Oogenesis

The formation of ova in the ovary from primordial germinal cells is said to be oogenesis.

Oogenesis is the biological process by which a mature female gamete (ovum or egg) is formed from primordial germ cells in the ovary. It is a form of gametogenesis that involves both mitotic and meiotic divisions, and is markedly different from spermatogenesis in its timing, symmetry, and outcome.

• The process begins in the fetal ovary during embryonic development.
• At the peak (around the 7th month of gestation), approximately 2–7 million oogonia are present.
• At birth, about 2 million primary oocytes remain.
• By puberty, only 60,000–80,000 primary follicles remain per ovary - the rest degenerate.
• Final completion of oogenesis occurs only after fertilisation.

Diagrammatic Section view of the ovary

Oogenesis is completed in three sequential phases, each occurring at different life stages:

Oogenesis

• Primary Germ Cells (PGCs) present in the germinal epithelium of the ovary undergo repeated mitotic divisions.
• Daughter cells are called oogonia (singular: oogonium) - they are diploid (2n = 46 chromosomes).
• Oogonia are the starting material; they are not yet gametes.
• The entire multiplication phase is completed in the embryonic stage of the female.

• Some oogonia stop dividing and undergo a period of intense growth.
• The enlarged cell is now called a primary oocyte— still diploid (2n = 46).
• Intracellular organelles including ER, Golgi apparatus, and mitochondria increase significantly in number.
• The primary oocyte enters Meiosis I but becomes arrested at Prophase I.
• Each primary oocyte becomes surrounded by a single layer ofgranulosa cells→ together form a Primary Follicle

During the maturation phase, the primary follicle transforms through a series of distinct developmental stages within the ovary:

 

Primary Follicle:

• Contains: primary oocyte (2n) surrounded by a single layer of granulosa cells.
• Granulosa cells provide nutrients and hormonal signals to the developing oocyte.

Secondary Follicle:

Granulosa cells proliferate into multiple layers around the oocyte.

A new outer covering called the theca layer forms, organised into two sub-layers:

1. Theca Interna- inner vascularised layer; secretes androgens
2. Theca Externa- outer fibrous connective layer

Tertiary Follicle:

• Characterised by the appearance of a fluid-filled cavity called the antrum.
• The primary oocyte within this follicle grows in size and completes Meiosis I(first maturation division).
• This division is unequal (asymmetric), resulting in: a large haploid secondary oocyte + a tiny first polar body.
• The secondary oocyte retains the bulk of the nutrient-rich cytoplasm.

Graafian Follicle (Mature Follicle)

• The tertiary follicle further develops into the mature Graafian follicle.
• The secondary oocyte acquires a glycoprotein membrane called the zona pellucida.
• The Graafian follicle ruptures and releases the secondary oocyte - this event is called ovulation.

A systematic view of how chromosome number changes at each stage of oogenesis: