A microscopic and biochemical study of fragmentation phenotypes in stage-appropriate human embryos

The occurrence of a pleiomorphic population of cytoplasmic fragments is a c ommon characteristic of early human embryos fertilized in vitro. Here, temp oral, spatial, fine structural, and biochemical aspects of fragmentation we re examined in fragmented monospermic and dispermic pronuclear to early cle avage stages human embryos classified as stage-appropriate during the first 3.5 days of culture. The morphodynamics of certain common patterns of frag mentation and the movement and composition of fragments were analysed by ti me-lapse video, mitochondrial fluorescent probes, and transmission electron microscopy, Plasma membrane and nuclear DNA integrity were assessed by ann exin V staining, and terminal deoxynucleotidyl transferase-mediated dUTP ni ck end-labelling (TUNEL) and single-cell alkaline gel electrophoresis ('com et') assays respectively. Developmental competence for affected embryos was related to outcome after embryo transfer. The results demonstrate that cer tain common forms of spontaneous fragmentation affecting early human embryo s are not lethal, and that clusters of apparent fragments are often transie nt structures, which disappear by resorption or lysis. The findings suggest that the occurrence and fate of fragments characteristic of these phenotyp es may be related to oncosis-like processes associated with transient and f ocal ATP deficiencies in blastomeres and mitochondrial deficiencies or abse nce in extracellular fragments.