An 8 day old 3D cluster of cells (embryoid body) formed from mouse embryonic stem cells which lack an essential mitochondrial enzyme. The cells were grown in factors to encourage development of the nervous system (neural differentiation). Clumps of neurones containing mature (beta-tubulin, green) and immature (nestin, red) neural cells are seen growing on the surface of the embryoid body. Imaged by fluorescence immunocytochemistry. Diameter of the embryoid body is 40 micrometres.