Microscopic study of muscle spindles in recti muscles of human fetal eye.

Muscle spindles of recti muscles of the eyebulb appear in the 11th week of intrauterine life. At first, the spindle manifests the stage of myotube with 2-3 intrafusal fibers and is surrounded by the monolayer of internal capsule. In the course of subsequent development, the number of intrafusal fibers increases. Internal capsule begins to demonstrate two layers (14th week) and, then, external capsule develops (17th week). In the 20th week, mature spindles have been observed with a multilayered external capsule.