Development of muscle spindles in man

Conclusions

• 1. The anlage of the muscle spindle is the “spindle nerve fiber complex.” It can be found in some muscles of a 6.5-week human embryo, but more regularly in the 7–8-week embryo.

• 2. Initial signs of differentiation of intrafusal elements, marked by the appearance of a nuclear bag in the equatorial region of the future spindle, are observed in the rectus abdominis, triceps brachii and other muscles in the 9-week human fetus, i.e., 3 weeks sooner than the earliest time recorded in the literature (Cuajunco, 1940).

• 3. A period of one week from the beginning of differentiation of the intrafusal bundle is needed for differentiation of all the component parts of the spindle to begin. Formation of the spindle capsule as a stratified sheath and formation of the perilymphatic space occur later in the course of human embryogenesis.

• 4. The muscle spindles of the human fetus are supplied with afferent endings of the flower-spray type.

• 5. Asynchronism is observed in the order of appearance and rate of differentiation of the muscle spindles in muscles serving different functions.