The wonder of life
The female reproductive systemWomen are born with their reproductive material; around 1–2 million oocytes are found in the ovaries. Until their menarche (first menstrual cycle), a woman has about 30,0000–40,0000 oocytes.
During each menstrual cycle, a woman loses several oocytes; some are infertile, yet about 400 of them are mature, and many of them can be fertilized. A woman can be fertile from the beginning of puberty until menopause, which comes at 45–55 years of age.
Every month, at sometime in the middle of the menstrual cycle, one or more follicles are stimulated in the ovaries. At least one oocyte usually matures; this is called ovulation. The oocytes move through the 10-cm-long oviducts towards the uterus, where they might be fertilized.
A mature oocyte lives about 12–24 hours. If it meets a healthy spermatozoon on its way to the uterus, it gets fertilized. The cell mass is divided several times on its way in the uterus and rests on its walls.
Every month, the endometrium is rebuilt and, if no fertilized cells are found, it is rejected with menstruation blood. As a result, 150 ml of blood are rejected in each menstrual bleeding in a woman’s life. It is quite possible to have menstrual bleeding without ovulation. This explains why women with low or liminal ovarian reserves may have menstruation. Menstrual bleeding does not mean that a woman is still fertile but that the hormonal framework, which supports the endometrial, still functions.
Ultraschallbild einer aufgebauten Gebärmutterschleimhaut in der Zyklusmitte. Die Höhe der Gebärmutterschleimhaut beträgt 8,1 mm und ist dreischichtig.
The female menstrual cycle
Different hormones are produced in each phase of the cycle. Natural provision dictates that a woman cannot get pregnant on just any day of the cycle—only during the fertile days. The hormones FSH are produced in order to prepare the endometrium for pregnancy, whereas progesterone is produced so the pregnancy survives the second part of the cycle.
The most fertile years of a woman’s life are in her 20s. During these years, there is a 20–25% chance of pregnancy per ovulation. Years of reduced fertility begin at 32 years old, and at the end of her 30s and the beginning of her 40s, it becomes increasingly harder to become pregnant by natural means. Past 35 years of age, natural conception is harder not only because of the poorer quality of her oocytes, but also due to higher risks. Among these risks, one can name certain anomalies (i.e., Down syndrome) or others connected with miscarriages.
The male reproductive system
Men, on the other hand, produce sperm constantly and until late age, thanks to spermatogenesis (from the Greek words sperma [semen] and genesis [birth]).
Primary spermatocytes (which are not yet fertile) double during puberty. Two spermatocytes are produced by a primary spermatocyte and, in a second meiotic phase, two spermatocytes produce four spermatids. In the last stage, ready sperm result from spermiogenesis (spermatids that can move further). The procedure of spermiogenesis, which produces sperm, lasts about 64–70 days. A man produces up to 100 million spermatozoa daily, and a single ejaculation contains seminal fluid and several millions spermatozoa (100–600 million). Two to six ml are released at a speed of 17 km/h. The percentage of spermatozoa in ejaculation is about 3–5%.
Darstellung der Spermiogenese
Ten to 20 million spermatozoa per millimeter, half of them active, are required for fertilization. Spermatozoa have a lifetime of several hours, whereas in the uterus they can survive up to seven days. The 0.06 μm long spermatozoa travel at a speed of 3–4 mm per second towards the oocytes, with the exclusive goal of fertilizing them. Only the fastest and healthiest will manage their way to the oocyte and fertilize it. Therefore, spermatozoa must follow the “odour”. Ovulation makes the mucus permeable, through which the oocytes transmit attractants that indicate the right oviduct to the spermatozoa.
As soon as the spermatozoon breaks through the zona pellucida of the oocyte, the latter changes its chemical composition. This ensures that no other spermatozoon will make it to the oocyte.
Der Moment der Befruchtung: Ein Spermium dringt in die Eizelle ein
Both these cell nuclei (oocyte and spermatozoon) now fuse, contributing half of the DNA set. Thus, a new complete set of chromosomes is formed. The father defines the sex of the child. The female set of chromosomes consists of X chromosomes. If the father contributes with a Y chromosome during the fusion a boy is formed, whereas if he contributes an X chromosome a girl is formed. All genetic information is fixed after fertilization, sex included.
The new cell mass is divided in the oviduct after 30 minutes and doubles with each division. On the fifth day, the cell is a blastocyst and reaches the uterus, where it “anchors”. From here, embryonic development begins.
An oocyte gets fertilized and wanders in the uterus.
Although parents do not feel this wonder yet, a new life has just begun.