Mitochondrial ultrastructure at the transmission electron microscope (TEM). (A) Immature oocyte (scale bar = 10 μm). (B) Immature oocyte mitochondria. Oocytes just starting vitellogenesis show a simple internal structure with few and short mitochondrial cristae (scale bar = 2 μm). Yolk (Y). (C) Mature oocyte (scale bar = 20 μm). (D) Mature oocyte mitochondria. Vitellogenic oocytes show well formed mitochondrial cristae in mitochondria usually measuring less than 600 nm (scale bar = 1 μm). Yolk (Y). (E) Spermatozoa show well formed cristae in mitochondria measuring generally more than 700 nm (scale bar = 2 μm; inset = 0.7 μm). Sperm midpiece containing mitochondria (dashed box); acrosome (a). Inset: spermhead (sh); mitochondria (m); axoneme portion (ax).