ABSTRACT
Although techniques such as preimplantation genetic diagnosis (PGD) have been used to aid in the selection for transfer of human embryos developing in vitro,1 the technology and expertise required limit their use to specialized laboratories; selection criteria in the routine IVF laboratory are largely based on parameters that the biologist can observe under the light microscope. These parameters include oocyte morphology (position and size of polar body, morphology of metaphase II plate, presence of imperfections in the oocyte cytoplasm),2 zygote morphology (the size and position of pronuclei, presence of a ‘cytoplasmic flare’, and nucleolar morphology),2 the rate of development (number of cell cleavages/day), and morphology of embryos (such as inequalities in cleavage and the presence of cell fragments). These morphological parameters are based on cellular physiology, and research in recent years has revealed many aspects of embryonic physiology that shed light on the basis for these observations.2
