The fine structure of the equatorial regions of developing muscle spindles in the rat.

Abstract

The fine structure of developing muscle spindles in the gastrocnemius muscles of young Wistar rats has been examined at stages from 18 days gestation to 14 days after birth. Initial simple contacts between large unmyelinated intramuscular nerve terminals and apparently undifferentiated single myotubes at 18 days gestation led to the encirclement of the myotube by the nerve terminal, with concurrent extension of the nerve sheath to form a unilamellar capsule around the innervated region. Subsequent proliferation and fusion of satellite myoblasts associated with the innervated myotube gave rise to a succession of further intracapsular myotubes. At birth the original myotube and its first formed companion had differentiated into recognizable ‘nuclear bag’ intrafusal fibres, and by the third post-natal day a second generation of smaller diameter ‘nuclear chain’ myotubes could be distinguished. These developing intrafusal muscle fibres remained in a tightly packed bundle, having numerous interlocking pseudopodial extensions of their superficial cytoplasm, until some 3–6 days after birth, the sensory nerve terminal enwrapping the external surface of the whole bundle. Lateral separation of the intrafusal fibres during the second post-natal week was accompanied by the acquisition of individual sensory innervation, the development of an axial sheath distinct from the now multilamellar capsule, and the first appearance of the subcapsular space. The fine structure of the developing sensory nerve terminals closely resembled that of mature spindle terminals, having dense amorphous cytoplasm containing many small mitochondria and 40–60 nm vesicles.