Development of neuromuscular junctions of fast and slow muscles in the chick embryo: a light and electron microscopic study.

The morphogenesis of neuromuscular injections (NMJ) was studied by electron microscopy in fast posterior and slow anterior latissimus dorsi muscles (PLD and ALD) of chick embryos. In 8 day embryos, the NMJ is already established in both types. In PLD at this stage, individual axons completely ensheathed by Schwann cell processes form NMJs with myotubes, whereas in ALD axon terminal lie adjacent to (and not separated by Schwann cell processes from) naked axons which are components of a nerve bundle. At 11-15 days, the number of profiles of axon terminals at each endplate increases in both PLD and ALD. In PLD, individual axon terminals are ensheathed by Schwann cells and often branch. In ALD, several axon terminals become ensheathed as a group by processes of a Schwann cell, along with a small number of adjacent naked axons. The individual axon terminals were confirmed by analysis of serial sections to originate from different preterminal axons. Thus, the increase in number of axon terminals in PLD may be due to extensive terminal branching, whereas in ALD it may be due to the arrival of other nerve fibres. From 16 days, each axon terminal in an endplate of ALD becomes individually ensheathed by Schwann cell processes. However, the property of polyneuronal innervation in each endplate is retained even in the adult muscle. The junctional sarcoplasm protrudes to separate individual axon terminals at certain developmental stages: the protuberances are thinner and more numerous in ALD than in PLD at 15-16 days. It is concluded that NMJ morphogenesis differs between PLD and ALD and that the differences reside mainly on the neuronal side.