Techniques of in vitro fertilization were first developed to aid couples who had not been able to conceive through normal sexual relations. In this technique, eggs were surgically removed from the mother and mixed with the father's sperm in the laboratory. If fertilization took place, one or more embryos were introduced into the mother's uterus in the hope that an embryo would implant and develop into a full-term infant. Almost immediately, these techniques were used in other animals, especially endangered species. It offered many advantages over natural reproduction. In pairs that showed little sexual interest in each other, eggs and sperm could be extracted, mixed in the laboratory, and viable embryos could be introduced into the female's uterus. Also, if there was little genetic diversity in a zoo population, sperm from a donor at another location could be sent and used. By the end of the twentieth century, in vitro fertilization was being coupled with surrogate motherhood. Here, after the embryos are formed, they are introduced into the uteri of females of similar, but not endangered, species. This increases the number of uteri available for the endangered species' reproduction.
ever, the pronuclei remain separate until after the first cleavage division. In a few others, like the copepod Cyclops, the pronuclei divide separately for several cleavage divisions.
The fusion of the sperm with the egg nucleus affects many other cellular processes. One of the most interesting is the displacement of some cyto-plasmic constituents. These constituents of the egg determine the fate of cells derived from the parts of the egg in which they were located and probably determine the plane of bilateral symmetry. Sperm attachment and entry often causes shifts in the position of the viscous cortical and subcortical cytoplasm, where many of the fate-determining chemicals are located.
See also: Asexual reproduction; Breeding programs; Cleavage, gastrulation, and neurulation; Cloning of extinct or endangered species; Copulation; Courtship; Determination and differentiation; Development: Evolutionary perspective; Estrus; Gametogenesis; Hermaphrodites; Hydro static skeletons; Mating; Parthenogenesis; Pregnancy and prenatal development; Reproduction; Reproductive strategies; Reproductive system of female mammals; Reproductive system of male mammals; Sexual development.
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