Biomedical Engineering Reference
In-Depth Information
been trimmed before suturing, the changes observed after the surgery were very
similar. The normally compact layers of the perineurium separated considerably
from each other and lost part of their basement membrane. Axons and Schwann
cells were observed between the layers of perineurial cells, a clear departure from
the structure of an intact perineurium. Myelinated axons penetrated through the scar
that had formed at the repair site and were observed to enter and leave the suture
line, bridging the short gap between the stumps; however, no bridging of the gap
between the ends of the divided perineurium in each stump with any tissue resem-
bling perineurium was observed, even after 42 days (Behrman and Acland 1981).
The dependence of regenerative potential of the perineurium on the type of in-
jury that had been sustained was shown in a study in which the perineurial sheath
was stripped off the sciatic nerves of rats over a 5-mm length, at a segment along
the length where the nerve consists of a single fascicle. Care was taken not to in-
jure the axons inside the intrafascicular space immediately underneath the stripped
perineurial sheath, and a record of specimens that had inadvertently suffered such
damage was carefully maintained. In several undamaged specimens, in which, no
degenerative changes were observed in axons present immediately underneath, it
was reported that an apparently normal perineurial sheath had been formed as early
as 10 days, extending along the length that had been injured. It was hypothesized
that the new perineurium had been synthesized by endoneurial fibroblasts migrating
from within the fascicle (Nesbitt and Acland 1980). Consideration of this data with
other related data described above (Behrman and Acland 1981) leads to the intrigu-
ing conclusion that the regenerative potential of the perineurium is very high pro-
vided that the injury is entirely confined to it; when the injury extends deeply to the
intrafascicular space (endoneurium) underneath it, the injury becomes irreversible.
The available evidence shows that the transected perineurium does not regen-
erate either in its original structure or at the original anatomical site; new peri-
neurium-like tissue is, however, synthesized around the minifacicles in the new,
compartmented nerve trunk that results from healing. Synthesis of a new perineu-
rium with altered structure outside its original anatomical site does not constitute
regeneration, at least in the sense in which the term was defined in Chap. 1; yet,
the morphology of the new tissue is approximately physiological. In view of the
evidence that the outcome depends on experimental conditions that require further
study, the perineurium will be referred below as a conditionally regenerative tissue.
Very little evidence has been collected specifically about the regenerative poten-
tial of the epineurium. Especially lacking are data following well-defined traumatic
injury, such as transection. Although several authors have reported the presence of
“neural scar” at each stump following transection, a clear association of such tissue
with scarring of the epineurium (epineurial fibrosis) was not made. Several investi-
gators have reported epineurial fibrosis following a variety of insults on peripheral
nerves, including chronic compression (Mackinnon et al. 1986), exposure to anes-
thetics (Barsa et al. 1982), and saline neurolysis (Frykmann et al. 1981).
We conclude that, following complete transection of a peripheral nerve, the en-
doneurial stroma, and possibly the perineurium as well, are not spontaneously re-
generated.
