Early changes in extrafusal and intrafusal muscle fibers following heterochronous isotransplantation

The ultrastructure of regenerating intrafusal and extrafusal fibers was stu died 18 h to 30 days after heterochronous isotransplantation, in which bupi vacaine-treated extensor digitorum longus (EDL) or soleus muscles from earl y postnatal rats were intramuscularly grafted into EDL muscles of adult inb red recipients. As in other models of mammalian muscle regeneration, surviv ing satellite cells gave rise to presumptive myoblasts, multiplying within the preserved basal lamina tubes at day 4 after grafting. Myoblasts fused t o form myotubes with central myonuclei by day 6 after grafting. Extrafusal myotubes differentiated into thin muscle fibers by day 8, which progressive ly increased in diameter and their nuclei became localized subsarcolemmally from day 13 onwards. The basal laminae of some intrafusal fibers already c ontained one or more nascent myotubes by day 4 after grafting. Regenerated intrafusal fibers lacked the typical nuclear accumulations and their number varied from one to eight fibers per spindle; additional fibers formed in t he periaxial space or between layers of the capsule. Regenerated muscle spi ndles usually had a thinner outer capsule and a reduced inner capsule and p eriaxial space. The present study demonstrates that extrafusal and intrafus al muscle fibers degenerate and regenerate after heterochronous isotranspla ntation in a manner similar to that in standard grafts. However, the time c ourse is slightly different. Degeneration was completed by day 5 after graf ting as in free grafts, but the regeneration of extrafusal and intrafusal f ibers started 1 or 2 days earlier, apparently because of the rapid and faci litated revascularization from the host muscle compared to that of standard grafts.