The Germ Cells
In human males, the Leydig cells of the testes produce mainly testosterone. The tubular cells of the testes produce the spermatozoa which pass through the epididymis, into another, the vas deferens to a storage vessel at its top end, the ampulla. These organs develop in primitive form in the twelve week-old embryo, and go on to completion as puberty approaches. Secretions from the seminal vesicles, and also from Cowper's glands and the prostate, transport the sperm during sexual intercourse. Meiosis occurs continually from puberty onwards, producing producing around a thousand million spermatozoa a week. They can survive up to forty days in the epididymis but, if unused, they die and are reabsorbed.
In human females, the ovaries produce estrogens and other steroids. The ovaries and ova develop in primitive form as diploid cells in the twelve week-old embryo, but meiosis does not occur. In other words, a woman's lifetime supply of ova is present from birth, each being surrounded by a cluster of cells called a Graafian follicle.
While in males, spermatogenesis produces gametes from all four haploid cells, in females only one goes on to form an egg, while the remainder form polar bodies.
At puberty, the menstrual cycle begins, and at about the fourteenth day of each cycle one, or more, ova mature and are released by the follicle. At this point, the first cell division of meiosis occurs. One of the two cells develops onward, while the other becomes the first polar body. They then move down the fallopian tubes and the oviducts, at the top of the uterus. They survive for about 36 hours, if is not fertilised, and then die.
Fertilisation generally takes place in the oviducts. It occasionally occurs in a fallopian tube, leading to an ectopic pregnancy, with serious consequences. If the egg is fertilised, the second division occurs. In addition, the second polar body is formed, and the first polar body may also divide. The empty follicle becomes a corpus luteum and begins to produce progesterone.
If there are difficulties with this process, fertility drugs are prescribed. However, they often have the effect of multiple births. Humans produce twins quite frequently. Recent research suggests that they occur quite commonly although only one survives until birth. Twins produced from two eggs are referred to as fraternal twins, or dizygous, to reflect the fact that they are from two eggs and therefore have a different genotype.
However, soon after fertilisation, fission of the egg, now a zygote, may occur, resulting in identical or monozygous twins.
Bland, J., (2003) About Gender: The Germ Cells
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08.04.98 Last amended 12.11.01, 26.12.03, 01.05.14