Physiology of human reproduction

The female genital system
It is comprised of 2 ovaries (a), 2 Fallopian tubes (b), the uterus (c) and the vagina (d).

The female genital system.

Ovarian reserve

The ovaries contain, very early during the development of the human fetus, all the ovules (or oocytes or eggs) necessary for the entire duration of the woman's reproductive life. The number of oocytes (reserve of follicles) is established as of the 5th month of gestation. Henceforth, it will only decline, rapidly until birth, then more slowly, becoming quasi-null at menopause.

The menstrual cycle

The menstrual cycle

At the onset of puberty, the menstrual cycles begin. This cycle generally lasts 28 to 30 days, with day 1 being the first day of menstrual bleeding. A succession of modifications, orchestrated by cyclically produced specialized molecules - the sex hormones - will change the ovaries and uterus. These hormones assure the communication among all the organs of the genital tract and the hypophysis, a gland located at the base of the brain.

The 3 successive phases of the menstrual cycle are described below.

1 The follicular phase (or the phase of follicular maturation)

Every month, among the high number of oocytes on stock in the ovary, about 50 will become sensitive to secreted hormones. Generally, all but one of these will degenerate (undergo atresia). The oocyte that reaches maturity is contained in a small sac (the follicle), bathed in a liquid (the follicular fluid). This sac gradually expands, pushing against the outer wall of the ovary, in response to follicle-stimulating hormone (FSH), which is produced by the hypophysis. The sac will reach about 2 cm in diameter just before ovulation, when the rupture of the ovarian wall releases the egg. The oocyte is a large cell that measures about 1/10th of a millimeter in diameter. Unlike all the other cells in woman's body, the oocyte has only half the chromosomes (that is 23) that contain all the genetic information that the woman will transmit to her descendents. The genetic or chromosomal content of the embryo will only be constituted after the addition of the 23 chromosomes harbored in the fertilizing spermatozoid. During the follicular phase, the ovaries also secrete hormones: estrogens. These hormones prepare the uterus to receive the embryo, should one be conceived.

2 2 The ovulation phase

Ovulation occurs around 14 days before the onset of menstrual bleeding, which means that a woman with a 28-day cycle will ovulate around the 14th day of her cycle. However, if she has a longer cycle, ovulation will occur later. Ovulation is provoked by another signal sent by the hypophysis, the luteinizing hormone (LH) peak. During ovulation, the oocyte is extruded from the follicle, whose wall breaks, and is aspirated into the Fallopian tube. The oocyte now starts its migration towards the uterus.

3 The luteal phase (or preparation of the uterus for the potential implantation of an embryo)

During this phase, the wall of the uterus (called the endometrium) is prepared for implantation of the embryo resulting from the fertilization of the oocyte by a spermatozoon. This process is dependent upon hormones released by the ruptured follicle from which the oocyte was expulsed: the follicle is transformed into a corpus luteum (which means yellow body) and starts to secrete a new hormone, progesterone.

The male genital system

The male genital system

It consists of the penis (1), the testicles (2), the epididymis (3), the deferent canals (4), the seminal vesicles (5) and the prostate. These tissues interact among themselves and with the hypophysis, in a manner similar to that seen in women.

Semen (the ejaculate) is constituted of seminal fluid secreted by several glands - the prostate and seminal vesicles - and spermatozoa, mobile cells produced in the testicles. Normally, 1 milliliter of semen contains 20 to 100 million spermatozoa, among which at least 40% are mobile.

For spermatozoa to be present in sperm requires not only that spermatozoids be produced in the testicles, but also that these cells can traverse the successive excretory pathways - epididymis, deferent canals and ureter.

Spermatozoa contain the genetic information that the man will transmit to his descendents. Just like oocytes, they contain only half the number of chromosomes (23) as other cells in the body that contain 46 chromosomes. In addition, each spermatozoon contains the chromosome that alone will determine the sex of the child: either the X chromosome for girls or the Y chromosome for boys.

Diagram of a spermatozoon

A spermatozoon is comprised of 2 parts: the head (1) and the tail or flagellum (2). This elongated cell is much smaller than an oocyte (0.05 mm long). The head is covered with a hood called the acrosome (3), which contains active molecules (enzymes), whose role is to pierce a tunnel through the layers surrounding the oocyte so that the spermatozoon can deliver its chromosomal contribution and thereby fertilize the egg. The flagellum's whip-like lashes assure the mobility of the spermatozoon in its progression towards the oocyte.

The stages of natural fertilization

The stages of natural fertilization

During sexual intercourse, spermatozoa are deposited at the far end of the vagina (1). They rapidly reach the cervical mucus (2), a medium in which spermatozoa can live for up to 3 days. The spermatozoa start their journey into and across the uterus (3), and then enter the Fallopian tubes (4). One-third of the way into the Fallopian tube, the spermatozoa meet the ovule (5) that had been expulsed from the ovary (6).

Stages of embryo development and implantation

If a spermatozoon and the oocyte meet (1), fertilization can occur. Only one spermatozoon can penetrate the zona pelucida, the protective wall surrounding the egg (2), and reach the oocyte membrane. The fertilized egg, now called a zygote, evolves rapidly while continuing on its way down the Fallopian tube to the uterus (4), (5) and (6). Sixteen to 18 hours after the encounter of the spermatozoon and the egg, the 2 nuclei (the organelles within each cell that contain the genetic heritage or chromosomes) are clearly visible, side-by-side, in the single cell. The fusion of these nuclei (3) leads to the formation of a new nucleus containing all of the embryo's genetic information (46 chromosomes).

Thirty hours after fertilization, the zygote starts its cellular divisions. These are repeated every 12 to 16 hours. This fertilized egg divides into smaller-and-smaller daughter cells, called blastomeres, until the morula stage (a spherical mass of blastomeres) is reached. Thus, the volume remains constant within the zona pellucida surrounding the ovule.

Once the stage of about a dozen blastomeres has been reached, these cells undergo a phenomenon of compaction. This term defines the passage from morphologically round cells to those resembling flat building blocks (squamous epithelium), which increases the areas of contact among them.

Towards the 5th day post-fertilization, another decisive transformation occurs. The cells comprising the morula begin to move into the ball of cells (invaginate) and form an internal cavity, called a blastocele, in which fluid accumulates. The embryo has reached the blastocyst stage: it is formed by a monolayer of cells around an internal cavity, which is the starting point for the development of the fetal appendages, including the placenta, and by a mass of cells which will give rise to the fetus.

Blastocyst on day 5.	Expanded blastocyst.

From day 5, the blastocele will continue to dilate progressively until the zona pellucida becomes thin and ruptures (hatched blastocyst). At this stage, the embryo is made up of about 100 cells.

The blastocyst (7) arrives in the uterus, where a favorable environment has been created under the action of the hormones released from the corpus luteum; it will penetrate into the uterine mucosa (endometrium), called implantation, where it will develop during the 9 months of gestation.

Thus, multiple conditions must be met for pregnancy to occur: the female and male genital organs must function at optimal levels so that ovulation and fertilization can proceed normally and an embryo can implant successfully in the uterus.

During the course of a natural cycle, a normally fertile couple has about a 25% chance of initiating a pregnancy. However, a wide variety of events can intervene and prevent conception, in which case, we can talk about infertility or low fertility.