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Spermatogenesis




  • File:Seminiferous tubule and sperm.jpgDefinition:

    The process by which sperms are formed within the seminiferous tubules from spermatogonia or sperm mother cells which lie on the basement membrane

    Spermatogenesis is divided in to two distinct phases :

    1. Spermatocytogenesis: A series of divisions during which spermatogonia form spermatids.

    2. Spermatogenesis: A phase where spermatids undergo metamorphosis forming spermatozoa. The entire process takes about 60 days in bull and 49 days in ram.

    Spermatocytogenesis:

    1. Phase – 1: (15 to 17 days duration) the first step in Spermatocytogenesis is mitotic division of spermatogonium. The dormant spermatogonium remains in the germinal epithelium near the basement membrane to repeat process later on. The active spermatogonium will under go 4 mitotic divisions eventually forming 16 primary sperrnatocytes.

    2. Phase -2: (15 days duration): Mitotic division of primary spermatocytes during which the number of chromosomes is halved (meiosis-I).

    3. Phase – 3 (few hours): Division of secondary spermatocytes in to spermatids (meiosis Vll).

    4. Phase – 4: The 4 spermatids from each primary spermatocyte or 64 from each active spermatogonium.

    Spermatogenesis:

    During the metamorphosis, the nuclear material compacts in one part of cell forming the head of spermatozoa, while the rest of cells elongate forming the tail. The acrosome a cap around the head of the spermatozoan will be formed. The golgi apparatus of spermatids, the cytoplasm from the spermatids is cast off during formation of tail. A cytoplasmic droplet will form the neck of spermatozoa. Newly formed spermatozoa will be released from sertoli cells and forced out through the lumen of seminiferous tubules in to Retetestis.

    Oogenesis (Ovigenesis)

    Oogenesis:

    It is the formation and maturation of female gamete. Oogenesis begins in the pre-natal period. In the female foetus, the germinal epithelium forms in to clusters in which one genocyte differentiates in to an Oogenium containing typical cell constituents. The Oogonia then undergo proliferation prior to or shortly after bird resulting in the fikled ovaries containing the sole reservoir of all future ova called Oocytes.

    The growth of Oocytes is characterized by
    i) The enlargement of cytoplasm by accumulation of different sizes of granules deutoplasm (yolk)
    ii) The development of an egg membrane – zona pellucida. iii) The mitotic proliferation of follicular epithelium adjacent tissue.

    There are two stages in the growth of Oocytes:

    First Phase: the growth is rapid and intimately associated with the development of ovarian follicle. Attainment to its mature size occurs at about the time of anstrum, formation begins in the follicle.

    Second Phase: The Oocytes doesn’t grow in size while the ovarian follicle responding to pituitary hormone increases very rapidly in diameter. During the later phases of follicular growth the Oocytes undergo maturation.

    The nucleus which has entered in the prophase of the meotic division during the growth of Oocytes prepares to undergo reduction division. The nuclei and nuclear membrane disappear and. chromosome coalesces in compact form. The centrosome then divides into two centrioles around which aster appear. These asters become more apart and spindle is formed between them. The chromosome in diploid pairs are set free in cytoplasm and become arranged on the equatorial plate of spindle (metaphase-I).

    The primary Oocytes now undergo two meotic divisions. In the first division two daughter cells arise each containing one half of chromosome complement. However, one cell acquires almost all the cytoplasm. This cell is also known as secondary Oocytes. The other much smaller cell is known as first polar body. At second maturation division, the secondary oocyte divides into ootids (n) and second pollar body (n).

    The two pollar bodies containing very little cytoplasm are entrapped in the zona pellucida of the oocytes and there they degenerate. The first polar body may also divide. The zona pellucida may contain one, two or three polar bodies.

    It should be pointed out that it is the secondary oocyte which is liberated at ovilation (primary oocyte in case of horse). The oocyte continues the process of maturation until fertilization when it becomes zygote. In the process of Oogenesis primary oocyte gives rise to one egg.

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