/. Embryol. exp. Morph. 89, 209-222 (1985) 209 Printed in Great Britain © The Company of Biologists Limited 1985 Muscle fibre types and innervation of the chick embryo limb following cervical spinal cord removal N. G. LAING AND A. H. LAMB Department of Pathology, Neuromuscular Research Institute, University of Western Australia, QEII Medical Centre, Nedlands 6009, Western Australia SUMMARY Several segments of spinal cord were removed from the cervical regions of stage-13 or -14 (day-2) chick embryos. After further incubation to day 17 or 18, the patterns of end-plate distribution and ATPase typing of muscle fibres in the anterior and posterior latissimus dorsi and the ulnimetacarpalis dorsalis, and the ATPase typing of the forearm muscles were examined. No differences from control embryos were found. The embryos had normal numbers of lateral motor column motoneurons in both the brachial and lumbar enlargements and the positions of motoneurons supplying the biceps as identified with retrograde horseradish peroxidase labelling were consistent with the normal patterns of motor projection into the limb. These results show that the fibre typing of limb muscles and their patterns of innervation are independent of descending inputs until just before hatching in the chick. INTRODUCTION Transection of the spinal cord of neonatal and adult mammals greatly affects the properties of skeletal muscle fibres supplied by spinal motoneurons caudal to the lesion. In neonates, slow muscles do not develop their normal properties, and in adults slow muscles develop fast muscle properties (Buller, Eccles & Eccles, 1960; Karpati & Engel, 1968; Grimby, Broberg, Krotkiewska & Krotkiewski, 1976; Gallego, Huizar, Kudo & Kuno, 1978; Rubinstein & Kelly, 1978). Fast muscle fibres are relatively unaffected. These effects are probably mediated by the alteration of function caudal to the lesion, which may give rise to increased or decreased peripheral motor activity, depending on species and maturity (Barcroft & Barron, 1937; Wang & Lu, 1940; Barron, 1941; Buller etal. 1960; Sims, 1962; Stelzner, 1975; Forssberg, Grillner & Halbertsma, 1980; Forssberg, Grillner, Halbertsma & Rossignol, 1980; Forehand & Farel, 1982; Smith, Smith, Zernicke & Hoy, 1982). In the chick embryo, alteration of behaviour caudal to spinal transection can first be demonstrated at approximately the tenth day of incubation (E10) (see Oppenheim 1975 for review) and alteration of the electrical activity of the cord caudal to the lesion can be seen at least as early as E13 (Provine & Rogers, 1977). Transection increases the duration and decreases the frequency of the cyclic bursts of activity seen in embryos showing that supraspinal inputs only modulate the spinally generated activity at these stages (Oppenheim, 1975; Key words: chick embryo, spinal cord transection, muscle fibre type, motoneurons.