Describe oogenesis?

Explanation

OOGENESIS:
The formation of ova or eggs from the diplod oogonial cells of the ovary is called oogenesis. Primordial germ cells give rise to oogonial cells and follicular cells (nurse cells). It is the oogonial cells that develop into ova. Oogenesis involves three phases, viz..
1. Multiplication phase 2. Growth phase 3. Maturation phase
1. Multiplication phase: The diploid oogonial cells of the ovarian follicles divide repeatedly by mitosis to produce more oogonial cells. Among them only a few will enter into growth phase and others are kept in reserve
2. Growth phase: During this phase, few oogonial cells synthesise yolk or vitelline in the cytoplasm and transform into primary oocytes. Hence this phase is also called vitellogenesis. During the growth of oogonial cells into primary oocytes, follicular cells or nurse cells or granulosa cells assist vitellogenesis
3. Maturation phase: This phase involves two successive divisions namely meiosis I and meiosis I. The meiosis is a reductional cell division resulting in the formation of a smaller cell often called the first polar body and larger secondary oocyte. The larger secondary socie undergoes meiosis II to form a larger cell called ootid or ovum and a Smaller cell called as second polar body. The first polar body may also undergo meiosis II to produce two more secondary polar bodies. So at end of oogenesis one ovum and three polar bodies are formed. These polar bodies will not survive. They undergo disintegration.