Division of Newborn Medicine, Brigham & Women's Hospital, Boston, Massachusetts 02115, USA.
During in vitro fertilization, embryos deemed clinically useless based on poor morphology are typically discarded. Here we demonstrate a statistical correlation between the developmental stage of such poor-quality embryos and the yield of human embryonic stem (hES) cell lines. Early-arrested or highly fragmented embryos only rarely yield cell lines, whereas those that have achieved blastocyst stage are a robust source of normal hES cells.