Differentiation of identifiable lobster neuromuscular synapses during development

The ultrastructure of physiologically identified low and high release synapses arising from a single axon on fibres of the distal accessory flexor muscle (DAFM) in a mature lobster was examined by serial section electron microscopy. Low release neuromuscular terminals located only on the proximal fibre were characterized by large synapses (mean area 2.084 micron2), small presynaptic dense bars (mean are 0.021 micron2) and hence a low (2.3%) ratio of dense bar area to synaptic area. In contrast high output terminals located only on the distal fibre had smaller synapses (mean area 0.625 micron2), large dense bars (mean area 0.066 micron2) and a high (23.9%) ratio of bar area to synaptic area. A similar ratio was consistently found for each synaptic type in several other examples of mature lobsters. Hence it was used as a criterion for determining the point at which differentiation occurs during development. In the first larval stage (24 h old) the innervation was localized and undifferentiated. In the fourth (2 week old) and twelfth (1 y old) stage lobsters, the innervation had proliferated to small bundles of proximal and distal fibres. During development synapses increase in their mean surface area in the proximal fibre while remaining constant in the distal fibre. The mean surface area of the dense bars is similar in all stages except for the proximal fibres of the twelfth stage where it is smaller by 50%. Similarly the ratio fo dense bar area to synaptic area is not significantly different for all stages except for the twelfth stage proximal fibres where it is half the value. Consequently differentiation of low and high release neuromuscular terminals occurs by the twelfth stage with an increase in the mean surface area of synapses and a decrease in the mean surface area of dense bars. This morphological differentiation is enhanced in the mature lobster.