Biomedical Engineering Reference
In-Depth Information
emphasizing the comparison of properties of experimental regenerates with the
highest limit of performance, the normal nerve. The experimental timescale for the
latter type of investigation often extends to 1 year or even beyond (Fields and Ellis-
man et al. 1986a, b; Le Beau et al. 1988; Fields et al. 1989; Archibald et al. 1991,
1995; Tountas et al. 1993; den Dunnen et al. 1993a, b; Chamberlain et al. 1998b).
Below we discuss the selection of an assay for each of these two very different ex-
perimental timescales.
6.1.3
Short-Term Quantitative Assays (
<
20 weeks): Frequency
of Reinnervation
Unlike studies with the synthesis of skin, outcome data from in vivo studies of
peripheral nerve regeneration have been mostly quantitative. However, although
extensive, often covering several weeks or even months of kinetic information, the
data often cannot be used to extract direct, useful relations between reactants em-
ployed and tissues synthesized. In addition, as mentioned in Chap. 2, the emphasis
on axon morphology (axonocentric viewpoint) and the relative neglect of data on
tissues that are not spontaneously regenerative, such as the endoneurium, has gener-
ated an imbalance in information germane to studies of induced regeneration.
A further reason for lack of data that could be used to directly extract useful
reactant-outcome relations is the large, almost random, proliferation of the types of
outcome assays. Data on morphology of PNS regeneration have been presented in
several ways, including number of unmyelinated or myelinated axons, axon diam-
eter distribution, thickness of myelin sheath, percent of defects bridged by nerve fi-
bers, cross section area of regenerated nerve trunk, level of vascularization, density
of SCs, presence of endoneurial collagen, number of axons per fascicle, density of
fascicles, and so on. Diverse types of data describing several electrophysiological
parameters, either of regenerated motor or sensory nerves, have also been reported;
these have included, among others, the velocity and amplitude of the electric signal
conducted through the regenerate. A large variety of neurological and behavioral
tests (e.g., walking track tests, sweat gland reinnervation, pin-prick testing) has also
been used to assess the extent of regeneration. The complexity increases rapidly to
over 30 outcome assays or so when one considers that investigators have exercised
additional degrees of freedom in planning their experiments, including choice of the
gap length between the transected stumps, time of observation (1-150 weeks), and
location of observation along the length of the regenerate (proximal stump, center
of gap, distal stump), as well as variation in selection of an animal species. Although
each of these outcome assays conveys valuable information, the lack of one or two
standardized and widely used assays quickly defeats a reviewer's attempt to make
use of the literature in order to identify the simplest protocols required to induce
synthesis of specific peripheral nerve tissues.
Fortunately, there is a widely used dimensionless measure of the synthesis of
a nerve trunk with myelinated axons along a gap: the frequency of reinnervation
across a tubulated gap (
%N
), reported as percent of defects (one defect per animal)
