Life is Sex (Part II)

How does a male/female mature?

The last article explained how you became male or female. It showed that without six specific molecules the human embryo either becomes female by default or an infertile male. In other words, without any one of these six chemicals, reproduction is impossible and so is the existence of humanity. But we know that although humans are male or female at birth, they aren’t able to reproduce yet. So it would seem this potential is built-in to be turned on when the time is right.

Most children begin to show signs of their sexual development to come by the end of the first decade. Over the following years they undergo sexual and bodily development in a process called puberty. Puberty is a constellation of physiological changes that enables humans to reproduce and also prepares them for their natural role in the family. Let’s see how it works.

The hypothalamus in your brain, and the pituitary located nearby, work together to control many hormones in your body. The hypothalamus sends out Growth Hormone-Releasing Hormone which stimulates the pituitary to release Growth Hormone (GH). As its name implies, GH is important in the overall growth and development of the body. The hypothalamus also sends out Thyrotropin-Releasing Hormone (TRH) which tells the pituitary to send out Thyroid Stimulating Hormone (TSH). It’s TSH that controls the production of thyroid hormone from the thyroid gland in your neck which mainly affects the body’s metabolic rate. And there are many other hormones these two glands send out as well.

It’s important to understand that the way the hypothalamus and pituitary work together to control the production of a given hormone is by feedback inhibition. Think of feedback inhibition like what happens when you pump gas. There’s a sensor in the hose you place in the opening to your gas tank that detects when it’s full which makes it automatically shut off the pump. In the same way, the hypothalamus and the pituitary have sensors that allow them to detect the blood level of the hormones they ultimately control.

Take for example thyroid hormone. If it rises above what is needed the hypothalamus reduces its output of TRH and the pituitary lowers its output of TSH. And when the thyroid hormone level drops below what’s needed the hypothalamus increases its output of TRH and the pituitary its output of TSH. The feedback of the thyroid hormone level on the hypothalamus and pituitary if too high inhibits their release of TRH and TSH respectively and if it’s too low reverses this inhibition.  

The production of the sex hormones is regulated in the same way. The hypothalamus sends out Gonadotropin-Releasing Hormone (GnRH). GnRH attaches to specific receptors on cells in the pituitary and tells them to send out the gonadotropins. These are Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH). It’s FSH and LH that attach to specific receptors on the cells in the testes or ovaries to produce the sex hormones, testosterone or estrogen, respectively. 

In the first decade of life the hypothalamus and the pituitary are very sensitive to the feedback inhibition of the sex hormones. This means that prior to puberty, very low levels of testosterone and estrogen are able to prevent the hypothalamus from releasing more GnRH and the pituitary more FSH and LH. This results in the blood levels of testosterone and estrogen being very low.

A few years before puberty the adrenals increase their output of androgenic (male) hormones. This causes a small growth spurt and the development of pubic and axillary hair. What actually triggers the beginning of puberty is poorly understood. However, what is known to happen is that the hypothalamus and pituitary start to become progressively less sensitive to sex hormones. The gradually diminishing feedback inhibition of the sex hormones on the hypothalamus and pituitary results in a gradual increase in their output of GnRH and FSH & LH. By the time puberty is in full swing the levels of these sex-hormones have increased significantly.

During male puberty more FSH and LH attaches to specific receptors in the testes. This causes an increase in the production of testosterone and the ability to produce sperm. Puberty in the male also results in an increase and coarsening of facial, chest, axillary, abdominal, extremity, and pubic hair, along with enlargement of the vocal cords and deepening of the voice. With the associated increase in the pituitary output of Growth Hormone, the male also experiences a linear growth spurt and the development of his musculoskeletal system. Along with the capacity for sperm production, puberty brings on enlargement of the penis, scrotum, and testes. Finally, testosterone not only plays a major role in sexual differentiation, development, and maturation, but also in the desire for sexual relations. In addition, testosterone is important in giving the male the ability to maintain an erection for adequate penetration into the vagina and ejaculation during sexual intercourse. All these developments prepare the boy to become a man and later a father.

During puberty in the female, more FSH and LH attaches to specific receptors in the ovaries. This causes an increase in the production of estrogen and the ability to develop an egg. Puberty in the female results in an increase in mainly pubic and axillary hair that is not as coarse as in the male. Breast development takes place so the potential mother will be able to provide breast milk for her infant. Like in the male, an increase in Growth Hormone results in a linear growth spurt and development of her musculoskeletal system. Puberty also brings on enlargement of the external genitalia and increased mucous production within the vagina and uterus. Along with the capacity for egg development, the increase in FSH, LH, and estrogen allows for ovulation, where the egg is released and can enter one of the fallopian tubes. 

Inside the fallopian tube the egg can meet up and join with the sperm to form new human life. This happens after the sperm has been deposited into the vagina by the male during sexual intercourse. After ovulation, the ovaries mainly secrete the pregnancy hormone, progesterone.  The estrogen before ovulation, and the progesterone after ovulation, attach to specific receptors in the lining of the uterus to make it thicken up and produce more mucous helping it to prepare for pregnancy. If pregnancy doesn’t happen, the levels of estrogen and progesterone drop precipitously. Without this hormonal support the lining of the uterus sheds in a process called menstruation. The first menstrual period generally marks the beginning of female fertility. It usually takes place on a monthly basis for the next thirty or forty years. All of this development prepares the girl to become a woman and later a mother. 

So, you can see that it takes a lot of sensors, hormones and receptors and undeveloped tissues to get puberty and sexual maturation done. But without it the human race would not exist.


Three Questions for Mr. Darwin

    1. Where did my body get the instruction manual on how and when to turn on puberty?

    2. Since many factors are needed how could sexual maturation have evolved gradually?

    3. How do glands know how much hormone they need to send out to do the job right?

Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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