Oogenesis: Human Reproduction ||Class 12 Biology

By -
     The process of formation of a mature female gamete is called oogenesis. It is the production of eggs (ova) that starts in ovaries of female foetus (of 25 weeks). It begins before birth but complete only after fertilization.

     It comprised of the following three main stages:
1. Multiplication Phase
2. Growth Phase
3. Maturation Phase

1. Multiplication Phase: Cells of germinal epithelium undergoes mitotic division producing undifferentiated germ cells called oogonia or egg mother cells within each foetus ovary. These cells start dividing and enter prophase-I of meiotic division and get primarily arrested at this stage, known as primary oocytes.

2. Growth Phase: Each primary oocyte then gets surrounded by a large of granulosa cells and called primary follicle. A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles.

3. Maturation Phase: During this phase, the secondary follicles soon transforms into tertiary follicles. These are characterised by a fluid cavity called antrum. The primary oocyte within this tertiary follicle grows in size. The cytoplasm of fully grown primary oocyte undergoes first meiotic division producing two unequal daughter nuclei in which larger cell is called secondary oocyte and a small cell is known as first polar body.
     The secondary oocyte starts undergoing second meiotic division and form the tertiary follicle which further changes into a mature follicle, called Graafian follicle. The secondary oocyte forms a new membrane called zona pellucida around it and starts its second meiotic division but remains suspended in metaphase-II, till sperms enter into it. At this stage, the Graafian follicle, ruptures to release the secondary oocyte from the ovary. The release of the secondary oocyte from the ovary is called ovulation.

Location: India

Comments

Post a Comment

If you have any doubts,please let me know

Popular posts from this blog

Spermatogenesis : Definition & Process ||Class 12 Biology

By -
Image
    Spermatogenesis is the process of formation of spermatozoa (sperms) in the testes which originate from the primordial germ cells (PCGs).     The process of Spermatogenesis initiates at puberty and continues even in the old age.       The process of Spermatogenesis comprised the following two main stages- 1. Formation of spermatids 2. Spermiogenesis 1. Formation of spermatids : It include the following three phases- i) Multiplication Phase ii) Growth Phase iii) Maturation Phase   i) Multiplication Phase : During this phase, the immature germ cells present in the Seminiferous tubules of the testes known as spermatogonia increases in number by mitotic divisions and so that the newly formed spermatogonium possesses the same number at chromosomes.  ii) Growth Phase : Each spermatogonium is diploid (2n) and contains 46 chromosomes. Some of the spermatogonia called primary spermatocytes.  iii)  Maturation Phase : Primary spermatocytes periodically undergo meiosis  division. A primary sper
Read more

Sperm: Structure ||Class 12 Biology

By -
Image
     A mature spermatozoa or a sperm cell is a microscopic, tadpole-shaped structure of about 2-5 micrometer in diameter and 50 micrometer long. A plasma membrane encloses the whole body of sperm.      Sperm consists of head, neck, middle piece and tail.       1. Head: It is flat, oval and composed of a large haploid nucleus and a small cap-like structure called acrosome. It lies at the tip of the nucleus and is formed from the Golgi complex. It contains hydrolytic enzymes and is used to contact and penetrate the egg (ova) during fertilization.       2. Neck: Neck is the smallest part, it is short structure that contains two centrioles- one parallel to the base of the nucleus and called proximal centriole and other distal centriole perpendicular to the previous one. The proximal centriole enters into the egg during fertilization and starts the first cleavage division of the egg which has no centriole. The distal centriole gives rise to the axial filament which forms the tail.       3.
Read more

Placenta: Structure & function ||Class 12 Biology

By -
Image
     The placenta is an organ attached to the lining of womb during pregnancy. Structure:       After implantation, finger-link projections appears on the trophoblast called chorionic villi which surrounded by the uterine tissues and maternal blood. The chorionic villi and the uterine tissue become interdigitated with each other and jointly form a structural and and functional unit between foetus and maternal body called placenta. Placenta is forms from the chorian and allantois. In human, the placenta averages 22cm (9 inch) in length and 2-2.5cm (0.8-1inch) in thickness. it has dark reddish-blue or crimson colour. It connects to the foetus by an umbilical cord. Function:      1. It facilitates the supply of oxygen and nutrients to the embryo.     2. Responsible for the removal of carbon dioxide and waste materials produced by the foetus.    3. The placenta is connected to the embryo through the umbilical cord which helps in the transport of substances to and from the embryo.    4. It
Read more