Stages in the development of cat muscle spindles.

The structure of developing spindles has been examined in cat peroneal muscles by light and electron microscopy, beginning at the 34- to 38-day foetal stage. By this stage alpha motoneurons have formed end-plates on primary myotubes. Secondary extrafusal myotubes then develop beneath the basal lamina of primary myotubes, and are innervated by motor axons early in their assembly. First-series secondary myotubes separate from primary myotubes prior to the development of subsequent series. The assembly of extrafusal fibres is completed by birth. Intrafusal fibres assemble in a similar manner. At the 34- to 38-day foetal stage developing spindles consist of a single primary myotube containing a small accumulation of myonuclei beneath the terminals of the Ia afferent axon. Simple motor nerve terminals also innervate this myotube, which will ultimately become the bag2 fibre of the mature spindle. Secondary intrafusal myotubes then assemble beneath the basal lamina of the primary bag2 myotube, in the order presumptive bag1, long-chain, intermediate-chain and typical-chain fibres. Their assembly begins at the equator, beneath the sensory terminals, and spreads to the poles. The bag1 and long-chain myotubes separate from the bag2 in the spindle pole prior to the development of the other chain fibres. The assembly of intrafusal fibres is completed by birth. The periaxial space begins to develop in the first postnatal week. The development of tandem spindles containing b2c units is described. The role of sensory and motor innervation in the assembly and differentiation of mammalian intrafusal fibres is discussed.