Apoptosis and morphology in mouse embryos by confocal laser scanning microscopy

Confocal laser scanning microscopy combined with a vital stain was used to study apoptosis in organogenesis-stage mouse embryos. Apoptosis has previously been visualized in whole embryos using the vital dyes acridine orange, Nile blue sulfate, and neutral red. In the present study, mouse embryos were harvested on Gestation Day 9 and stained with the vital lysosomal dye LysoTracker Red. Following incubation in the stain, embryos were fixed overnight in 4% paraformaldehyde, dehydrated in a graded methanol series, and cleared in benzyl alcohol/benzyl benzoate. The resulting embryo is almost transparent and retains specific LysoTracker Red staining. To achieve optical sectioning through embryos, it was necessary to use low-power objectives. With this procedure, the entire embryo can be optically sectioned and reconstructed in three dimensions to reveal areas of dye staining. Our results demonstrate specific regions undergoing programmed cell death in normal development and increased LysoTracker staining in embryos exposed to hydroxyurea. This procedure allows for the optical imaging of whole Day 9 ( approximately 22 somites) embryos that were greater than 700 microm thick in the z axis and can be applied to studies involving neural tube formation or other aspects of organogenesis.