Study of human post-implantation conceptuses, normal and abnormal.

Important data relative to organogenesis and teratogenesis can be learned from the study of early embryos derived from induced abortions, which are legal in Japan. Nearly 1/3 of the conceptuses from the unrecognized period of pregnancy (less than 35 days) possess abnormalities, most of which would normally cause them to be aborted. The commonest findings were degeneration and necrosis and neural tube defects. The timetables for normal organogenesis were included in the "Atlas of Prenatal Histology," published in 1983. The relationship between normal organogenesis (or teratogenesis) and gestational age exhibits a certain amount of variability; also it is not usually possible to determine the exact ovulation age of the embryo. The occurrence of abnormalities in the early intrauterine embryo is far more common than the number of such abnormalities seen in newborns, again indicating that most abnormal embryos are aborted. Early intrauterine embryos also showed a high correspondence of internal to external abnormalities, especially in the case of midline defects, indicating that the embryonic midline acts as a developmental unit. Embryonic tissues from human embryos grafted subcutaneously or intratesticularly into athymic mice were accepted and grew and differentiated to a considerable extent, showing that such tissues could be effectively used experimentally.