If the secondary oocyte is fertilized, the cell continues through the meiosis II, completing meiosis, producing a second polar body and a fertilized egg containing all 46 chromosomes of a human being, half of them coming from the sperm. Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at the periphery of the tube and the spermatozoa at the lumen of the tube. Immediately under the capsule of the tubule are diploid, undifferentiated cells. These stem cells, called spermatogonia singular: spermatagonium , go through mitosis with one offspring going on to differentiate into a sperm cell, while the other gives rise to the next generation of sperm.
Meiosis begins with a cell called a primary spermatocyte. At the end of the first meiotic division, a haploid cell is produced called a secondary spermatocyte. This haploid cell must go through another meiotic cell division. The cell produced at the end of meiosis is called a spermatid. Four sperm result from each primary spermatocyte that goes through meiosis. Stem cells are deposited during gestation and are present at birth through the beginning of adolescence, but in an inactive state.
During adolescence, gonadotropic hormones from the anterior pituitary cause the activation of these cells and the production of viable sperm. This continues into old age. Learning Objectives Distinguish between spermatogenesis and oogenesis. Key Points Gametogenesis, the production of sperm spermatogenesis and eggs oogenesis , takes place through the process of meiosis. 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. 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.
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. All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase of meiosis I until puberty.
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. 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:.
The uterine tubes move closer to the ovaries physical approximation. The fimbriae on the ends of the tubes beat more rapidly increased fluid current.
Those cells secrete glycoproteins around the primary oocyte to form zone pellucid. The fluid-filled spaces between granulosa cells merge together to form a central fluid-filled space called the antrum. These are referred to as secondary follicles. Those secondary follicles develop during each monthly cycle under the influence of follicle-stimulating hormone and luteinizing hormone.
LH surge induces this stage and meiosis-I complete here. Inside the follicle are formed two haploid cells of unequal sizes. A polar body forms one of the daughter cells which receives less cytoplasm. This cell is not involved in the development of an ovum. The secondary oocyte is known as the other daughter cell. Meiosis-II occurs in the two daughter cells. The polar body replicates to form two polar bodies whereas the secondary oocyte arrests in the meiosis-II metaphase stage.
Oocyte formation occurs in ovaries. To form a follicle each oocyte is neighbored by follicle cells. As the menstrual cycle begins, primary oocytes begin to grow larger, and the number of follicle cells increases, causing the follicle to grow larger too.
Some nursing oocytes usually degenerate and leave one follicle only to mature. Fraternal twins, which are genetically distinct, can be born here. The primary oocyte begins its primary meiotic division when a follicle reaches maturity and becomes secondary oocyte.
Shortly after, in the Fallopian tube, the follicle splits and secondary oocytes are released even though the second meiotic division has not occurred.
That release from ovaries of a secondary oocyte is known as ovulation. On fertilization Meiosis-II is complete.
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