Gametes Health Dictionary

An adjective describing organisms, cells or nuclei that have a single set of unpaired CHROMOSOMES. Human beings have haploid gametes (see GAMETE) following MEIOSIS.... haploid

An embryonic cell with the potential to develop into a spermatozoon or ovum, which, on maturity, are called gametes. The term also describes

a gamete or any cell that is undergoing gametogenesis (the process by which gametes are formed).... germ cell

(Further information about the subject and the terms used can be found at http:// www.hfea.gov.uk/glossary)

This technique is used when normal methods of attempted CONCEPTION or ARTIFICIAL INSEMINATION with healthy SEMEN have failed. In the UK, assisted-conception procedures are governed by the Human Fertilisation & Embryology Act 1990, which set up the Human Fertilisation & Embryology Authority (HFEA).

Human Fertilisation & Embryology Act 1990 UK legislation was prompted by the report on in vitro fertilisation produced by a government-appointed committee chaired by Baroness Warnock. This followed the birth, in 1978, of the ?rst ‘test-tube’ baby.

This Act allows regulation monitoring of all treatment centres to ensure that they carry out treatment and research responsibly. It covers any fertilisation that uses donated eggs or sperm (called gametes) – for example, donor insemination or embryos (see EMBRYO) grown outside the human body (known as licensed treatment). The Act also covers research on human embryos with especial emphasis on foolproof labelling and immaculate data collection.

Human Fertilisation & EmbryologyAuthority (HFEA) Set up by the UK government following the Warnock report, the Authority’s 221 members inspect and license centres carrying out fertilisation treatments using donated eggs and sperm. It publishes a code of practice advising centres on how to conduct their activities and maintains a register of information on donors, patients and all treatments. It also reviews routinely progress and research in fertility treatment and the attempted development of human CLONING. Cloning to produce viable embryos (reproductive cloning) is forbidden, but limited licensing of the technique is allowed in specialist centres to enable them to produce cells for medical treatment (therapeutic cloning).

In vitro fertilisation (IVF) In this technique, the female partner receives drugs to enhance OVULATION. Just before the eggs are released from the ovary (see OVARIES), several ripe eggs are collected under ULTRASOUND guidance or through a LAPAROSCOPE. The eggs are incubated with the prepared sperm. About 40 hours later, once the eggs are fertilised, two eggs (three in special circumstances) are transferred into the mother’s UTERUS via the cervix (neck of the womb). Pregnancy should then proceed normally. About one in ?ve IVF pregnancies results in the birth of a child. The success rate is lower in women over 40.

Indications In women with severely damaged FALLOPIAN TUBES, IVF o?ers the only chance of pregnancy. The method is also used in couples with unexplained infertility or with male-factor infertility (where sperms are abnormal or their count low). Women who have had an early or surgically induced MENOPAUSE can become pregnant using donor eggs. A quarter of these pregnancies are multiple – that is, produce twins or more. Twins and triplets are more likely to be premature. The main danger of ovarian stimulation for IVF is hyperstimulation which can cause ovarian cysts. (See OVARIES, DISEASES OF.)... assisted conception

The process by which male and female gametes (spermatozoa and oöcytes respectively) fuse to form a zygote which develops, by a complex process of cell division and di?erentiation, into a new individual of the species. In humans, fertilisation occurs in the FALLOPIAN TUBES. Sperm deposited in the upper vagina traverse the cervix and uterus to enter the Fallopian tube. Many sperm attempt to penetrate the zona pellucida surrounding the oöcyte, but only one is able to penetrate the oöcyte proper and this prevents any other sperm from entering. Once the sperm has entered the oöcyte, the two nuclei fuse before the zygote begins to divide.... fertilisation

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

reproduction of cells by division first of the chromosomes (karyokinesis) and then of the cytoplasm (cytokinesis). Cell division to produce more body (somatic) cells is by *mitosis; cell division during the formation of gametes is by *meiosis.... cell division

In humans there are 23 pairs of CHROMOSOMES. Male and female di?er in respect of one pair. In the nucleus of female cells, the two members of the pair are identical and are called X chromosomes. In the male nucleus there is one X chromosome paired with a dissimilar, di?erently sized chromosome called the Y chromosome. In the sex cells, after MEIOSIS, all cells in the female contain a single X chromosome. In the male, half will contain an X chromosome and half a Y chromosome. If a sperm with an X chromosome fertilises an ovum (which, as stated, must have an X chromosome) the o?spring will be female; if a sperm with a Y chromosome fertilises the ovum the o?spring will be male. It is the sex chromosomes which determine the sex of an individual.

Sometimes during cell division chromosomes may be lost or duplicated, or abnormalities in the structure of individual chromosomes may occur. The surprising fact is the infrequency of such errors. About one in 200 live-born babies has an abnormality of development caused by a chromosome, and two-thirds of these involve the sex chromosomes. There is little doubt that the frequency of these abnormalities in the early embryo is much higher, but because of the serious nature of the defect, early spontaneous ABORTION occurs.

Chromosome studies on such early abortions show that half have chromosome abnormalities, with errors of autosomes being three times as common as sex chromosome anomalies. Two of the most common abnormalities in such fetuses are triploidy with 69 chromosomes and trisomy of chromosome 16. These two anomalies almost always cause spontaneous abortion. Abnormalities of chromosome structure may arise because of:

Deletion Where a segment of a chromosome is lost.

Inversion Where a segment of a chromosome becomes detached and re-attached the other way around. GENES will then appear in the wrong order and thus will not correspond with their opposite numbers on homologous chromosomes.

Duplication Where a segment of a chromosome is included twice over. One chromosome will have too little nuclear material and one too much. The individual inheriting too little may be non-viable and the one with too much may be abnormal.

Translocation Where chromosomes of different pairs exchange segments.

Errors in division of centromere Sometimes the centromere divides transversely instead of longitudinally. If the centromere is not central, one of the daughter chromosomes will arise from the two short arms of the parent chromosome and the other from the two long arms. These abnormal daughter chromosomes are called isochromosomes.

These changes have important bearings on heredity, as the e?ect of a gene depends not only upon its nature but also upon its position on the chromosome with reference to other genes. Genes do not act in isolation but against the background of other genes. Each gene normally has its own position on the chromosome, and this corresponds precisely with the positon of its allele on the homologous chromosome of the pair. Each member of a pair of chromosomes will normally carry precisely the same number of genes in exactly the same order. Characteristic clinical syndromes, due to abnormalities of chromosome structure, are less constant than those due to loss or gain of a complete chromosome. This is because the degree of deletion, inversion and duplication is inconstant. However, translocation between chromosomes 15 and 21 of the parent is associated with a familial form of mongolism (see DOWN’S (DOWN) SYNDROME) in the o?spring, and deletion of part of an X chromosome may result in TURNER’S SYNDROME.

Non-disjunction Whilst alterations in the structure of chromosomes arise as a result of deletion or translocation, alterations in the number of chromosomes usually arise as a result of non-disjunction occurring during maturation of the parental gametes (germ cells). The two chromosomes of each pair (homologous chromosomes) may fail to come together at the beginning of meiosis and continue to lie free. If one chromosome then passes to each pole of the spindle, normal gametes may result; but if both chromosomes pass to one pole and neither to the other, two kinds of abnormal gametes will be produced. One kind of gamete will contain both chromosomes of the pair, and the other gamete will contain neither. Whilst this results in serious disease when the autosomes are involved, the loss or gain of sex chromosomes seems to be well tolerated. The loss of an autosome is incompatible with life and the malformation produced by a gain of an autosome is proportional to the size of the extra chromosome carried.

Only a few instances of a gain of an autosome are known. An additional chromosome 21 (one of the smallest autosomes) results in mongolism, and trisomy of chromosome 13 and 18 is associated with severe mental, skeletal and congenital cardiac defects. Diseases resulting from a gain of a sex chromosome are not as severe. A normal ovum contains 22 autosomes and an X sex chromosome. A normal sperm contains 22 autosomes and either an X or a Y sex chromosome. Thus, as a result of nondisjunction of the X chromosome at the ?rst meiotic division during the formation of female gametes, the ovum may contain two X chromosomes or none at all, whilst in the male the sperm may contain both X and Y chromosomes (XY) or none at all. (See also CHROMOSOMES; GENES.)... sex chromosomes

n. a mature sex cell: the *ovum of the female or the *spermatozoon of the male. Gametes are haploid, containing half the normal number of chromosomes.... gamete

n. any of the cells that are in the process of developing into gametes by undergoing *gametogenesis. See also oocyte; spermatocyte.... gametocyte

n. the process by which spermatozoa and ova are formed. In both sexes the precursor cells undergo *meiosis, which halves the number of chromosomes. However, the timing of events and the size and number of gametes produced are very different in the male and female. See oogenesis; spermatogenesis.... gametogenesis

the substance postulated by 19th-century biologists (notably Weismann) to be transmitted via the gametes from one generation to the next and to give rise to the body cells.... germ plasm

n. a male or female reproductive organ that produces the gametes. See ovary; testis.... gonad

n. a change in the genetic material (*DNA) of a cell, or the change this causes in a characteristic of the individual, which is not caused by normal genetic processes. In a point (or gene) mutation there is a change in a single gene; in a chromosome mutation there is a change in the structure or number of the chromosomes. All mutations are rare events and may occur spontaneously or be caused by external agents (*mutagens). If a mutation occurs in developing sex cells (gametes) it may be inherited. Mutations in any other cells (somatic mutations) are not inherited.... mutation

rules of inheritance based on the breeding experiments of the Austrian monk Gregor Mendel (1822–84), which showed that the inheritance of characteristics is controlled by particles now known as *genes. In modern terms they are as follows. (1) Each body (somatic) cell of an individual carries two factors (genes) for every characteristic and each gamete carries only one. It is now known that the genes are arranged on chromosomes, which are present in pairs in somatic cells and separate during gamete formation by the process of *meiosis. (2) Each pair of factors segregates independently of all other pairs at meiosis, so that the gametes show all possible combinations of factors. This law applies only to genes on different chromosomes; those on the same chromosome are affected by *linkage. See also dominant; recessive.... mendel’s laws