Simulation · Biology · Class 12
One oocyte, one ovum; one spermatocyte, four sperms
From the lesson Gametogenesis: oogenesis in Human Reproduction. Change the values and watch what happens.
One oocyte, one ovum; one spermatocyte, four spermsBiology · Class 12
The idea behind it
NCERT §2.3
- Oogenesis starts before birth: during embryonic development, a couple of million oogonia (gamete mother cells) form in each foetal ovary, and no new oogonia are added after birth.
- Oogonia begin meiosis and halt in prophase I; these arrested cells are primary oocytes. A primary oocyte surrounded by a layer of granulosa cells is a primary follicle.
- Many follicles degenerate between birth and puberty, so each ovary has only about 60,000–80,000 primary follicles left at puberty.
- A secondary follicle forms when a primary follicle is surrounded by additional layers of granulosa cells and acquires a new theca. Once a fluid-filled cavity called the antrum appears, the follicle is tertiary, and its theca has become organised into theca interna and theca externa.
- Inside the tertiary follicle the primary oocyte grows and finishes meiosis I. The division is unequal: it gives a large haploid secondary oocyte that keeps most of the cytoplasm and a tiny first polar body.
- The tertiary follicle matures into the Graafian follicle, in which the secondary oocyte lies enclosed by a covering called the zona pellucida.
- At ovulation the Graafian follicle ruptures and releases the secondary oocyte (the ovum). Meiosis II of this cell is completed only if a sperm enters it.
- Net result: one primary spermatocyte gives four sperms, but one primary oocyte gives only one functional ovum, with the rest of the chromosomes discarded in polar bodies.