HOW YOU BECOME A MALE – INTRODUCTION | Medsarticles. Medical Weblog

HOW YOU BECOME A MALE – INTRODUCTION

Deep in the dark, moist, warm innermost recesses of the vagina, three hundred million spermatozoa (sperms) are ejaculated from the pulsing male penis at orgasm. One of these three hundred million will, with luck, succeed in making the 12-centimetre journey through the uterus of the woman and will penetrate the ’shell’ of a single egg which lies waiting in her oviduct. The egg, or ovum, which has been expelled from the woman’s ovary at ovulation, has been gently propelled into her oviduct by the finger-like fronds which surround its internal opening.

The journey of the sperms is only possible within a period of two days on each side of ovulation, for it is only during this short time that the sperms can penetrate the mucus which fills the canal which leads into the uterus. Only at this time does the mucus alter from an impenetrable mesh to long strands through which twisting, turning channels form. Through these helical tunnels several million sperms pass, of the three hundred million ejaculated, propelled by the thrashing of their long threadlike tails. But only those sperms whose heads are of the right size can get through; sperms with abnormally big heads are trapped.

Of the several million which reach the uterus, only a few thousands will survive the journey through its cavity, and even fewer will survive the journey along the oviduct. One, and one alone, will penetrate the shell of the ovum. As its head fixes deeply into the substance of the egg cell, it loses its tail, and the free head fuses with the nucleus in the ovum. A new individual, male or female, has been formed.

Each of the sperms ejaculated into the vagina carries in its head in twisted material, like a bank of computer tapes, all the genetic information needed to make the new individual unique. The twisted material is separated into strands called chromosomes. This genetic inheritance from father and mother, and their father and mother, and their father and mother, combines in an almost infinite variety of ways, suppressing some inherited characteristics, exaggerating others, so that family resemblances appear in the new individual, but not so much that the individual is identical with its parents or ancestors.

The core genetic material in the head of each sperm fuses with the core genetic material in the ovum to make a new mix – a new individual who will be formed as the fertilized egg cell divides repeatedly in complex ways. Once the individual has been created, each cell in his or her body has the genetic material within it capable of forming another new individual, but through aeons of evolution this facility has been suppressed, and only two specialized cells are capable of this function. These are the female and the male cells, the ova and the spermatozoa. Each human body cell, except the sex cells, carries 46 chromosomes in its nucleus. Forty-four of these determine the individual’s appearance, 2 determine its sex. The sex cells contain only 23 chromosomes, half the human number, so that when the sperm head (containing 23 chromosomes) fuses with the ovum (also containing 23 chromosomes), the normal human number of 46 is restored. Each ovum carries one sex chromosome, called an X chromosome because of its shape. Each spermatozoon also carries one sex chromosome, but about half of the spermatozoa carry an X chromosome, and the other half carry a smaller chromosome which resembles a Y.

If an X-carrying spermatozoon fertilizes the ovum, the resulting new individual becomes a female. If a Y-carrying spermatozoon fertilizes the ovum the resulting individual is a male.

Since only spermatozoa carry either an X or a Y chromosome the sex of the unborn child – at this stage only the size of a pinpoint – is determined by the child’s father. So if a man sires many daughters but no son, he cannot blame his wife. The sex of each of his children is his responsibility!

With the fusion of the nucleus of the sperm head and that of the ovum, a new life has begun. Quite rapidly the single cell accumulates energy and divides, and then divides again, so that 2, then 4, then 8, then 16, then 32, then 64 cells are formed. Each of these cells contains in its nucleus a chromosome count of 46 of which two are sex chromosomes, one X and one Y. In genetic shorthand this is written as 46XY or 46XX.

Occasionally, for incompletely understood reasons, something goes wrong to upset this seemingly simple system and extra X or Y chromosomes are added, or are taken away, or otherwise distorted. Such cell lines may continue and the individual, when born, may be sexually abnormal.

The dividing cells form a sphere, which looks rather like a mulberry within 3 days, and a day later the sphere has entered the cavity of the uterus and has become attached to its lining.

Growth occurs rapidly, and the sphere changes shape. One part of it, where several layers of cells collect, forms the embryo and, later, the foetus, while another part forms the placenta.

Three weeks after fertilization, the embryo, which now looks reptilian, has developed a gut cavity. Along its back surface, two ridges appear, one each side of the midline. These ridges will form the sex glands, or gonads. Into these ridges sex cells migrate from a nearby area, and rapidly divide and divide again.

At this stage of development it is impossible to tell the sex of the embryo by looking at the sex glands, but the cells of the sex glands have been programmed by the sex chromosome they have inherited. If the cells contain a Y sex chromosome, the gonads develop into testes and, provided the embryonic testes function properly, the remainder of the sexual anatomy will develop as a male. This is because the Y chromosomes in each of the cells which make up the embryonic testes induce it to manufacture quantities of the male sex hormone, testosterone, and a much smaller quantity of the female sex hormone, oestrogen. If the sperm which fertilized the egg carried an X chromosome, not a Y chromosome, the gonads will become ovaries, which produce quantities of oestrogen and a much smaller quantity of testosterone. In other words, the testes and the ovaries produce both male and female sex hormones, but in different quantities. The embryonic testes also produce another substance called the female duct-inhibiting substance which is important in determining the sex of the embryo.

There is, of course, more to the sexual apparatus of a male than testes. From each testis, on each side of the body, a twisted, hollow tube runs to join a similar tube from the other testis, at what will eventually become the prostate gland, and opens into a pit at the rear end of the embryo called the cloaca. By now, about 7 weeks after conception, the embryo also has a set of female ducts -oviducts, uterus, and upper vagina. Under the influence of testosterone, the male ducts grow and the female duct-inhibiting substance-causes the female ducts to wither away. If it so happens, as occurs rarely, that the embryo has a testis on one side and an ovary on the other, the side with the testis will produce a male duct and that with the ovary will produce a female duct. The child will be born a hermaphrodite.

Normally, testosterone and the female duct-inhibiting substance secreted by the testes make the male ducts grow and the female ducts wither. The embryo is well on the way to becoming a male.

He has to go through another stage of development before he does. The male ducts, at this period of development, terminate in the cloaca where the gut also ends. Just in front of the cloacal pit (that is on the embryo’s front side) a small lump appears, and two swellings grow backwards to make a raised edge to the pit. Looking from the outside, it is impossible to tell if the embryo is a female with a big clitoris, or a male with a small penis and a split behind it.

Quite soon the sex of the embryo becomes clear. If the embryo is a male, the cloacal cells absorb the testosterone which is circulating in the blood and convert it into a new product called dihydrotestos-terone. This, in turn, converts the tissues of the cloaca into male genitals. By the 14th week after conception, the lump at the front has become a tiny penis, and the folds at each side of the pit have joined together to form a scrotum. At this stage of development it is empty. Much later in pregnancy the testes are drawn down into the empty scrotum from their previous position in the foetus’s abdomen, and at birth the baby is obviously a boy.

After birth only small amounts of the sex hormones are produced by the gonads – testes in males and ovaries in females – until puberty occurs.

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