Understanding Gametogenesis - AP Biology

Spermatogenesis is a continuous process starting at puberty. Sperm production takes place in the seminiferous tubules. There, each spermatogonium cell (2n) divides equally via mitosis to produce two identical primary spermatocytes (2n). Each primary spermatocyte undergoes meiosis I to produce two equally sized secondary spermatocytes (n). Each secondary spermatocyte then undergoes meiosis II to produce a total of four spermatids (n). The spermatids differentiate before moving to the epididymis.

Oogenesis, on the other hand, begins prior to birth and is a stop-start process. While still within the embryo, each oogonium (2n) produces two identical primary oocytes (2n) via mitosis; the primary oocytes remain dormant in small ovarian follicles until puberty. After the onset of puberty, hormones periodically stimulate the follicles to complete meiosis I and produce secondary oocytes (n). It is important to note, however, that meiosis I produces only one secondary oocyte—the other "daughter cell" is actually the first polar body, which is almost always rapidly degraded (though sometimes this polar body also undergoes meiosis II and produces a third polar body). This occurs because the primary oocyte divides unevenly, leaving the secondary oocyte with virtually all of the cytoplasm during cytokinesis. Secondary oocytes are released during ovulation, but they do not complete meiosis II until fertilization, when they are penetrated by a sperm. Meiosis II results in another polar body, again due to uneven division of cytoplasm during cytokinesis.

During mitosis cells divide a single time and retain the exact same genetic material, producing two identical copies of the parent cell. During meiosis, cells divide twice and cross over during anaphase I. This produces a unique combination of chromosomes (recombinants) not seen in the parent cell.

The mature human ovum is formed through the developmental process known as oogenesis, which takes place during the first three months of fetal development. Ovarian follicles each contain one oogonium, which becomes a primary oocyte, then a secondary oocyte. No further division occurs until puberty.

Mitosis is a type of nuclear division in which one copy of each chromosome moves into each of two daughter nuclei to create identical daughter cells. Meiosis is also a type of cell division, in which a diploid cell divides twice to produce four haploid cells. Ovulation is the release of the secondary oocyte from the ovary, and fertilization is the union of a male sperm and female egg to produce a zygote.

During spermatogenesis, spermatogonia become primary spermatocytes, then seconday spermatocytes. These divide to form two spermatids, which transform into functional spermatozoa. Through the process of meiosis, the chromosome number is reduced from the diploid number (46) to the haploid number (23).

Mitosis refers to the replication of somatic cells, creating identical daughter cells from a diploid parent. Migration occurs as the spermatogonia, spermatocytes, and spermatids move from the outermost edge of the seminiferous tube to the central cavity of the tuble. Proliferation is the repeated reproduction of new parts. Gametogenesis is defined as the development of gametes.

Gametes are the haploid sex cells that are produced in the gonads (ovaries in females and testes in males). These are the primary sex organs.

A cell with 22 chromosomes and 1 sex chromosome is clearly haploid (n=23) and so it must be a sex cell and not a somatic cell (2n=46). Since both male and female gametes may contain an X chromosome, that information is not enough to tell us whether this cell comes from a male of female. However, since we are told the cell came from the ovary, we know it is the female gamete, an ovum. A muscle cell is a type of somatic cell, all of which are diploid.

Gametocytes are eukaryotic germ line cells. They can undergo mitosis to form more gametocytes or undergo meiosis to form gametids. Male gametocytes are called spermatocytes and female gametocytes are called oocytes.

Spermatogonia are diploid undifferentiated male germ cells located in the seminiferous tubules of the testes. Spermatogonia are important in the process of spermatogenesis; they turn into primary spermatocytes via growth and maturation.

Mature sperm cells that are haploid, motile, and produced during spermatogenesis are called spermatozoa. Spermatids are haploid; however, they are immature. Spermatids will turn into spermatozoa once they mature.

Spermatogenesis—the formation of spermatozoa—occurs in the seminiferous tubules of the testes. Seminiferous tubules are composed of Sertoli cells and spermatogenic cells. There is a high concentration of testosterone present in these tubules. High testosterone concentrations support sperm development. The epididymis is the site of sperm maturation. Last, the seminal vesicles are small glands that produce the majority of the seminal fluid.

Spermatozoa are stored in the epididymis prior to ejaculation. Prior to entering the epididymis, spermatozoa are immotile; however, as they travel through the epididymis the cells mature and gain motility.

Spermiogenesis is the final stage of spermatogenesis. In the process of spermiogenesis, spermatids develop into spermatozoa and form axoneme—the internal structure of the spermatozoa tails. It develops by way of microtubule growth from a single centriole that constructs the internal structure of the sperm tail. Mature spermatozoa have axonemes comprised of nine outer microtubule doublets and two central microtubules. These structures aid in motility.

Spermiogenesis is the final stage of spermatogenesis. In mature spermatozoa, the acrosome is the head of the cell and, in humans, contains digestive enzymes for ovum penetration. The acrosome develops during spermiogenesis when the Golgi apparatus surrounds the nucleus.

Mitochondria cluster around the axoneme during spermiogenesis. They supply the energy needed for sperm movement, which is achieved through movement of the flagellum.

Sertoli cells are located in the seminiferous tubules and aid in spermiogenesis. There are many ways that Sertoli cells facilitate the process of sperm maturation including the following: the contribution of testicular fluid, phagocytosis of excess cytoplasm, and the protection of sperm from the immune system. Leydig cells—interstitial cells—are responsible for secretion of testosterone.

There is a high concentration of testosterone within the seminiferous tubules. The secretion of androgen-binding protein by Sertoli cells concentrates testosterone around cells undergoing spermiogenesis, The testosterone aids in the process of sperm maturation by removing excess organelles from the developing spermatozoa.

The seminiferous tubules and developing spermatozoa within them are temperature-sensitive; they cannot tolerate high temperatures—including the temperature of the human body. In order to manage this, the testes are located outside of the human body in a sac of skin called the scrotum. This lowers the temperature of the seminiferous tubules to optimal levels.

Oogonia are diploid germ line cells that are precursor ova cells. They are produced during a process called oocytogenesis before and sometimes shortly after birth; there are believed to be a finite number of oogonia in each female at the time of birth.

Primary oocytes are diploid germ line cells that form from oogonia. Primary oocytes are immature ova. In order to develop further, primary oocytes enter into ootidogenesis, or meiosis to produce secondary oocytes. This process is arrested at prophase I late in fetal development. The period of arrest during ootidogenesis is called the dictyate stage and is characterized by a lack of cellular translation. This arrest is caused by blocked mRNA binding sites, which prevents translation initiation factors from binding. The dictyate stage ends before puberty by an increase in the secretion of luteinizing hormone (LH).