Veterinary Physiology-III

FOLLICULOGENESIS

• Factors that control the initial follicular growth are not known.
• When any of the primordial follicles is released from the reserve, it continues to grow until ovulation or until atresia stage.
• The development of the follicle is initiated by the growth of oocyte.
• The follicle that has an early growing oocyte but still surrounded by a single layer of follicular cells is termed as primary follicle. The primary oocyte in primary follicle increase in size; at the same time the follicular cells multiply into several layers of cells. This is called maturing follicle or growing follicle (preantral follicle or secondary follicle) which still has not developed theca or antrum.
• On further development, an antrum (cavity) form by fluid secreted by granulosa cells which collect between the granulosa cells and separating them. When the antrum has formed, the follicle is classified as a Graafian follicle (antral/tertiary follicle).The mature tertiary follicle appears as a fluid-filled bulging on the surface of the ovary.
• Two types of cells surround the maturing oocyte – granulosa cells on the inside surrounded by theca cells on the outside with a basement membrane in between .
• The initial phase of follicular growth is termed as hormone-independent phase – oocyte increases in size and activity with large amount of RNA synthesis.
• Follicular cells grow and divide to become granulosa cells which produce a glycoprotein layer around the oocyte forming the zona pellucida. The granulosa cells maintain contact with oocyte by cytoplasmic processes.
• This initial growth is followed by hormone-dependent phase with synthesis of FSH and oestradiol receptors on the granulosa cells.
• The granulosa cells produce oestradiol by converting androgens synthesised by theca cells. LH stimulates androgen synthesis by theca cells. The androgens diffuse into granulosa cells. FSH stimulates aromatase enzyme of granulosa cells to convert androgen to oestrogen. This process is described as two-cell two-gonadotropin model of synthesis of follicular oestradiol.
• The follicle becomes more responsive to FSH (FSH induces synthesis of its own receptors) and the oestrogens enhance growth and division of granulosa cells which starts secreting fluid into the antrum of the follicle.
• Oestradiol inhibits progesterone biosynthesis in the follicle.
• When the follicle reaches its maximum size, FSH induces LH receptors in the granulosa cells. When LH surge occurs, LH stimulates the granulosa cells to synthesize progesterone.
• The granulosa cells surround the antrum. In addition, a hillock (mound) of granulosa cells is located at one side of the antrum which is called cumulus oophorus. The ovum rests upon the cumulus oophorus with other granulosa cells extending around the potential ovum.
• The granulosa cells surrounding and in immediate contact with the ovum are termed the corona radiata. Both theca interna and granulosa cells are involved in production of oestrogen.
• The granulosa cells are the progesterone producing cells in the corpus luteum. They also secrete other compounds that have been identified in follicular fluid which help to regulate the functions of the ovary. When ovulation occurs the follicle ruptures expelling the liquor folliculi, some granulosa cells, and the oocyte into the body cavity near the opening to the oviduct.
• At the time of expulsion, the oocyte is surrounded by the corona radiata and a sticky mass containing other granulosa (cumulus) cells which aid the oviduct in picking up the oocyte and moving it down the oviduct. In some species the corona radiata is present at the time of fertilization. In other species these cells are shed quickly and are not present when fertilization occurs.

Picture showing follicle on the ovary
(Courtesy: Dr.S.Selvaraju, Scientist, NIANP, Bangalore)