Gametogenesis

Gametogenesis is the production of haploid sex cells (in humans, ovum and spermatozoa) that each carry one-half the genetic compliment of the parents from the germ cell line of each parent.

The production of ovum is termed oogenesis and the production of spermatozoa is called spermatogenesis. Both oogenesis and spermatogenesis provide a mechanism through which genetic information may be passed to offspring. The fusion of spermatozoa and ova during fertilization results in a zygote with a fully restored diploid genome.

The production of male and female gametes is a highly complex and coordinated sequence of a mitotic division, two meiotic divisions, cytoplasmic apportionment (divisions) and cellular differentiation. Any chronic alteration in the sequence of morphological and biochemical transformations required to produce gametes usually results in sterility for the affected parent.

Spermatogenesis provides the haploid gametes necessary to pass on paternal genetic information. Oogenesis provides the haploid gamete necessary to pass on maternal genetic information and extranuclear genetic information (e.g., mitochondrial DNA).

In eukaryotic organisms the gametes are derived from primordial germ cells, which enter the gonads during early development. During embryogenesis, the primordial germ cells are determined early in development by the presence of a cytoplasmic component termed germ plasm. Once germ cells are determined they follow a different maturation and, of course, genetic function, than do the remaining somatic cells of the body. Primordial germ cells are the stem cells that, via mitosis, supply both spermatogonia and oogonia.

In humans, spermatogenesis starts with a diploid (2N) spermatogonium that carries the full genetic compliment of 46 chromosomes (22 autosomal pairs, one X and one Y sex chromosomes). The spermatogonium represents the germ cell line from which all sperm cells are derived. Sequentially, the process of spermatogenesis via mitosis produces a primary spermatocyte that is also diploid (2N) and then via meiosis, two secondary spermatocytes that are haploid (N). The haploid secondary spermatocytes carry 22 autosomes and either an X or a Y sex chromosome. The secondary spermatocytes each undergo a second meiotic division to form a total of four haploid spermatids. Subsequently, nurtured by surrounding somatic cells, through the process of cellular differentiation the four spermatids produce 4 sperm cells capable of motility and fertilization. Although there is variation between sperm cells as to the exact nature of their genetic information (i.e., what alleles they carry or which chromosome trace back to a maternal or parental line) in sharp contrast to female gamete production all the terminal male gametes (the sperm cells) have roughly the same cytoplasmic volume and contents and the same amount of genetic material.

In human females the germ cell line is represented by the diploid (2N) oogonium that carries the full female genetic compliment of 22 autosomal pairs and two X chromosomes. Mitotic division yields a diploid primary oocyte. Meiotic divisions then produce one female ga mete—the ovum. In humans, the first meiotic division is suspended in the diplonema stage during embryonic development. Meiosis resumes, one ovum at a time following puberty and during the ovulatory period of the menstrual cycle. Maturation proceeds with the production of haploid (N) secondary oocytes with 22 autosomal chromosomes and an X sex chromosome (the sex chromosome must be an X chromosome because normal human females carry two X chromosomes and no Y chromosomes). Also formed is a haploid polar body that is nearly devoid of cytoplasmic contents. This is a fundamental difference between male and female Gametogenesis. In males, there is a nearly equal division of cytoplasm to the gametes, in females the cytoplasmic contents are preserved for the eventual "egg" or ovum. Extraneous genetic material is removed via polar bodies. Another meiotic division results in the production of an ootid and yet another polar body (the eventual number of polar vies associated with an ovum may equal as many as three if the first sloughed off polar body undergoes a subsequent division). Cellular differentiation of the ootid yields an ovum ready for fertilization. In many cases, however, the last maturational processes are accelerated because in human females, meiosis II is usually completed after fertilization.

During ovum maturation, there is a tremendous increase in ribosomal related component so that the cellular machinery is present to handle the tremendous amount of translation and protein synthesis required in the rapid cellular divisions that follow the formation of the zygote.

Germ cell line manipulation (e.g., gene "knockouts) is a powerful potential tool to manipulate an organism's genome. Each generation of sexually-reproducing organisms is dependent upon the continuation of the germ cell line The germ line is also the vehicle of genetic transmission and alteration of the genome via mutations and recombination (i.e., evolution).