How Babies are Made - The Basics

The Hormones

Reproduction is like an orchestra - and the reproductive organs need to be synchronised to perform at just the right time for them to work properly. It is the fertility hormones which play the conductor's role.

Hormones are chemicals the body makes to carry messages from one part of the body to another . There are two major female hormones - estrogen and progesterone - which are produced by the ovaries.

The cycle of ovarian hormone production has two phases. In the first half called the follicular phase, estrogen plays a dominant role. During this phase the egg matures inside the ovary in its follicle. The egg; the surrounding cells (which nurture the egg and are called granulosa cells and theca cells); and the fluid (called follicular fluid) which accumulates in progressively larger amounts during this phase, is called a follicle. The follicle secretes a large amount of estrogen (produced by the granulosa cells) into the bloodstream, and the estrogen circulates to the uterus where it stimulates the endometrium to thicken.

The second phase of hormone production begins at ovulation, midway through the cycle, when the follicle changes into the corpus luteum. This produces estrogen ; and also large quantities of progesterone throughout the second half of the cycle. Travelling through the bloodstream to the uterus, progesterone complements the work begun by estrogen by stimulating the endometrium to mature and making it possible for a fertilized egg to implant in it. In case pregnancy does not occur, production of estrogen and progesterone falls 10 to 14 days after ovulation as the corpus luteum dies, and the endometrium is shed from the body as the menstrual period.

How is the release of hormones regulated by the body ? This is a complex self-regulating system, which uses negative feedback control loops, much like a thermostat for an oven does. As the temperature increases, the thermostat shuts off the heater to reduce its heat output. When the temperature falls below the thermostat's setting, the thermostat signals the heater to turn up the heat again, thus maintaining the desired temperature. A similar signaling relationship exists between the pituitary gland and the ovaries in women; and the testes in men . For example, as the concentration of gonadotropins in the blood rises, this signals the woman's ovaries to increase hormonal output of estrogen. In turn, when the blood levels of estrogen rise , the pituitary gland slows its release of gonadotropins, thus maintaining the desired equilibrium.

Fig 5. A schematic of the hormonal changes during the menstrual cycle. The interplay of the pituitary and ovarian hormones regulate the changes which occur in the uterine lining.

The Reproductive System of a Man

The male reproductive system begins in the scrotum, the sack behind the penis. This contains two testicles, which make men's sex cells, called sperm; and the male sex hormone, called testosterone. The testicles feel solid, but a little spongy, like hard boiled eggs without the shell. They hang from a cord called the spermatic cord. It's normal for one testicle to hang lower than the other; and for one testicle to feel slightly larger than the other.

The testicles make sperm best at a temperature a few degrees cooler than normal body temperature. This is why nature designed a scrotum - so that the testes can hang outside the body to keep them cool.

The testicles start making sperm when a young man reaches puberty. This is in response to the male sex hormone, testosterone , which starts being produced at this time. The testes keep making sperm for the rest of the man's life.

The testes have two components, the seminiferous tubules, where sperms are produced, and the "interstitium" or the tissue in between the tubules, which contain the Leydig cells which produce the male sex hormone,  testosterone, which causes the male sexual drive.

Most of the testis is composed of the tightly coiled microscopic seminiferous tubule, which if uncoiled would reach a length of 70 cm. The sperms are produced inside the seminiferous tubule, and these converge and collect into a delta (like the mouth of a river) near the upper part of the testis called the rete testis which then empties through a series of very small ducts out of the testis towards the epididymis. The epididymis is an amazing structure - it is a very long tiny tubule ( about 5-6 meters long), which runs back and forth in convolutions and loops to form a tiny compact structure with a head , body and tail that sits like a cap on the top of and behind the testis . The tail of the epididymis then leads to the vas deferens - a thin cord like muscular tube, which is part of the spermatic cord and which ends at the ejaculatory duct in the prostate. Here is joined by the seminal vesicle ducts and they all open into the prostatic part of the urethra - which in turn leads to the urethra in the penis.

Mature sperm take about 75 days to develop in a process called spermatogenesis which takes place in the seminiferous tubules. The primordial germ cells in the testis, called the spermatogonia , which are "immortal" stem cells , divide repeatedly to form primary spermatocytes. These undergo meiotic ( reduction) division to form secondary spermatocytes, which differentiate to form spermatids , which then  ultimately mature to form spermatozoa. Sperm production takes place as though it were on an assembly line - with the more mature sperms being passed along toward the center of the tubule from where they swim towards the efferent ducts of the testis towards the epididymis. The spermatogenic cells are supported and nourished by large cells called the Sertoli cell,  which help to support sperm maturation. This can be a very "temperamental" assembly line - things often go wrong, causing low sperm counts.

When the sperm leave the testis, they are not yet able to swim on their own. They acquire the capacity to do so in their passage through the epididymis - which is like a swimming school for the sperm. They spend between 2 to 15 days here during which they attain maturity and fertilising potential. Sperm are propelled along this tunnel by frequent contractions of its thin muscular wall. Most of the mature sperm are then stored at the end of the epididymis - where they wait to be rushed through the vas deferens and ejaculated at the time of orgasm.