What are the changes in the oogonia during the transition of a primary follicle to Graafian follicle?

Explanation

(i) Increase in size due to synthesis and accumulation of yolk. (ii) Nucleus becomes bloated with nucleoplasm to form germinal vesicle. (iii) Increase in number of mitochondria, and amount of ER and Golgi body. (iv) Secretion of egg envelopes like vitelline membrane, zona pellucida etc. (v) Reductional division (meiosis-I) to transform diploid primary oocyte to haploid secondary oocyte and release of polar body. (vi) Development of antrum with liquor folliculi in membrana granulosa.