Hereditary "draft lamb" disease of Border Leicester sheep: the ultrastructural pathology of the skeletal muscles.

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A hereditary and congenital disease of Border Leicester lambs clinically indistinguishable from the "draft lambs" of Innes et al. (1949) but with no cerebellar lesion, is defined in terms of the ultrastructural pathology of the muscle, intramuscular nerves and neuromuscular junctions. The pathognomonic feature of these congenitally affected lambs was the occurrence of large intrafascicular muscle cells. On the basis of the mitochondrial content and Z disc thickness these were considered to be one of two subtypes of type I cell. There was evidence of muscle cell growth in all cell types between birth and 3 mth of age, particularly in the form of myofibrillar splitting. The myofibrils of the large cells were very large and incompletely split. The mitochondria in these cells were of two kinds, roughly rounded and tubular. The latter showed conspicuous branching. A few muscle cells appeared immature but no significant degenerative changes were detected. In some neck muscles intramuscular nerves were seen and in 28 per cent. of the axons lamellated electron-dense bodies occurred. The periodicity of the lamellations was 40--60 nm. No other defects were observed in their vicinity but some axon terminals contained distended mitochondria which themselves contained membranous bodies. The axon terminals also contained morphologically normal synaptic vesicles and there was evidence that these were transmitting their transmitter substance. The lesions in hereditary "draft" lamb disease of Border Leicester sheep may result from functional defects of the nervous system which have a delaying effect on the development of some muscle cells, whilst others capable of normal growth hypertrophy. Alternatively, there may be a primary defect in the muscle cells themselves either alone or together with an unrelated neuropathy.