Fig. 4. Nuclear density declines at all fibre sizes in the absence of myoD. Mean fibre cross-sectional area (A), nuclei per unit fibre length (B), surface area domain size (SADS, C) in wild-type (number of animals per group, n=9, n=5) and myoD-null (D^–/–) mice (n=4, n=5) at 2 months and 22-24 months, respectively. **P<0.01. Error bars indicate animal to animal s.e.m. (D) Fibres pooled into small (<1500 µm²), medium (1500-2500 µm²) and large (>2500 µm²) groups show the fibre size is not lower in ageing D^–/– than wild-type mice. (E,F) Relationship of nuclei/unit fibre length (E) and SADS (F) to fibre size in 22-24-month-old D^–/– and age-matched controls. (G,H) Nuclei/unit fibre length decreases (G) and SADS increases (H) in 22-24-month-old D^–/– mice in all fibre size ranges compared with both age-matched controls. Number of fibres in 22-24-month-old D^–/– mice are 60, 23, 30 for small, medium and large fibres and 54, 37, 29 for 22-24-month-old controls. *P<0.05, **P<0.01, ***P<0.001.