Question

What determines the size of the Graafian follicle?

Answer 1

The process of formation of a mature female gamete is called oogenesis which is markedly different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells called oogonia are formed within each fetal ovary; no more oogonia are formed or added after birth.

Complete answer-
With the onset of puberty, a primary follicle begins to mature with each ovarian cycle. The follicular cells become cuboidal, divided by mitosis to form a stratified epithelium, the granulosa layer. So, the primary follicles get surrounded by more layers of granulosa cells and a new theca, called secondary follicles. Granulosa cells rest on a basement membrane and the surrounding stromal cells form theca folliculi. The secondary follicle soon transforms into a tertiary follicle which is characterized by a fluid-filled cavity antrum, which appears between the granulosa and the granulosa cells. Initially, the antrum is crescent shaped, but with time it greatly enlarges. The fluid of the antrum is called liquor folliculi. As the follicles grow, the theca folliculi get organised into theca interna and theca externa.
The primary oocyte within the tertiary follicle develops and grows in size and finishes its first meiotic division at puberty. It is an unequal division leading to the formation of an outsized haploid secondary oocyte and a small polar cell.
The secondary oocyte retains the bulk of nutrient-rich cytoplasm of the primary oocyte. The tertiary follicle changes into the mature follicle or Graafian follicle.
The secondary oocyte forms a new membrane called Zona pellucida surrounding it. This thick coat of zona pellucida is composed of glycoproteins and synthesized by oocytes. Later, the granulosa, the cells lying in close vicinity of the ovum and zona pellucida, becomes elongated to form the corona radiata. In the presence of LH hormone, the Graafian follicle now ruptures to release the secondary oocyte developing from the ovary by the process of ovulation. After ovulation, the ruptured follicle left in the ovary is converted to a structure called corpus luteum, which secretes mainly progesterone.

Note:
Generally, only one ovum is liberated in each menstrual cycle by alternate ovaries. Each ovary releases six ova in one year. Only about 400-450. Ova are produced by human females over the entire span of their reproductive life, which lasts about 35-40 years.