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Ultrastructural studies were undertaken on sections of skeletal muscle from normal pigs and pigs with splayleg from herds with high or low incidence of the disease. Studies were also made of the growth of the muscle from 1 to 7 days after birth. No consistent qualitative differences between the muscles from normal and affected pigs were detected. During the first week of post-natal life, muscle cell cross-sectional areas, intracellular lipid and motor endplate size and complexity increased. Concurrently, the degree of myofibrillar hypoplasia and the area of the extramyofibrillar space (EMS) were reduced. The EMS was occupied by a granular material with a sub-structure of electrondense particles 3 to 7 nm in diameter and was considered to be predominantly glycogen. Evidence is presented that muscle cells were being formed in the early post-natal period. No significant degenerative changes were found in muscle cells from any pig.

SUMMARY Cross-sections of semitendinosus muscle from Landrace pigs with splayleg and from unaffected litter-mates were examined with a Quantimet 720 image analyzer to determine the extent and distribution of myofibrillar hypoplasia (MFH). MFH was not uniformly distributed, so that measurement of a large number of muscle samples was necessary. Even though the extent of MFH was found to be the same in the two groups of pigs there was a very highly significant difference between the arrangement of the myofibrils within the fascicles of affected and unaffected animals.

Light microscopical studies were undertaken on 1 μm thick araldite sections of skeletal muscle from normal pigs and pigs with splayleg from herds with high or low incidence of the disease. The progressive clinical improvement occurring during the first week of life was accompanied by an increase in muscle cell cross-sectional area, a reduction in the degree of myofibrillar hypoplasia and an increase in intracellular lipid. Similar changes occurred in normal pig muscle. Clinical splayleg occurred without significant abnormality in either the extrafusal muscle cells or in the intrafusal cells of neuromuscular spindles. Thus, light microscopical study of sections of skeletal muscle is not generally useful to determine whether or not a pig had splayleg. At present this can only be determined consistently by clinical observation.