SUPEROVULATION AND PRESERVATION OF MAMMALIAN EGG
Authors: Hafez E. S. E. 
Published: October 2009

Immature animals can be superovulated as soon as an antrum develops within the ovarian follicle. From birth to puberty the sensitivity to gonadotropins increases gradually and at rates which are species and probably breed specific. Eggs obtained from immature donors are functionally equivalent to those of adults. To superovulate adult animals, different sources of FSH are administered at an optimal time during the follicular phase of the estrous cycle. In cattle the rupture of the first follicle starts about 24 hr after HCG injection. The ovulation of subsequent follicles may continue, at least in prepuberal calves, for the next 48 hours. The physiological mechanisms which cause ovulation are quite independent of those which cause estrus. There are quantitative and qualitative differences in the responses of animals to crude and purified PMSG. The superovulatory responses of mature animals vary with season, breed, live weight, stage of the estrous cycle at injection, and individual variations in cycle length, age, genetic constitution, the amount of hormone administered previously, the postpartum interval, and plane of nutrition. Refractoriness to repeated injections also occurs in several species. Fertilization rate is lower following superovulation in farm animals than in laboratory animals. Low fertilization rate is partly due to lack of sperm transport after vaginal or cervical insemination. Overstimulation of the ovary causes reduced rates of egg recovery. This is due to: a) the effect of endogenous estrogen production on the rate of egg transport; b) the trapping of some eggs in follicles; and c) fimbriae too small to surround all ovarian surface. Different gonadotropins and combinations of them have been used to induce mild superovulation and multiple pregnancy is cattle and sheep. The incidence of early embryonic mortality is related to the number of ova shed following gonadotropin treatment: the higher the number of induced ovulations, the lower the ability of the animal to maintain pregnancy, regardless of the number of embryos that develop in the early stages of gestation.

Methods have been described for the manipulation, selection, preservation and transfer of fertilized eggs. The integrity of an egg is important for its survival both in vivo and in vitro. Microscopically visible as well as nonvisible defects may be present; such eggs may or may not implant. The eggs of several species have successfully been stored for a few days at sub-normal temperatures. The survival rate depends on the species, the developmental stage of the egg, physical and biochemical properties of the storage media, storage temperature, the rate of cooling and rewarming the eggs and techniques of storage and egg transfer. Fertilized eggs are more resistant to cold storage than unfertilized eggs. Optimal temperature for storage ranges from 5° to 10° C. The period which the egg can be stored without losing its viability and without causing a harmful effect varies with the species, stage of development of the egg, and the storage medium and temperature. Eggs of one species can be transferred into the oviduct or uterus of another species, and after short-term storage, may be recovered for subsequent transfer to an appropriate recipient. Stage of development of transferred eggs should be synchronous with the reproductive stage of the recipients. Eggs could also be transferred to ectopic sites, e. g., the ovarian bursa of the anterior chamber of the eye, spleen or kidney capsule. Surgical and non-surgical methods have been used to transfer eggs to the uterus. Areas for future research are recommended.