Xm. Wang et al., EARLY DEVELOPMENT AND DEVELOPMENTAL PLASTICITY OF THE FASCICULUS GRACILIS IN THE NORTH-AMERICAN OPOSSUM (DIDELPHIS-VIRGINIANA), Developmental brain research, 98(2), 1997, pp. 151-163
The first objective of the present study was to ask when axons of the
fasciculus gracilis reach the nucleus gracilis in the North American o
possum (Didelphis virginiana). When Fast Blue (FB) was injected into t
he lumbar cord on postnatal day (PD) 1 and the pups were killed 2 days
later, labeled axons were present within a distinct fasciculus gracil
is at thoracic and cervical levels of the cord. When comparable inject
ions were made at PD3 or 5 and the pups were allowed to survive for th
e same time period, a few labeled axons could be followed to the cauda
l medulla where they were located dorsal to the presumptive nucleus gr
acilis. In order to verify these observations and to determine if any
of the axons which innervate the nucleus gracilis early in development
originate within dorsal root ganglia, we also employed cholera toxin
conjugated to horseradish peroxidase (CT-HRP) to label dorsal root axo
ns transganglionically. When CT-HRP was injected into the hindlimb on
PD1 and the pups were maintained for I day prior to death and HRP hist
ochemistry, labeled axons were present within the fasciculus gracilis
at thoracic and cervical levels, but they could not be traced into the
medulla. When comparable injections were made on PD3, and the pups we
re maintained for 2 days, labeled axons were present within the caudal
medulla. Our second objective was to determine whether axons of the f
asciculus gracilis grow through a lesion of their spinal pathway durin
g early development. In one group of animals, the thoracic cord was tr
ansected at PD5, 8, 12, 20 and 26 and bilateral injections of Fast Blu
e (FB) were made four segments caudal to the lesion 30-40 days later.
After a 3-5 day survival, the pups were killed and perfused so that th
e spinal cord and brainstem could be removed and sectioned for fluores
cence microscopy. In all of the cases lesioned at PD5, axons of the fa
sciculus gracilis were labeled rostral to the site of transection and
they could be followed to the nucleus gracilis. Evidence for growth of
fasciculus gracilis axons into the caudal medulla was also seen in ca
ses lesioned at PD8. In contrast, labeled axons were not observed rost
ral to the lesion when it was made at PD12 or at later stages of devel
opment. In order to verify that some of the axons which crossed the le
sion originated within dorsal root ganglia, the thoracic cord was tran
sected at PD5 in another group of animals and 7 days later, injections
of CT-HRP were made into one of the hindlimbs. After a 3 day survival
, labeled axons could be traced through the lesion site and into the c
audal medulla. We conclude that axons of the fasciculus gracilis reach
the nucleus gracilis by at least PD5 in the opossum and that they gro
w through a lesion of their spinal pathway when it is made at the same
age or shortly thereafter. The critical period for such growth appear
s to end between PD8 and PD12.