The application of electron microscopy in the evaluation of two- to four-cell human embryos cultured in vitro for embryo transfer.

Eighteen two- to four-cell embryos, cultured in vitro for 32-65 hr after insemination, were examined by transmission electron microscopy to assess their normality and developmental potential. These stages are now being widely used for embryo transfer in in vitro fertilization clinics. They were obtained by inseminating preovulatory oocytes aspirated at laparoscopy, with or without ovarian stimulation, by methods which have yielded normal pregnancies. The organization of seven embryos was apparently normal and their blastomeres had cellular organelles usually present in fertilized ova. Details of their ultrastructure including subtle changes observed on prolonged culture are described. Other embryos showed some normal and obvious abnormal features, such as partial fragmentation and multinucleated blastomeres, or evidence of degeneration.