Biomedical Engineering Reference
In-Depth Information
Significant evidence for the synthesis of a perineurium in vivo was obtained in
a 26-week study of the transected 10-mm gap in the rat sciatic nerve, bridged by
a silicone tube (Azzam et al. 1991; Fig.
6.3
). In this study, the newly synthesized
nerve trunk comprised an outer circumferential layer of connective tissue and an
inner core in which all myelinated and nonmyelinated axons were gathered into
minifascicles surrounded by sheaths of perineurial cells. The structure that was in-
terpreted as a perineurium consisted of a sheath surrounding bundles of axons with
SCs in each minifascicle. The sheath comprised 3-5 lamellae of flattened cells with
tight junctions and endocytotic vesicles, each cell possessing a BM on its inner
and outer surfaces. Collagen fibrils between the lamellae were not reported in this
study. Strong support for the interpretation of the sheath as a physiological perineu-
rium was obtained in the same study by observing that the sheath was a competent
permeability barrier (Azzam et al. 1991). In the same study, the region of the trunk
lying between the outer circumferential connective tissue layer and the inner neural
core was explored in a search for the single perineurial sheath of large diameter,
surrounding the entire cylindrical trunk that might have hypothetically been regen-
erated in its original location. No distinct perineurium was observed at this location
(Azzam et al. 1991).
A description of newly synthesized perineurial sheaths was reported in a study
in which a 5-mm gap in the rat sciatic nerve was bridged by wrapping a ribbon of
plastic film (not further identified) around the stumps, thereby creating a tube that
contained a short segment of the severed nerve at each end. After 4 weeks, slender
cell processes with all the characteristics of perineurial cells were observed sur-
rounding bundles of nerve fibers. The cells were almost completely lined by BM,
their cytoplasm contained numerous endocytotic vesicles, and their thin processes
were in contact with one another at their edges by means of tight junctions; there
was also an abundance of collagen fibrils between the cell layers (Scaravilli 1984).
Regeneration of an epineurial sheath has not been reported; however, it should
be recalled that many of the studies with tubulated nerves were conducted with
unifascicular nerves that lack a prominent epineurium.
In several studies, a thick sheath of tissue, with the obvious appearance of a very
tight, multilayer wrapping, was reported surrounding the entire nerve trunk that was
regenerated inside silicone tubes (Lundborg et al. 1982a). Authors have occasion-
ally referred to this outer cover as a perineurial sheath. The structure of the sheath
was examined and found to consist of circumferential cells that lacked, however,
the encasement in BM and intercellular tight junctions characteristic of a mature
perineurium (Williams et al. 1983). The cells comprising the multi-laminated outer
tissue of nerve trunk regenerated in a silicone tube were eventually identified as
contractile cells (myofibroblasts), a characteristic of the early as well as chronic
response of the nerve to transection (Chamberlain et al. 2000a). Further discus-
sion of the significance of the myofibroblast capsule around the nerve trunk in the
mechanism of nerve regeneration appears later (Chap. 8).
In conclusion, confirmed synthesis of a perineurium inside a nerve trunk regen-
erated in tubulated configurations has been reported. The morphology of the peri-
neurial sheath, as well as its permeability barrier, has both been reported to be close
