Editor’s note: Physicians have a special place among the thinkers who have elaborated the argument for intelligent design. Perhaps that’s because, more than evolutionary biologists, they are familiar with the challenges of maintaining a functioning complex system, the human body. With that in mind, Evolution News is delighted to offer this series, “The Designed Body.” For the complete series, see here. Dr. Glicksman practices palliative medicine for a hospice organization.
In previous articles I’ve explained that the human embryo is destined to become female by default unless several chemicals swing into action to make it become a normal male. But that’s only part of the story, because for the first several years of life, humans, whether male or female, cannot reproduce. Puberty involves an as yet unexplained reduction in the feedback inhibition of the hypothalamus and pituitary so they can increase their output of their respective hormones. This leads to the testes being able to produce sperm and more testosterone and the ovaries being able to develop and release an egg and produce more estrogen and also the pregnancy hormone, progesterone.
Once the sexual organs have matured so the male can produce sperm and the female can release an egg into the fallopian tube, all that is needed for new human life to come about is for them to join together to form a zygote. The natural way that human reproduction occurs is by the male and female physically coming together in sexual intercourse. This very intimate physical union requires the man to deposit semen containing sperm near the cervical opening of the woman’s uterus. Over the next several hours, aided by the cervical mucus, the sperm use their flagella to swim through the body of the uterus toward the fallopian tubes. If one of the woman’s ovaries has released an egg around that time then one of the sperm may be able to penetrate its outer shell to form a zygote in a process called fertilization. Over the next several hours the zygote develops into an embryo which over the next several days moves into the body of the uterus and implants in its endometrial lining. Once implantation takes place the embryo continues to develop and grow into the fetus in a process called gestation. It then exits the mother’s body about nine months later as a newborn baby.
In my last article we looked at the two tasks the male must perform to reproduce and what can go wrong to prevent it. The male must produce enough healthy active sperm which is dependent on having, not only properly working testes, but the right amount of hormones and properly working receptors and he must have enough pelvic blood flow and nervous function to penetrate deep into the vagina and ejaculate his semen. Now let’s look at what it takes for a woman to reproduce and what can go wrong to prevent it.
From the above it is evident that the female’s fertility is mainly dependent on three tasks: developing and releasing an egg from the ovary, getting the egg to enter the fallopian tube while assisting the sperm to reach it for fertilization, and providing nutritional support for the developing new human life once it implants in the endometrial lining of the uterus.
All women have their full complement of immature eggs (ova) in their ovaries at birth. These are contained in sacs, with surrounding support tissue that are called follicles. The first task of the female, developing and releasing an egg from the ovary, is dependent on producing enough of the gonadotropins, Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH), enough estrogen and having enough properly functioning specific receptors.
At the beginning of a woman’s menstrual cycle the blood level of estrogen is low. This tells the hypothalamus to send out more Gonadotropin Releasing Hormone (GnRH) and the pituitary more FSH and LH. In particular, by attaching to specific receptors the rise in FSH stimulates some of the follicles in the ovaries to mature and make estrogen as well. At this point the ovary is said to be in the follicular phase. Throughout this phase the cells in the maturing follicles form more receptors for both FSH and estrogen which results in a positive feedback that makes the follicles even more sensitive to FSH and estrogen. This increased sensitivity enables them to produce more estrogen and causes the eggs to mature further. The dominant follicle that will ultimately release an egg (ovum) is the one that has been able to produce the most FSH and estrogen receptors and therefore has received the most hormonal stimulation.
However, during the follicular phase as the estrogen level rises higher and each of the developing follicles vies for the right to release their egg, something very surprising takes place in the pituitary. Remember that prior to puberty the release of FSH and LH is normally inhibited by a rising estrogen level. This in fact is how the body is able to keep control of its estrogen level. However, as the ovary nears the end of the follicular phase, when the level of estrogen is rising higher, this actually stimulates the pituitary to suddenly release more LH (and to a lesser degree FSH as well) in what is called the LH surge. This actually represents a positive feedback which as yet is still poorly understood. Clinical experience teaches that the LH surge is absolutely necessary for the dominant follicle to release the egg from the ovary so it can migrate toward the fallopian tube and have a chance of meeting up with a sperm in a process called ovulation.
There are several conditions that can result in anovulation, where the ovaries are not able to send out an egg toward the fallopian tube. One category involves inborn errors which results in the ovaries not developing and maturing properly. The vast majority however are acquired disorders which usually are intermittent and with medical intervention can be resolved. Consistent monthly ovulation is dependent on the delicate and complex interplay of the hypothalamus, the pituitary and the ovaries. Disruption of this hormonal balance by chronic emotional stress, malnutrition, significant fluctuations in weight, serious or recurrent illness and excessive physical exercise are some of the commoner reasons for anovulation.
Another not uncommon condition is polycystic ovary syndrome (PCOS). PCOS involves inappropriate negative feedback of the sex hormones on the pituitary gland. This causes a relatively low level of FSH which limits the cyclical development of the follicles in the ovaries so that a dominant one is not able to be released in ovulation. Also, there are numerous different glandular disorders, such as ones that affect the thyroid, the adrenals or the pituitary, which can lead to anovulation as well. Finally, it must be remembered that each woman begins her life with a full complement of egg follicles in her ovaries. Since, after puberty, during each month several follicles mature and vie for ovulation, this means there are that many less follicles available for ovulation and estrogen production in the future. A woman’s fertility therefore eventually runs out and with it she no longer ovulates or has menstrual periods and has very low levels of estrogen. This is called menopause and usually takes place after thirty to forty years of menstruating.
If the female has had sexual intercourse around the time she has ovulated then her second task of getting the egg to enter the fallopian tube while assisting the sperm to reach it for fertilization comes into play. By attaching to specific receptors, the high levels of estrogen prior to ovulation makes the cells in the cervical opening of the uterus secrete lots of watery mucus. This watery mucus assists the sperm as they swim up through the body of the uterus to the fallopian tubes. At the same time the high levels of estrogen also causes the fallopian tubes to increase the movement of their cilia (small hair-like projections) and muscle contraction in an effort to try to coax the egg to enter. Once inside the fallopian tube the egg is swept along toward the body of the uterus by ciliary action and muscle contraction. It is here, within the relatively confined space of the fallopian tube, that the sperm usually meet up with the egg and fertilization takes place. The resulting zygote is then also swept along the fallopian tube into the body of the uterus on its way to implantation.
Some of the commoner causes of female infertility involve disorders of the fallopian tubes. Sexually transmitted diseases, like gonorrhea and chlamydia, cause pelvic infections which results in damage to the fallopian tubes and abnormal function. This causes them to either not be able to capture the egg, let the egg and sperm meet, or let the zygote pass through to the body of the uterus. Another not uncommon cause of fallopian tube malfunction, resulting in female infertility is a condition called endometriosis. This disorder involves the growth of tissue from the lining of the uterus (endometrium) in abnormal places, such as around the fallopian tubes and the ovaries. The presence of this abnormally placed endometrial tissue causes obstruction and damage of the fallopian tubes resulting in malfunction.
Recall, the second task of the female involves not only the fallopian tubes but also the cervical opening of the uterus where the sperm enter on their way to trying to fertilize the egg. Sexually transmitted diseases, like gonorrhea and chlamydia, can also cause inflammation and scarring of the cervix. This can lead to narrowing of the cervical canal and abnormal mucus production both of which can prevent the sperm from moving up into the uterus. In addition, certain hormone problems can cause the cervix to not produce the right amount or kind of mucus to adequately help the sperm move into the uterus.
If a sperm is able to fertilize an egg in the fallopian tube and the resulting zygote is able to move into the body of the uterus, then the third task of providing nutritional support for the developing new human life once it implants in the endometrial lining of the uterus becomes necessary. The increasing amounts of estrogen the ovary releases prior to ovulation attaches to specific receptors in the endometrial lining of the uterus and signals it to proliferate. This causes the endometrial lining to grow and develop resulting in it secreting large amounts of clear mucus which aids the sperm in their struggle to reach the fallopian tubes.
After ovulation the remaining cells of the dominant follicle become the corpus luteum (yellow body) and begin to form more LH receptors on their plasma membranes. The predominance of LH receptors on these cells results in the production of mostly progesterone, and to a lesser extent, estrogen as well, from continued FSH stimulation. Progesterone attaches to specific receptors on the endometrial lining and signals them to proliferate further and to secrete thicker and more nutrient-rich mucus in preparation for the implantation of the embryo. The corpus luteum normally has a lifespan of only about 10 to 14 days at which time a precipitous drop in the production of estrogen and progesterone takes place. This sudden drop in the levels of sex hormones results in the endometrial lining no longer being supported and it degenerates and dies. The endometrial tissue is then shed, with blood, out of the uterus and into the vagina and from there out of the woman’s body in a menstrual period.
However, if pregnancy does take place the embryo produces a hormone called human Chorionic Gonadotropin (hCG) which acts like LH and is able to keep the corpus luteum alive and functioning until the placenta forms and takes over. From here on gestation takes place whereby the embryo develops into a fetus and continues to grow and develop within the uterus until it comes out into the world as newborn baby several months later.
For a healthy pregnancy to continue the embryo must implant in the lush endometrial lining of the uterus. The presence of uterine defects, such as abnormal shape, a dividing wall (septum), benign muscle tumors (fibroids), and abnormal mucosal growths (polyps) can interfere with either implantation or continued gestation resulting in infertility. If the corpus luteum does not secrete enough progesterone for an adequate amount of time the endometrial lining will not be prepared to properly nurture the embryo. Finally, one other rare cause of luteal phase insufficiency is the complete absence of progesterone receptors on the gland cells of the endometrium. Without these receptors the progesterone secreted by the corpus luteum cannot stimulate the endometrium, it cannot grow and develop properly and the uterine lining will be unable to perform the third task of female fertility.
In summary, human reproduction involves not only having the right tissues and organs in place, but also having them working together in a well-coordinated fashion. The female cannot be fertile unless at least one of her ovaries can release an egg, her fallopian tube can capture it and move it towards the sperm that have been assisted by the cervical mucus to swim toward it, and then provide a supportive haven for the implantation and gestation of new human life. These all require not only having the right tissues and organs in place, but also having the right amount of hormones and receptors that respond in the right way and at the right time. Any one permanent abnormality that leads to any one chronic malfunction is likely to make human reproduction impossible.
Of course, it goes without saying that all of the parts working together in a coordinated fashion, as directed by specific hormones and their receptors, to enable either the male or female to reproduce demonstrates not only irreducible complexity but natural survival capacity as well. However, the word sex comes from the Latin secare which means to separate or divide. This means that for every life form that reproduces sexually not only must all of its organ systems that allow for metabolic control but also its male and female components must have developed simultaneously. As for human life, whether it came about by the more plausible explanation of intelligent design or whether one believes the Darwinian narrative, it all had to start with just one male and just one female.
Image: Detail of Portrait of an Unknown Lady, by William Segar [Public domain], via Wikimedia Commons.