Golden retriever muscular dystrophy (GRMD), a degenerative myopathy due to the absence of dystrophin, is genetically homologous to human Duchenne muscular dystrophy (DMD). Spontaneous death of GRMD neonates within the first 2 weeks of life occurs frequently. This report describes the microscopical muscle lesions that developed in 12 GRMD puppies aged 1-8 days of age, and makes a comparison with three normal age-matched siblings and two older GRMD dogs. Immunohistochemical methods were used to confirm dystrophin deficiency in GRMD puppies. Muscle lesions were assessed on sections stained with haematoxylin-eosin-saffron, Gomori's trichrome and alizarin red S, and their severity was graded semi-quantitatively. Muscle fibre types were determined immunohistochemically on the basis of the pattern of expression of developmental, slow and fast isoforms of myosin. Muscle lesions in the GRMD puppies were characterized by massive necrosis, affecting most muscles of the proximal limbs, trunk and neck at birth. Lingual lesions began to develop in utero, and respiratory muscles underwent terminal diffuse necrosis resulting in death from acute respiratory failure. However, GRMD puppies do not invariably die in the neonatal period. Muscle in 2-month-old GRMD dogs showed signs of regeneration (immunohistochemical immaturity of muscle tissue), which suggested that all GRMD dogs suffer from massive post-natal myonecrosis, whether fatal or not. Muscle lesions in neonates consisted mainly of hyalinization, hypertrophy, calcification and necrosis, followed by regeneration. Such "phase I" lesions due to the absence of dystrophin are found in all species in which dystrophin deficiency has been described (human beings, dogs, cats and mice), whereas the endomysial fibrosis and myofibre atrophy found in 2-month-old GRMD dogs constituted "phase II" lesions, which are specific to GRMD and human DMD.
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