Spermatogenesis

Stages of Spermatogenesis - Mitosis and Meiosis

Introduction:

The process of sperm cell development is known as spermatogenesis. In order to produce spermatozoa, rounded immature sperm cells go through successive mitotic and meiotic divisions and a metamorphic change. Following is the process involved in spermatogenesis: 

  • 1. Mitosis and Meiosis:

  • Mitosis:

    The process of cell duplication is known as mitosis. Two daughter cells are formed with accurately the similar DNA and chromosomal content of the original diploid (2N) mother cell. Human cells comprise of 46 chromosomes. Out of 46 chromosomes, 22 pairs of homologous chromosomes and one pair of sex chromosomes will be present.
    Mitosis covers just one step in the eukaryotic cell cycle: G1 > S > G2 > M > C.

  • • During the dominant G1 phase, the cells grow and during the S phase replication of chromosomes take place.

  • • During G2, replication of organelles and synthesis of microtubules take place.

  • • The combined stages of G1, S, and G2 include the interphase.



















  • The process of mitosis: Throughout mitosis, Chromosomes compact, the nuclear envelope vanishes, and spindle fibers start to appear from microtubules (prophase). Centromeres of duplicate sister chromatids line up next to the spindle equator (metaphase). Chromatids split and wander towards opposite poles (anaphase). The mitotic equipment is disassembled, independent nuclear envelops are found, and the chromosomes loosen (telophase). The last and final stage of the cell cycle is the cell division known as cytokinesis (C).

    Meiosis:

    The process of reductional cell division which results in the formation of four gametes possessing half (1N) number of chromosomes found in somatic cells is known as meiosis. Haploid gametes bond at fertilization to produce a diploid zygote. In the case of mammals, the heterogametic male (XY) determines the sex of the embryo. Nearly one-half of the spermatozoa comprise either X or Y chromosome. Here, XX is the sex chromosomal complement of mammalian females and so, ova can only give an X chromosome to the children. X chromosome, on which the genes are carried, reduces spermatogenesis which is inactivated in XY somatic cells.


    Steps involved in meiosis are outlined as follows:
    Interphase: Replication of DNA (4N)

    Prophase I

  • Leptotene: Condensation of chromatin

  • Zygotene: Pairing of homologs

  • Pachytene: Crossing-over and recombination

  • Diplotene: Synaptonemal complexes dissociate

  • Diakinesis: Chiasmata vanish and homologs start to repel

  • Metaphase I: One face of each homolog centromere attaches to a spindle fiber
    Anaphase I: Homologous pairs split and start to move
    Telophase I: Chromosomes travel to each pole, cell division (2N)
    Prophase II: Spindle fibers reschedule and chromosome recondense
    Metaphase II: Chromosomal pairs line up along spindle equator
    Anaphase II: Sister chromatids divide and move to opposite poles
    Telophase II: Every daughter cell nucleus has a single set of chromosomes (1N)

    In two critical aspects, meiosis differs from that of mitosis. Chromosomes pair along their length and come in contact in discrete areas of synapsis (chiasmata), during the prophase of meiosis I. the recombination of segments of chromosomes lets repeated genetic variability i.e. fast evolutionary progress and provides a method for correcting damage in the DNA helix. Next, non-identical sister chromatids don’t replicate between serial nuclear divisions. Meiosis II is necessarily mitotic.

  • 2. Spermatocytogenesis:

  • Throughout the process of spermatocytogenesis, the primitive cells are known as spermatogonia proliferate by mitosis. Various types of spermatogonia like A-0 through A-4, intermediate [IN], and B have been identified. Discriminating anatomist alone can actually differentiate among types of spermatogonia. The process of mitosis ends when a B spermatogonium yields two primary spermatocytes.

    During meiosis, the diploid number of primary spermatocytes is divided. During the phase of meiosis I, a primary spermatocyte is converted into secondary spermatocytes. During the meiosis II phase the cells, in turn, are transformed into (1N). the second mitotic division is quick and so very few secondary spermatocytes can be recognized in histological sections. spermatids and spermatocytes are likely to be larger than their ancestral spermatogonia.



    Males have an infinite capacity to produce germ cells. This is accomplished by replenishment early in mitosis in A spermatogonia. Even though the mechanics of renewal are not completely understood, it seems that a stem cell (A-0) divides into and A-1 spermatogonia and an effective copy of itself. The cell A-1 becomes granted to spermatocytogenesis and the A-0 cell is reserved for future divisions.
    Endogenous damage to the genetic material is predictable in proliferative cells. A battery of repair devices guarantees that the DNA fidelity of gametes is sustained. 

  • 3. Spermiogenesis:

  • During spermiogenesis, the spermatids undergo a dramatic change in form into the streamline spermatozoa modified for fertilization. It engages nuclear condensation, the formation of the acrosomal cap, and tail development. Using the Golgi apparatus, the acrosome is derived from. Centrioles i.e. the points of the organization of spindle fibers migrate to a post-nuclear area after the completion of the phase of meiosis. The distal centriole provides a pattern for the accumulation of cytoskeletal elements consisting of the contractile lattice of the tail. Mitochondria become concentrated into the cover of the middle piece. During the phase of spermiogenesis, the cells don’t divide.

    Spermiation:

    The process by which spermatozoa are released from the seminiferous epithelium into the lumen of the tubule is called spermiation. Most of the cytoplasm and organelles of the spermatid is not discarded within the seminiferous epithelium in the residual body form. A small amount of cytoplasmic material, the cytoplasmic droplet, stays attached around the middle piece or within the neck region as the spermatozoon makes its way into the epididymis.