Gamete

A gamete is a specialized reproductive cell. The cells usually have one half as many chromosomes in their nuclei as the majority of body cells, which are known as somatic cells. All sexually-reproducing plants, animals, and microbes produce gametes sometime during their life span.

During the second and third quarters of the nineteenth century the scientists J. L. Prevost, J. B. Dumas, T. Schwann, and R. Virchow were especially influential in the evolving consensus that sperm were cells, and that these cells united with other cells, ova or egg cells, to form a fertilized cell (a zygote) that went on to form a new organism.

Gametes usually form in the gonads, organs which form the sex cells. In flowering plants, the gonads are found in the flowers. The male gonads are the anthers, seen as the enlarged tips of the stamens. The anthers produce pollen (male gametes) in flowering plants. The female gametes are formed in the base of the flower, in the ovules, located in the ovary of the pistil.

In vertebrates such as fish, amphibians, reptiles, birds, and mammals the male gonads are the testis, where very large numbers of gametes (spermatozoa) are formed. The female gonads (ovaries) of these animals produce low numbers of gametes known as eggs, or ova. Usually the number of mature egg cells produced in females is far fewer (a handful) than the number of sperm cells formed in the males (counted in the millions).

In most organisms the gametes are produced by a special double cell division process, a reduction division known as meiosis, in which new cells (gametes) end up with half as many chromosomes as the original cell. The fusion of the egg and sperm at fertilization restores the normal chromosome number. In the case of bees, the fertile female queen bee is fertilized by male gametes from a male bee called a drone. But a drone develops from an unfertilized egg, which is a single gamete (ovum)! Therefore the sperm of the drone must be produced by ordinary cell division (mitosis) instead of the meiotic cell division which usually is involved in the formation of fertile gametes.

Currently there is much interest in manipulating the gametes of domestic animals in breeding programs to promote characteristics yielding economic advantages in agriculture. Selected female cattle are given hormones to cause multiple ovulation (release of ova) which are then artificially fertilized with male gametes (bull semen) in the uterus. After a few days the tiny embryos are flushed out of the waters and stored as frozen embryos for later insertion and gestation in surrogate mothers. A similar technique has been developed for use in humans.

Another new biotechnology that is being tried in a few human cases is intracytoplasmic sperm injection (ICSI). This technique involves a skilled technician with a micro pipette and microscope capturing a single sperm and injecting it directly into the cytoplasm of a female gamete, the ovum. When successful, the resulting embryo must then be implanted into a uterus prepared to receive and nourish the new offspring.