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Gametogenesis is the process of formation of gametes by primary sex organs (Testis in males and Ovary in females). Gametes are sperm and ovum in males and females, respectively. So, testis produce sperms in males and ovary produce ovum in females.
- Gametogenesis is of two types: Spermatogenesis and Oogenesis.
- Spermatogenesis is the process of formation of sperms by the immature male germ cells (spermatogonia) in testis at the onset of puberty.
- Spermatogenesis takes about 65–75 days in humans to complete. It begins with spermatogonia, which contain diploid (2n) number of chromosomes.
- Spermatogonia are type of stem cells. Some spermatogonia remain undifferentiated to serve as reservoir for future purpose.
- Other spermatogonia present on the inside wall of seminiferous tubules multiply by mitotis, and undergo developmental changes, and differentiate into Primary spermatocytes. Like spermatogonia, primary spermatocytes are diploid (2n) and have 46 chromosomes.
- Now, each primary spermatocyte replicates its DNA and then meiosis begins. Primary spermatocyte
completes meiosis I (first meiotic division known as reduction division) which forms two equal, haploid cells called Secondary spermatocytes (n), which have only 23 chromosomes each.
- Meiosis I in one primary spermatocyte (2n) produces two secondary spermatocytes (n).
- After this, secondary spermatocytes undergo meiosis II (second meiotic division) to produce four equal, haploid spermatids (n), i.e. spermatids have 23 chromosomes each.
- Meiosis II in two secondary spermatocyte (n) produces 4 spermatids (n). Or we can say one secondary spermatocyte produces 2 spermatids.
- So, single primary spermatocyte produces four spermatids via two rounds of cell division (meiosis I and meiosis II).
- Then spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. Each haploid spermatid develops into a single Sperm. During spermiogenesis, spherical spermatids transform into elongated, slender sperm.
- After spermiogenesis, sperm heads become embedded in the sertoli cells. Release of sperms from sertoli cells and from the seminiferous tubules occurs by the process called spermiation. At this point, sperm are not yet able to swim.