Early human development.

Abstract

This presentation covers original research conducted since 1977 on excess human embryos obtained from several invitro fertilization (IVF) centres around the world. Most of the embryos portrayed were developed from normal, monospermic, bipronuclear ova or from dispermic, tripronuclear ova, after fertilization. With respect to morphology, early dispermic embryos resemble normally fertilized embryos, although they are genetically triploid. The process of human fertilization has been already presented on CD-ROM. Two visual atlases have been published recently on human IVF and embryo development (Sathananthan et al., 1991; Sathananthan, 1996). The preimplantation development of the human embryo begins at fertilization and ends at the implantation of the blastocyst in the uterine endometrium on day 6 after fertilization. Preimplantation development occurs in the Fallopian tube or oviduct and in the uterus during which time the embryo cleaves by repeated mitosis to form a hollow blastocyst. There is little growth and differentiation during cleavage and the embryo is partly dependent on the secretions produced by the oviduct and uterus, since the human egg has no yolk. The embryonic genome is thought to be activated between the 4–8-cell stages of development and the first signs of cell differentiation are evident in the blastocyst, when an outer trophoblast and an inner cell mass or embryoblast have formed. Recent work suggests that somatic and germline cells may be allocated from the 4-cell stage (Edwards and Beard, 1997). The blastocyst finally hatches out from the zona pellucida, or shell, within the uterus and immediately invades the uterine endometrium by means of its trophoblast, after which it is dependent on the mother’s blood for sustenance and eventually develops a placenta. Molecular events during human implantation, especially those involving embryonic interleukins and epithelial β3 integrins, are described by Simon et al. (1998). Human embryos can be developed to blastocysts and even to hatching in vitro. This process of hatching is one of the wondrous events of early development. Embryos are normally developed to the 2–4-cell stages for embryo transfer during IVF and are replaced either in the uterus or Fallopian tube.