The development of muscle spindles in the rat

Abstract

The morphogenesis of muscle spindles in rat lower hindlimb muscles has been investigated using the electron microscope. The earliest detectable spindles are seen in the 19.5-day foetus and consist of a single myotube bearing simple nerve terminals of the large primary afferent axon from the nearby unmyelinated spindle nerve trunk. The capsule forms by an extension of the perineural epithelium of the supplying nerve fasciculus, and is confined initially to the innervated zone. Myonuclei accumulate in this region, so that the first intrafusal muscle fibre to develop is a nuclear-bag fibre. Myoblasts, that are present within the axial bundle throughout its development, fuse to form a smaller, less-differentiated myotube by the 20-day foetal stage. This matures in close association with the initial fibre, and by birth (21-22 days gestation) has formed the smaller intermediate bag fibre that has been identified histochemically and ultrastructurally in the adult. Nuclear-chain fibres develop in the same way; myoblasts fuse to form satellite myotubes that mature in apposition to one or more of the other fibres, lying within a common basement membrane. By the 4-day postnatal stage the full adult complement of 4 intrafusal muscle fibres is present, although the ultrastructural and histochemical variations, seen in the adult, are not present. The fusimotor innervation begins to arrive by birth, but is not fully established until the 3rd postnatal week, when the ultrastructural and histo chemical maturation of the axial bundle is complete. The maturation of the capsule and periaxial space occurs at the same time. It is suggested that the sequential development of the intrafusal fibres is a reflection of the decreasing morphogenetic effect of the afferent innervation, whereas the role of the fusimotor innervation is in ultra- structural and histochemical differentiation.