Organisms that reproduce sexually produce sex cells, or gametes, that carry a single set of chromosomes, the haploid number for the species. In contrast, body, or somatic, cells are the regular cells that form the organism’s tissues and organs. Somatic cells of sexual organisms have the diploid number of chromosomes, one set from each parent. Both cell types undergo mitosis, but only the primary reproductive cells, or gametocytes, also undergo meiosis.
In the fetuses of humans and other mammals, the vast majority of new cells are somatic. Germ cells are special. Precursors to the primary germ cells, known as gametogonia, migrate to the area in the developing fetus that will become the gonads -- male testes or female ovaries. The gametogonia first go through many rounds of mitosis and cell division. Secretions from special cells that surround the gametogonia stimulate their division into primary gametocytes. These primary reproductive cells generate female gametes, or eggs, if the organism has two X chromosomes, but XY individuals will develop sperm.
Somatic cells differentiate into various types that compose the body’s tissues and organs. In a differentiated cell, some genes are permanently “turned off.” This prevents, for example, a brain cell from turning into a liver cell. Conversely, gametocytes remain totipotent, meaning they can turn into any kind of differentiated cell. Each diploid gametocyte produces four mature haploid gametes through two cycles of meiosis. In humans, an egg’s second meiosis cycle doesn't occur until fertilization. Through the crossing-over feature of meiosis, parts of paired chromosomes are exchanged, increasing genetic variation. Mitosis lacks this feature.
Male gametocytes, or spermatocytes, differ from body cells because they can enter into meiosis and create spermatozoa. In meiosis cycle I, a diploid primary spermatocyte located in the seminiferous tubules of the testes divides into two haploid secondary spermatocytes. The two cells become four spermatids via meiosis II. The spermatids mature into spermatozoa through the final stage of spermatogenesis, known as spermiogenesis. The cells form tails and concentrate their chromosomes into acrosomal caps. The sperm undergo some final packaging and take up residence in the epididymis, a tightly coiled tube within a testicle.
The primordial female germ cell, or oogonium, differentiates from early germ cells in the first one to two months following fertilization. The nucleus of an oogonium has random granular material and a prominent nucleolus not found in body cells. The oogonia divide via mitosis into primary oocytes around the time of birth. When a female matures, her menstrual cycle stimulates a primary oocyte to undergo meiosis I and divide into a haploid secondary oocyte and a nonfertile polar body. Meiosis II begins, but then halts until fertilization, which results in an ootid and second polar body. The ootid matures into an ovum before mixing its chromosomes with those of a sperm, thereby creating a zygote. Ova are very large cells that do not resemble body cells.
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