Muscles Developmental - Pathology

()Map MUSCULOSKELETAL SYSTEM (Map) MUSCLES

Congenital diaphragmatic defects

• Clefts may lead to herniation of abdominal organs into thoracic cavity in any species

Myofibrillar hypoplasia

• In piglets
• = Splayleg
• May spontaneously resolve
• Unknown cause
• Responsible for significant deaths

Hyperplasia of muscle fibres

• In calves and lambs
• “Double muscling”
  • Increased number of myofibres in affected muscle (thighs, rump, loin)
  • Predisposes to dystocia

Muscular dystrophies

• Inherited group of degenerative muscular diseases
• Progressive muscle weakness and wasting
• Usually due to a genetic fault -> muscular protein deficiency
  • Duchenne MD in humans due to dystrophin deficiency also present in some animals
  • Dystrophin gene mutations reported in the Golden Retriever, Rottweiler, German shorthaired pointer and Irish terrier etc.
• Inadequate regeneration, compensatory hypertrophy
• More about Muscular dystrophy

Glycogen storage diseases

• Caused by a deficiency of an enzyme
• See general pathology
• Type II glycogenosis
  • Deficiency of acid maltase
  • In Shorthorn cattle
  • Muscular weakness and incoordination
  • Glycogen stored in skeletal muscle, heart and CNS
  • Type I and II muscle fibres contain glycogen vacuoles
• Type III glycogenosis
  • Deficiency of debranching enzyme
  • In dogs
  • Causes hypertrophic cardiomyopathy and hepatomegaly
  • Glycogen in skeletal and cardiac muscle, neurons nad hepatocytes

Steatosis

• Also called lipomatosis
• In cattle, sheep and pigs
• Fat replaces lost muscle fibres
• Pale muscle, bilaterally symmetrical
• No clinical significance
• Noticed at slaughter or necropsy

Myasthenia gravis (MG)

• Acquired
  • Autoimmune disease
    • Antibodies directed against acetyl choline receptors
  • Associated with thymomas, megaoesophagus and dysphagia
  • In adult dogs
• Congenital
  • Inherited deficiency in acetyl choline receptors
  • Rare
  • Newfoundland, Jack Russel Terrier, Springer Spaniels – genetic predisposition
  • In dogs – 4 DLA genes recognized: DLA-12, DLA-88, DLA-79 and DLA-64
  • Associated with HLA gene in humans
  • No antibodies against acetyl choline receptors in serum
  • Non-specific muscle disuse atrophy and fibrosis or no changes on histology
• Both forms manifest as weakness which worsens on exercise

Canine dermatomyositis

• Hereditary in Collies and Shetland sheepdgs
• Lesions resemble canine polymyositis
• Also degeneration of blood vessels -> muscle microinfarcts
• Usually generalised but temporalis and other masticatory muscles may be involved the most
• Microscopically: inclusion bodies in endothelial cells
• Best place for biopsy is anterior and most superficial temporalis muscle
• Also involves skin, may involve the oesophagus