Question

Question 12: What is oogenesis? Give a brief account of oogenesis.

Class 12 - Biology - Human Reproduction Page 56

Answer 1

Answer:

Ooogenesis is the phenomena of formation of haploid female gametes known as ova from diploid oogonia in the ovary, Graffian follicles to be precise. This process is discontinuous which is initiated during the period of foetal development that is terminated only after puberty sets in.

The process of Oogenesis takes place in three phases:

Multiplicative phase –

Follicle cells are differentiated from the germinal epithelium of the ovary due to repeated mitosis division. Few follicle cells enlarge and are termed as egg mother cells and undergo mitosis to multiply which are referred to as oogonia.

Growth phase – one of the oogonium of the egg nest differentiates while the rest change into surrounding nutritive follicular epithelium. There is an increase in the size of the differentiated isolated oogonium as it gets nourished from the girdling follicle cells thereby transforming into a diploid primary oocyte.

Maturation phase – The diploid primary oocyte in this phase passes through two maturation divisions. Meiosis I – the first meiotic division splits the diploid primary oocyte into two haploid cells wherein the larger one is the secondary oocyte and the minor one is the polar body(polocyte). In meiosis II or the second meiotic division, the secondary oocyte splits to form one large ootid and a tiny second polar body. Furthermore, the first polar body splits through mitosis to form two polar bodies. The ootid matures into a functional haploid ovum. Therefore, one primary oocyte produces one large ovum and three polar bodies which inturn degenerate. They degenerate as they do no participate in reproduction thus leaving behind one functional ovum.

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