Size and location of the motoneuron pool supplying normal and orthotopically transplanted muscles.

Following intramuscular placement of horseradish peroxidase (HRP) into the 129ReJ mouse extensor digitorum longus muscle, 15 +/- 1 (S.E.M.) labeled motoneurons were found in the antero-lateral motor column, between the exits of spinal roots L3 and L4. When HRP was placed in 100-day orthotopic whole muscle transplants of the extensor digitorum longus muscle, the location of the motoneurons supplying the graft was similar to that of control muscle; however, the number of motoneurons innervating the grafts was variable and usually reduced (7 +/- 1 S.E.M.).