Reproduction

Meiosis

Reproduction in higher animals, including humans, depends on the fusion of a spermatozoon from the male parent and an ovum from the female. These reproductive cells are also called gametes. Each gamete must receive half as many chromosomes as somatic cells so that when they unite in fertilization the full complement of chromosomes is obtained. As the sex cells mature, two processes of cell division occur: the first is mitosis, in which each daughter cell receives a complete set of chromosomes. Following this a two-stage cell division occurs which is peculiar to reproductive tissue and is called meiosis (Fig. 25.1). The first division is similar to mitosis and gives rise to two daughter cells, each containing the original number of chromosomes; the second division follows the first very quickly and results in four gametes, each containing half the number of chromosomes. In humans normal body cells have 46 chromosomes (in 23 pairs) while each gamete has only 23 single chromosomes. Following fusion of the gametes the resulting cell, a zygote, has 46 chromosomes (in 23 pairs). Cell division of a zygote is by mitosis and the resulting multicelled organism is called an embryo.

Fig. 25.1 Comparison of meiosis and mitosis.

The chromosomes consist of a chain of genetic material (DNA), and it is the genes which pass on the characteristics of the parent cell, so that daughter cells are always similar to parent cells. In this way, some characteristics of children are derived from parents, including characteristics such as hair colour, height and degree of intelligence. In any pair of genes, one will exert a stronger influence than the other. The stronger gene is termed dominant and the weaker one recessive. Hereditary characteristics depend upon the dominance of the genes.

Sex determination

One pair of chromosomes from the father and one pair from the mother are the sex chromosomes which will determine the sex of the child. In the female the sex chromosomes are the same and are called XX. In the male they are different and are called XY. One chromosome from each pair will determine the sex of the child. If the child has an X chromosome from the mother and an X chromosome from the father, it will be a girl (XX). If the child has an X chromosome from the mother and a Y chromosome from the father, it will be a boy (XY) (Fig. 25.2).

Fig. 25.2 Sex determination.

The central dogma of genetics

DNA consists of two strands held together in a double helix by hydrogen bonds (Fig. 25.3). The primary structure of the bases in DNA is the basis of the genetic code, which gives rise to all of our characteristics. The code consists of triplets (known as codons), each of which leads to the incorporation of an amino acid into a protein molecule.

Fig. 25.3 The quaternary structure of DNA is maintained by hydrogen bonds between the two strands.

DNA can replicate and conserve the genetic code during cell division (see Chapter 2) and can also be transcribed into ribonucleic acid (RNA) (Fig. 25.4). RNA also contains the genetic code and is used as a template for the building of unique protein molecules. This process is, obviously, under very close regulation in living cells to ensure that the right protein molecules are made as required. Disturbance of this process and the unregulated production of particular cells are called ‘cancer’.

Fig. 25.4 The central dogma of genetics. Replication and transcription take place in the nucleus. Translation takes place in the cytoplasm.

The male genital organs

The male genital organs (Fig. 25.5) consist of six components:

Fig. 25.5 The male reproductive organs.

The testes (Fig. 25.6) are the reproductive glands in the male. They are suspended in the scrotum by the spermatic cords but they develop high up in the abdomen close to the kidneys and gradually descend through the inguinal canal into the scrotum shortly before birth. Occasionally one, or both, glands fails to descend and remains in the abdomen or in the inguinal canal; surgery may then be required to relocate it (or them).

Fig. 25.6 The structure of the testis.

As the testis descends, it brings down with it a pouch of peritoneum called the tunica vaginalis. This forms a serous covering for the testis but the pouch should otherwise be obliterated. If it is not obliterated, it forms a possible site for hernia. This causes an inguinal hernia, a loop of bowel or some other organ falling into the pouch, which forms the hernia sac. Each testis consists of 200–300 lobules, each containing up to three tiny convoluted tubules called the convoluted seminiferous tubules. The epithelial lining of their walls contains cells that develop into spermatozoa by a process of cell division. The tubules are supported by loose connective tissue, which contains groups of interstitial cells; these cells secrete the male hormone testosterone.

The epididymis is a fine, tightly coiled tube packed into the form of a long narrow body, which is attached to the back of the testis. The seminiferous tubules of the testis open into it and it leads into the deferent duct.

The deferent duct is a continuation of the duct of the epididymis. It passes through the inguinal canal and runs between the base of the bladder and the rectum to the base of the prostate gland where it is joined by the duct of the seminal vesicle.

The seminal vesicles (Fig. 25.7) are two pouches lying between the base of the bladder and the rectum. They secrete an alkaline fluid containing nourishment, which forms a large part of the seminal fluid.

Fig. 25.7 Relations of the seminal vesicles.

The ejaculatory ducts are formed by the union of the ducts of the seminal vesicles and the deferent ducts. They commence at the base of the prostate and end at the opening of the prostatic utricle in the urethra.

The penis is a tubular organ plentifully supplied with large venous sinuses that can fill with blood causing erection of the organ. It contains the urethra, which is common to both the urinary and reproductive systems in the male. At the tip of the penis is an enlargement called the glans penis, in the centre of which lies the urinary meatus. The glans is normally covered by a loose double fold of skin called the prepuce or foreskin. It should be possible to draw the foreskin back over the glans penis but sometimes the opening in it is too small. This is known as phimosis and is treated either by stretching the foreskin or by circumcision, i.e. cutting it away, in more severe cases.

The prostate surrounds the commencement of the urethra in the male. It is about the size of a chestnut and contains the urethra and the ejaculatory ducts. It consists partly of glandular tissue and partly of involuntary muscle and produces a secretion called semen, which is alkaline and provides nourishment for the sperm.

The bulbourethral glands are situated on either side of the membranous portion of the urethra. The ducts open into the spongy portion of the urethra and the glands secrete a substance that forms part of the seminal fluid.

The seminal fluid is composed of substances secreted by the testes, the seminal vesicles and the prostate; it contains the spermatozoa.

The spermatozoa are minute cells, each with a tail-like projection joined to the cell by a constricted portion called the neck (Fig. 25.8). The tail has a lashing movement that enables the cell to move after the semen leaves the male reproductive tract. As a result, when the spermatozoa are deposited in the vagina they can make their way up the uterus and uterine tubes in search of the ova. They are produced in enormous numbers and it is estimated that, on average, 300 000 000 are deposited in the vagina at one time, though only one is necessary to fertilize the ovum.

Fig. 25.8 (a) A mature ovum. (b) A spermatozoon, drawn to the same scale as (a).