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We analyzed the fiber-type composition of the soleus muscle in rats and mice to determine whether the adult proportion of fiber types is fixed soon after birth or whether it changes during postnatal maturation. We examined muscles from animals varying in age from 1 week to 1 year using monoclonal antibodies that distinguish between fast and slow isoforms of myosin heavy chains. In cross sections of unfixed muscle containing profiles of all myofibers in the muscle, we counted the fibers that stained with antibodies to fast myosin, and in adjacent sections, those that stained positive with an antibody to slow myosin. We also counted the total number of fibers in each section. Rat soleus contained about 2500 myofibers, and mouse about 1000 at all ages studied, suggesting that myogenesis ceases in soleus by 1 week after birth or sooner. In mouse soleus, the relative proportions of fibers staining positive with fast and slow myosin antibodies were similar at all ages studied, about 60%-70% being fast and 30%-40% slow. In rat soleus, however, the proportions of fast antibody-positive and slow antibody-positive fibers changed dramatically during postnatal maturation. At 1 week after birth, about 50% of rat soleus fibers stained with fast myosin antibodies, whereas between 1 and 2 months this value fell to about 10%. In mouse, about 10% of fibers at 1 week, but none at 1 year, reacted with both fast and slow antibodies, whereas in rat, fewer than 3% bound both antibodies to a significant degree at 1 week. It is puzzling why, in rat soleus, the majority of apparently fast fibers present at 1 week is converted to a slow phenotype, whereas in mouse soleus the predominant change appears to be the suppression of fast myosin expression in a subset of fibers that expresses both myosin types at 1 week. It is possible that this may be related to differences in size and the amount of body growth between these two species.

The stability of neuromuscular junctions (NMJs) was studied in soleus muscles of adult mice by labeling acetylcholine receptors in vivo with rhodamine alpha-bungarotoxin. Identified NMJs were examined in living animals by low-light-level fluorescence microscopy on 2 or 3 occasions separated by up to 6 months. Many NMJs appeared identical each time they were viewed except for overall enlargement probably related to growth of the animal. Forty-four percent of NMJs, however, changed their shape over 6 months; these changes consisted mostly of small deletions or additions to part of the initial configuration. NMJs in adult soleus appeared to be less malleable than suggested by earlier studies but more plastic than NMJs in another muscle, the mouse sternomastoid, in which virtually no remodeling was observed using similar methods to the present study (Lichtman et al., 1987a). Thus, the degree of remodeling at NMJs may vary among different muscles, perhaps depending on their pattern of use.

Eight sensory structures (campaniform sensilla), appearing identical in the light and scanning electron microscopes, are found in specific locations on the wings of Drosophiia. Their axons enter one of 2 central tracts, a medial one or a lateral one. The topographic arrangement of the sensilla on the wing is not reflected in this central projection pattern. There is, however, a strict correlation between the time when a sensillum develops and the path its axon follows: The 4 sensilla whose axons form the medial projection are born and differentiate early during the development of the wing, while the other 4 sensilla, all of which project laterally, arise during a second wave of differentiation.This time-related projection pattern remains stable in the face of a variety of genetically induced alterations in the precise number and location of sensilla.The sensory systems of insects, like those of vertebrates, are frequently organized in topographic fashion (e.g., Johnson and Murphey, 1985; Murphey, 198 1;Strausfeld, 1976). Because differentiation in a sensory field often proceeds in a topographically ordered way, there can be a simultaneous correlation between the position of a sensory element in the periphery, its age, and its central projection (Murphey et al., 1980;Walsh and Guillery, 1984).We report here an analysis of the central projections of a set of 8 identified sensory elements in the wing of the fiuitfly Drosophila melanogaster, whose cuticular components appear identical by light and scanning electron microscopy, and whose cell bodies normally differentiate in a sequence that does not correspond with their position. We find that in this system the region of the CNS to which a neuron projects is related to the time of its birth and differentiation, rather than to its peripheral position. This time-related axonal distribution pattern remains stable even when the numbers or locations of the neurons contributing to it are altered by genetic manipulation. A preliminary account of these results has been published (Wigston et al., 1984).Ongoing studies indicate that the physiological properties of these receptors are not identical, in spite of the structural similarity of the cuticular elements, so that a three-way correlation between time of differentiation, central projection, and physiological function is established (Dickinson and Palka, 1985, and unpublished observations).Received Oct. 2, 1985; revised Dec. 9, 1985; accepted Dec. 18, 1985. We thank W. A. Harris. C. E. Holt. E. R. Ma-o. R. K. Murnhev. and members of our laboratory for commenting bn the ma&s&ipt; Eric &e&r painstaking selection of useful genetic variants; Usha Rani for patient technical help; and especially Peter Kareiva for carrying out the statistical analysis.

We transplanted external intercostal muscles from one of several thoracic (T) levels to the neck of adult rats. The cervical sympathetic trunk, which innervates the superior cervical ganglion, was cut, and its proximal end was apposed to the muscle. Preganglionic axons in the trunk reinnervated muscle fibers in the transplants. We determined the segmental origin of synaptic inputs to transplanted muscles by recording intracellularly from muscle fibers while stimulating individual ventral roots which supply axons to the trunk. In one series of experiments, T2 or T8 muscles were transplanted from the thorax to the neck of the same rat. While T2 and T8 muscles were reinnervated to a similar extent, they differed in the segmental origin of the innervation they received: T2 muscles received more inputs from rostral segments (T1 and T2) than did T8 muscles, and T8 muscles received more inputs from caudal segments (T4 to T6) than did T2 muscles. This difference between reinnervation of T2 and T8 muscles was detected both 2 to 4 weeks and 10 to 14 weeks after surgery. In a separate series, using rats of an inbred strain, T3, T4, or T5 muscles were transplanted from one rat to a separate host. Again, the average segmental origin of inputs to transplants from different levels differed systematically: it was most rostral to T3 muscles, intermediate to T4 muscles, and most caudal to T5 muscles. Finally, T3 and T5 muscles were soaked in a myotoxin, Marcaine, before reimplantation. This treatment kills muscle fibers but not myoblastic satellite cells; therefore, muscle fibers were replaced by regeneration. Marcaine-treated T3 and T5 muscles were successfully reinnervated but did not differ significantly in the segmental origin of their inputs. Our results show that adult mammalian muscles can be selectively reinnervated, and they raise the possibility that the selectivity is based on some positional quality that matches axons and muscles from corresponding segments. However, while differences among muscles survive denervation and transplantation, their expression or accessibility may change during regeneration.

The segmental pattern of motor innervation of hindlimb muscles in the axolotl was studied before and after reinnervation. To ascertain the specificity of reinnervation, the four spinal nerves innervating the hindlimb were severed and allowed to regenerate. The segmental origin of axons reinnervating particular muscles was then determined by intracellular recording from muscle fibers. Muscles were reinnervated in a specific manner: From the outset, the axons reinnervating each muscle originated largely from segmentally appropriate spinal nerves in the proper proportions, suggesting that a reliable mechanism of selective synapse regeneration exists even in mature axolotls. To examine the selectivity of reinnervation, individual muscles were transplanted to novel positions within the limb and the specificity of their reinnervation determined. Even after being moved to new positions, muscles were reinnervated for the most part by axons of appropriate segmental origin. Therefore, cues must exist on or within limb muscles that regenerating motor axons recognize and use to discriminate between different muscles during synapse formation. These results suggest that one of the mechanisms that promote the reestablishment of correct connections during reinnervation of axolotl limbs may be the selective formation of synapses with appropriate target cells.

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SUMMARY1. The supracoracoideus (s.c.) muscle of the axolotl shoulder is innervated by two nerves, the anterior and posterior s.c. nerves. The posterior nerve was induced to make synapses outside its normal territory in the muscle by removing a segment of the anterior nerve. Intracellular recording indicated that the efficacy of transmission from posterior nerve terminals outside their normal territory increased over several weeks prior to the return of the anterior nerve.2. The anterior nerve reinnervated its muscle by 40-50 days after the operation, and quickly made synapses throughout the muscle. The posterior nerve territory subsequently returned to its original size and location over 3-6 months.3. Transplantation of either of two completely foreign nerves into s.c. muscles with enlarged posterior nerve territories resulted in a similar return of the posterior nerve territory to its normal size when anterior nerve regeneration was prevented.4. These results suggest that the advantage which newly regenerated native nerves have over sprouted foreign nerves is not the quality of 'nativeness' but rather the smaller number of synapses they support. In this view, sprouted nerves compete less effectively because they initially support more synapses per neurone than regenerating nerves.