Development of excitatory innervation in the lobster claw closer muscle

The development of excitatory innervation to the claw closer muscles of 1st (larval), 4th (juvenile), and adult stage lobster was examined by thin serial section electron microscopy. This was possible since neuromuscular terminals of the excitatory axon are distinguishable from those of the inhibitory axon by the shape of their synaptic vesicles in all three stages. In the adult cutter claw closer muscle, innervation of a dorsal and a ventral fiber which is supplied by the fast closer excitor (FCE) and slow closer excitor (SCE) axons respectively, was qualitatively as well as quantitatively similar. Consequently no attempt was made to distinguish between innervation by FCE and SCE axons in the subsequent analysis of 1st, 4th, and adult stage claw closer muscles. Excitatory innervation in the 1st larval stage is limited to four discrete locations in the entire cross-sectional area of the closer muscle. It subsequently spreads to each individual muscle fiber in the adult, thus demonstrating the tremendous proliferation of innervation during development. Concomitantly the mean size of synapses increases significantly from the 1st stage to the 4th stage to the adult lobster. This increase in synaptic size may occur by both the fusion and enlargement of existing smaller synapses. In contrast, the mean size of presynaptic dense bars and their mean number per synapse remained fairly constant in each of the 1st, 4th, and adult stages. However, a relatively greater proportion of adult synapses possessed two or more dense bars compared to their larval and juvenile counterparts. Development of innervation of the lobster claw closer muscle therefore consists of a substantial proliferation of axonal tissue and enlargement of synaptic size.