Re-innervated extensor digitorum longus, soleus and plantaris muscles of the rat were studied after denervation performed at various postnatal ages. The muscle fibres, which normally run from tendon to tendon as independent units, were found to be very frequently connected by myomuscular junctions, both in the form of terminal insertions of one fibre into another and of lateral bridges which may join two or more muscle fibres at one or more levels. Positive reaction for AChE activity was demonstrated at the level of the junctions. Incubation for myosin ATPase activity showed that myomuscular junctions are only found between fibres of the same histochemical type, which in re-innervated muscles are usually aggregated in 'type groupings'. Ultrastructural features were similar in both forms of myomuscular junctions. The appearance is that of an interdigitation of muscle projections from neighbouring fibres, each projection being covered by a basement membrane with attached collagen fibrils. The finger-like projections at their endings contain vesicles and elongated cisternae filled with granular dense material. It is postulated that the synchronous activity of neighbouring fibres within the compact motor units of reinnervated muscles is a causal factor initiating the formation of myomuscular junctions.
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