Development of the mature distribution of synapses on fibres in the frog sartorius muscle.

Abstract

Most of the fibres in mature frog sartorius muscle possess two or more synapses separated by up to one-third the length of the muscle. The aim of the present work was to determine how the relative distances between these synapses changes during development in the frog (Limnodynastes tasmaniansis), as the fibres increase in length from 2 mm (stage 56) to 20 mm (1 year postmetamorphosis). At the earliest stage investigated (fibres 2.0-4.0 mm in length; stages 56-57) about 80% of the fibres were innervated at two endplates. The percentage of fibres with two endplates then remained approximately constant with further development. The polyneuronal innervation of endplates was almost eliminated by stage 57. Muscle fibres with two endplates had each situated on average about one-third the length of the fibre from a tendinous insertion; these relative positions did not change throughout development. Thus the distance between endplates increased linearly with an increase in fibre length. The size of terminals and the complexity of their branching also increased continually throughout development, independently of the location of the terminals on the fibres. The observations suggest that the distance between terminals increases during development because of the intercalation of new plasma membrane and basal lamina associated with the increase in length and diameter of fibres.