Introductory remarks on the milieux of the egg and the early embryo

in Reproduction
J. D. BiggersLaboratory of Human Reproduction and Reproductive Biology, Department of Physiology, Harvard Medical School, 45 Shattuck Street, Boston, MA 02115, U.S.A.

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Extensive experimental work on the developmental biology of the mammalian embryo (Burki, 1986; Hogan et al., 1986; Rossant & Pedersen, 1986) was made possible by the discovery of usable culture methods that were perfected about 20 years ago for the mouse and, to some extent, the rabbit (Biggers, 1987). This work on preimplantation embryo development has far outstripped the study of the microenvironments encountered by an embryo as it moves along the female genital tract to the uterus. This fact is perhaps surprising, since the culture media used to study development are substitutes for these environments. There are three reasons for this lack of attention to the composition of the genital tract secretions during the initial stages of pregnancy. Firstly, the available culture media are adequate for much of the developmental work to proceed. Secondly, the paucity of the secretions makes it very difficult to obtain samples for biochemical analysis. For example, the composition of the uterine secretions has received relatively little attention, although those interested in animals which implant superficially had speculated for years that the histotrophe produced in these species has a nutritional role (Amoroso, 1952). Thirdly, the oviduct has been traditionally thought of as a superfluous organ that acts only as a conduit for the embryo as it passes from the site of fertilization to the uterus. This view was particularly fostered by the results of the Estes operation, which was discussed by Hunter (1977), and later by Adams (1979) at the last Symposium held by the Society for the Study of Fertility on the milieu of the ovum. This operation, first described by Dudley in 1900 and later by Estes in 1909, was the method sometimes used until the 1960s to treat sterility due to tubal occlusion (Biggers, 1984). The ovary, attached to its pedicle, was moved into the uterine cavity where it was hoped ovulation would occur. It was argued that because a few pregnancies occurred following this operation the passage of the preimplantation embryo through the oviduct was unnecessary and that the oviduct is not involved in the nurture of the embryo. Biggers (1979) has pointed out that this clinical evidence is particularly weak to reach such a conclusion.


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