Hoechst and TUNEL staining of blastocysts not capable of zona pellucida (ZP) hatching. A) Frequency histogram of non-ZP hatched blastocysts with different number of apoptotic cells evaluated by the nuclear staining, Hoechst 33342, cultured in control or composite-conditioned medium (C-CM) in three independent experiments. B) Pictures of representative blastocysts cultured in control (upper) or composite-conditioned medium (below) stained with Hoechst 33342 (left) and TUNEL method (right). The arrows indicate TUNEL-positive apoptotic cells. Note a few TUNEL-positive cells in the ICM of a control blastocyst and several TUNEL-positive cells mostly concentrated in the ICM of a blastocyst cultured in C-CM. Magnification approximately 100×.