The fine structure of the outer capsule of the snout muscle spindle in the Japanese lesser shrew-mole.

The fine structure of the outer capsule of the snout muscle spindles of the Japanese lesser shrew-mole (42 spindles from 9 animals) was described electron microscopically. The capsule of the snout muscle spindle is composed of 5 to 6 layers of the so-called capsule cells derived from the perineural epithelium of the peripheral nerve which supplies the spindle. The capsule cells revealed the presence of numerous pinocytotic vesicles, granular endoplasmic reticulum, abundant ribosomes, mitochondria, dense bodies, microvesicles and Golgi complex. It would be highly suggestive of their secretory or transporting activity. The various types of plication within the capsule of the muscle spindle were detected in the equatorial region. By means of such projections the functioning surface of the outer capsule has been enormously increased.