B. Fragmentation

Small portions of cytoplasm enclosed by a cell membrane but usually not containing DNA are often formed during cell division. Fragmentation is therefore defined as the presence of anucleate structures of blastomeric origin (Keltz et al., 2006) and evaluation of the degree of fragmentation is included in almost every embryo scoring system. The degree of fragmentation is most often expressed as the percentage of the total cytoplasmic volume. The relative degree of fragmentation is defined as mild (<10%, Figs 223–225), moderate (10–25%, Figs 226–235) and severe (>25%, Figs 236–242).

It is often difficult to make the distinction between a large anucleate fragment and a small (nucleated) cell. Johansson et al. (2003) showed that portions of cytoplasm that were <45 µm in diameter on Day 2 and <40 µm in diameter on Day 3 did not contain DNA, and the authors suggested a standardization of defining fragments as all structures below these sizes.

It has been shown that a high degree of fragmentation correlates negatively with implantation and pregnancy rates (Racowsky et al., 2000), while the presence of minor amounts of fragmentation has no negative or possibly even a positive impact (Alikani et al., 1999). Two distinctly different types of fragmentation have been documented by time-lapse analysis in human embryos: definitive fragmentation, characterized as stable persistent fragments clearly detached from blastomeres and pseudo-fragmentation, characterized by a transient appearance during, or shortly after, cell cleavage, but not detected during later development (Van Blerkom et al., 2001).

Increasing fragmentation also results in reduced blastocyst formation and can influence allocation of cells during differentiation (Hardy et al., 2003). The spatial distribution of the fragments in the perivitelline space (PVS) can be differentiated into two patterns, i.e. scattered (Figs 224, 226, 227, 230–236, 239 and 240) or concentrated (Figs 223, 225, 228, 229, 237 and 238). The scattered appearance was found to be correlated with an increased incidence of chromosomal abnormality (Magli et al., 2007). The higher the degree of fragmentation, the more difficult it is to differentiate between scattered and concentrated fragmentation (Figs 241 and 242). Fragmentation is considered to be an essential parameter to include in the evaluation of developing embryos, as embryos with very strong and persistent fragmentation are less likely to be viable.