Ovulation - Anatomy & Physiology

Introduction

The key event in the process of ovulation is the Pre-ovulatory surge in Luteinising Hormone (LH). This surge in LH is required to initiate the downstream processes of ovulation. These processes result in increased follicular pressure and weakening of the follicle wall eventually leading to follicle rupture and the release of the Oocyte from the ovary surface into the oviduct.

Consequences of Pre-ovulatory LH surge

Hyperaemia

• LH causes the release of Prostaglandin E2 (PGE2) and Histamine. These both cause a general increase in blood supply to the Ovaries and an increase in the local blood supply to the dominant follicles.
• The local increase in blood supply ensures that the dominant follicles recieve all the nutrients they require to reach maturation and ovulation.
• The increase in blood supply also causes formation of oedema within the follicles due to increased hydrostatic pressure within the ovarian capillaries. This oedema causes an increase in intra-follicular pressure.

Progesterone Synthesis

• LH causes a shift from oestrogen to progesterone production by follicular Granulosa cells. This local increase in progesterone stimulates Theca cells to produce the enzyme Collagenase, which breakdown the collagen within the Tunica Albuginea weakening the follicle wall.

Increase in Prostaglandin F2a

• PGF2a stimulates Granulosa cells to release lysosomes containing digestive enzymes. These enzymes contribute to the degradation of the Tunica Albuginea collagen and so the weakening of the follicle wall.
• PGF2a also causes contraction of the ovarian smooth muscle. This increases the pressure within the follicle.