Motor innervation within supernumerary legs of cockroaches

An i d e n t i f i e d cockroach motoneuron, mesothoracic c e l l 30, t h e f a s t e x c i t a t o r y motoneuron t o t h e coxal depressor muscles, has been found t o make connections w i t h i n a p p r o p r i a t e muscles a f t e r regenerating t o a l e g from which i t s c o r r e c t muscles had been excised. Thus t h e s p e c i f i c i t y o f i n n e r v a t i o n shown by motoneurons regenerating t o a normal, i n t a c t l e g cannot be due s o l e l y t o a one-to-one matching o f motoneurons w i t h muscles.I n many vertebrates muscles can be innervated by f o r e i g n motor nerves e i t h e r during e a r l y stages o f regeneration (Schmidt and Stefani, '76; Cass e t al., '73) o r i f t h e c o r r e c t nerve i s r e s t r a i n e d from growing back t o i t s muscle (Sperry and Arora, '65). This c o n t r a s t s w i t h t h e s i t u a t i o n i n t h eHere I r e p o r t t h a t an i d e n t i f i e d cockroach motoneuron, deprived of t h e o p p o r t u n i t y o f i n n e r v a t i n g i t s c o r r e c t muscle, can innervate an i n c o r r e c t muscle. The neuron under study was t h e f a s t e x c i t a t o r y motoneuron t o t h e depressor muscles o f t h e mesothoracic l e g ( f i g .

Incorrect connections made by a regenerating cockroach motoneuron

Whitington

1977

Pattern regulation of mesoderm in transplanted and supernumerary limbs of the cockroach. I. The final morphology

Steinbach

Josephson

1980

Muscles, nerves, and tracheae were examined histologically in clusters of three limbs arising from single mesothoracic leg sockets in the cockroach, Periplaneta americana. The three-limbed preparations were produced by amputating a normal leg and replacing it with a contralateral leg from a donor animal, with the anterior-posterior (A-P) axis of the graft reversed relative to the A-P axis of the host. After a few molts, two supernumerary limbs develop from the hostigraft junction, one anterior and one posterior to the graft. Three to nine months after the operation, femurs from the supernumerary and graft limbs possess identifiable muscles which are, however, not as well developed as those of normal limbs. Supernumerary coxae rarely possess identifiable musculature, but distal regions of the transplant coxae contain muscles of nearly normal morphology. Near the hostigraft junction, the coxal muscles are disorganized, producing a proximal to distal gradient in muscle organization within the coxa. The extensive disorganization near the hostigraft junction is primarily a result of the reversed axis, since in grafts of ipsilateral limbs without axis reversal, there is considerably less muscle disorganization near the hostigraft junction. Large nerves and large tracheae were often located in supernumerary and graft femurs and in graft coxae in approximately the same positions as in normal limbs.

Pattern regulation of mesoderm in transplanted and supernumerary limbs of the cockroach. II. Time course of muscle dedifferentiation

Steinbach

1980

The extent and time course of muscle dedifferentiation was followed histologically in transplanted mesothoracic limbs of the cockroach Periplaneta americana. The limbs were grafted to contralateral leg sockets and were oriented such that the anterior-posterior axis of the graft was reversed relative to that of the socket. The muscles in such a graft dedifferentiate before the first postoperative molt. Muscle dedifferentiation is characterized by the loss of muscle fibers and the appearance of mononuclear cells in the graft. Dedifferentiation begins proximally near the hostlgraft junction and progressively spreads to distal levels until the only remnants of the original muscle organization are aggregates of mononuclear cells around distal apodemes. Muscles in the thorax which were damaged during transplantation also dedifferentiate before the first postoperative molt. Redifferentiation of identifiable muscle can begin as early as the second intermolt period. The dedifferentiation of transplanted mesoderm is not due to the axis reversal, since the muscles in a graft without axis reversal dedifferentiate to the same extent as in grafts with reversed transverse axes. The results are interpreted as indicating that developmental information from nonmuscular tissues in the limb is required for reformation of identifiable muscles.