Coloured transmission electron micrograph (TEM) of spermatids in an advanced phase of spermiogenesis. The excess of cytoplasm (yellow) will be soon eliminated. Several spermatids show the sperm head with acrosomal cap and nucleus (blue). The rest show cross-sections of the middle piece (green), with mitochondria, fibrous sheet and flagellum. In the lumen, there are microvilli of Sertoli cells (light blue) and some flagella (red).