Normal somatic human cells are diploid possessing a 2N amount of DNA in the form of 46 chromosomes arranged in 23 homologous pairs. One chromosome in each homologous pair comes from each parent. Of these chromosomes 44 are autosomal and 2 are sex chromosomes. Somatic cells reproduce by normal cell division known as mitosis, which yields daughter cells also with a 2N amount of DNA. The daughter cells produced by mitosis are genetically identical.
Gametes (oocytes and spermatozoa) are the descendants of primordial germ cells that originate in the wall of the yolk sac in the embryo and migrate to the gonadal region. Gametes are specialized haploid reproductive cells possessing 1N amount of DNA in the form of 22 autosomal chromosomes and one sex chromosome for a total of 23 chromosomes.
Mitosis and Meiosis
Primordial germ cells differentiate into gametes by a specialized two-phase cell division process known as meiosis, which produces four haploid (1N) cells from one diploid (2N) germ cell. Replication of DNA and crossover occur during meiosis I. Centromeric division (and reduction of chromosome number) occurs during meiosis II. The random distribution of chromosomes between the resulting daughter cells in this process results in the independent assortment of chromosomes, and together with crossover are mechanisms for ensuring genetic variability among offspring.
Female Gametogenesis (Oogenesis)
In females, most of gametogenesis occurs during embryonic development. Primordial germ cells migrate into the ovaries at week 4 of development and differentiate into oogonia (46,2N). Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (2N,4C). All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase of meiosis I until puberty.
During a woman’s ovarian cycle one oocyte is selected to complete meiosis I to form a secondary oocyte (1N,2C) and a first polar body. After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,1N) and a second polar body.
Male Gametogenesis (Spermatogenesis)
In males, gametogenesis begins at puberty and continues into advanced age. Primordial germ cells (46,2N) migrate into the testes at week 4 of development and remain dormant. At puberty, primordial germ cells differentiate into type A spermatogonia (46,2N). Type A spermatogonia divide by mitosis to form either more type A spermatogonia (to maintain the supply) or type B spermatogonia.
Under the influence of estrogen released during the first half of the menstrual cycle, three changes take place in the uterine tubes to facilitate its capture of the egg:
1. The uterine tubes move closer to the ovaries (physical approximation)
2. The fimbriae on the ends of the tubes beat more rapidly (increased fluid current)
3. The number of ciliated cells in the epithelium of the fimbriae increase (increase in ciliation)
Transport of Sperm in Female
Sperm are deposited in the upper vagina and must overcome several obstacles to reach an egg in the ampulla of one of the uterine tubes.
Sperm lose their ability to fertilize an egg after 3 - 3˝ days. The egg itself is viable for only about 24 hours.
Table 2 - Karyotypes of Germ Cells and Gametes
Aneuploidy is an abnormal number of chromosomes that can result from either unbalanced chromosomal translocations or nondisjunction during meiosis II. Most chromosomal abnormalities are incompatible with life, however, some combinations do result in live offspring, and trisomies involving chromosomes 13, 14, 15, 21 and 22 (groups D and G chromosomes) are relatively common birth defects. Down syndrome results from trisomy 21 that occurs in approximately 1/500 live births, and is characterized by growth retardation, mental retardation, and specific somatic abnormalities. Aneuploidy of the sex chromosomes can also occur, and certain karyotypes are associated with characteristic syndromes.
Table 3 - Syndromes Associated with Aneuploidy of the Sex Chromosomes