nervous system. As a result of the release of nitric oxide by these neurons, arterioles in the penis dilate, causing the erectile tissue to become engorged with blood and turgid.

This increased pressure in the erectile tissue compresses the veins, so the blood flows into the penis but cannot flow out. Some mammals, such as the walrus, have a bone in the penis that contributs to its stiffness during erection, but humans do not. The result of erection and continued sexual stimulation is ejaculation, the ejection from the penis of approximately 5 milliliters of semen containing an average of 300 million sperm.

Successful fertilization requires such a high sperm count for fertility because the odds againts any one of the sperm cell successfully completing the jurney to the egg and fertilizing it are extraordinarily high and the acrosomes of several sperm need to interact with the egg before a single sperm can penetrate the egg.

Males with fewer than 20 million sperm per milliliter are generally considered to be sterile. Despite their large numbers, sperm comprise only about 1% of the volume of the semen ejculated; the remainder of the semen is made up of the fluids secreted by the seminal vesicles and prostate gland.

Hormonal Control of Male Reproduction

We know that the anterior pituitary gland secrets two gonadotropic hormones, FSH and LH. Although these hormones are named for their actions in the female, they are also involved in regulating male reproductive function. In males, FSH stimulates theSertoli cells to faciliate sperm development, and LH stimulates the Leydig cells to secrete testosterone.

The principle of feedback inhibition applies to the control of FSH and LH secretion. The hypothalamic hormone, gonadotropin releasing hormone (GnRH), stimulates the anterior pituitary gland to secrete both FSH and LH. FSH causes the Sertoli cells to release a peptide hormone called inhibin that specifically inhibits FSH secretion.

Similarly, LH stimulates testosterone secretion, and testosterone feeds back to inhibit the release of LH, both directly at the anterior pituitary gland and indirectly by reducing GnRH release.

The importance of negative fedback inhibition can be demonstrated by removing the testes, the major source of both inhibin and testosterone: in their ansence, the secretion of FSH and LH is greatly increased.