Veterinary Physiology-III

OVULATION

• Ovulation is mechanism by which the ovum is released from mature graffian follicle in response to LH surge.
• A few follicles leave the primordial state each day and begin to develop
• Developing follicles either will ovulate or may undergo atresia depending on the time of their development, as the follicles continue to develop during all phases of the oestrous cycle including the luteal phase. Several dominant follicles develop during the luteal phase of the cycle at about 10 day intervals
• Selection of follicles for ovulation depends on the timing of the regression of the CL. Those follicles developing after the regression of the CL become ovulatory follicles
• Accumulation of fluid in the antrum of the follicle exerts pressure on the ovulation wall resulting in bulging and thinning of the surface of the ovary
• The granulosa cells loose cell to cell contact as follicle approaches ovulation, probably by dissociate of gap junctions
• The oocyte along with cumulus cells separate from the pedicle of the cumulus and freed into the follicular fluid of antrum
• Connective tissue of thecal layer dissociated during preovulatory period
• Gonadotrophin stimulated preovulatory increase in follicular prostaglandins produced of granulosa cells stimulates ovarian contraction and activate thecal fibroblast to release proteolytic enzymes that digest the follicular wall and basal lamina.
• A vascular stigma / papilla appears on the apex of the follicle. As the protrusion progresses, follicular wall layer stretch and thin until it breaks and the follicular contents are released.
• FSH also increased before ovulation, which induces LH receptor synthesis in granulosa cells.
• LH Surge is caused by the positive feed back stimulation of estrogen on hupothalamus and anterior pituitary. Increasing size of follicle produce more amount of estrogen which stimulates release of LH and FSH.
• Preovulatory LH surge occurs about 24 hours before ovulation and it causes maturation of the oocyte
• LH surge blocks the production of meiosis inhibiting factor and luteinization inhibiting factor. Meiosis is resumed and completed before ovulation
• In cow and buffalo, ovulation occurs after the end of estrus period and in bitch, mare, sow before the end of estrus period.
• In domestic animals ovulation occurs at any point on the ovarian surface except in mare wherein ovulation occurs at ovulation fossa .