Biomedical Engineering Reference
In-Depth Information
for in vitro studies of myelination; in contrast, the sympathetic superior cervical
ganglion supports outgrowth of axons that are predominantly nonmyelinated and
provides a model for study of ensheathment, rather than myelination, of axons by
SCs (Obremski and Bunge 1995).
Synthesis of a myelin sheath around axons in vitro has been reliably obtained
by exposing the axons and SCs in culture to a surface plated with type I collagen
(Bunge and Bunge 1978). The presence of an ECM component as a hypothetical
requirement for myelination of axons by SCs was supported by the data (Moya et al.
1980). In another study, a reconstructed BM was added as a reactant in a neuron-
SC culture that led to synthesis of a myelin sheath (Carey et al. 1986). A variety of
culture media were studied in an effort to clarify the requirements for myelination
and several observations were made suggesting that axons and SCs suffice for my-
elination (Carey and Todd 1987; Eldridge et al. 1987, 1989; Clark and Bunge 1989).
The hypothesis for an ECM requirement was further challenged in a study of the
embryonic dorsal root ganglion neuron that was cultured with SCs, in which a fully
defined medium was used, rather than serum; as a result, synthesis of extracellular
matrix was effectively suppressed. Myelination was nevertheless observed to occur
in this study, showing that ECM synthesis, including synthesis of a BM, may be
linked to myelination but is not a prerequisite for the synthesis of a myelin sheath
in vitro (Podratz et al. 1998).
BM has been synthesized in vitro when both neurons and SCs were present in
culture (Bunge et al. 1980, 1982; Cornbrooks et al. 1983; Carey and Todd 1987;
Eldridge et al. 1987, 1989; Clark and Bunge 1989). In the absence of neurons, SCs
have been observed to synthesize components of the BM (McGarvey et al. 1984);
even synthesis of a filamentous matrix resembling a BM has been observed (Baron-
Van Evercooren et al. 1986). The presence of neurons was, however, not required
for synthesis of BM when fibroblasts were cultured with SCs; even the presence of
fibroblasts was not required when purified laminin was added to the neuron-free SC
culture (Obremski et al. 1993; Obremski and Bunge 1995).
The preceding discussion has been limited to in vitro conditions. However, in
vivo synthesis of conducting nerve fibers has been described in a very large number
of experimental protocols, most of which consisted of a tubulated nerve gap gener-
ated by transection. Under these conditions, not only nerve fibers but also other tis-
sue components of a peripheral nerve were synthesized. The description of these in
vivo pathways for the synthesis of nerve fibers will be presented in a later section,
in the context of procedures for the synthesis of an entire nerve trunk.
In one noteworthy protocol conducted in vivo, however, a specialized experi-
mental configuration was used to ensure that SCs would be isolated from axons.
The silicone chamber bridging a 10-mm gap in the rat sciatic nerve was prepared
by tubulation of the transected nerve stumps. The chamber was exogenously filled
with a collagen matrix seeded with SCs; however, the tube was closed at both ends
by a Millipore filter that excluded axons and allowed the entry of exudate from the
stumps. Even though axons were absent from this in vivo configuration, it was ob-
served that BM was synthesized around the SCs (Ikeda et al. 1989).
