Biomedical Engineering Reference
In-Depth Information
3.5
Size of Experimental Volume
Having considered the identity of tissues that must be deleted from the experimental
volume, we now focus on the quantity of tissue that requires deletion. A lower limit
is set by the need to demonstrate a regenerative effect, or its absence, with sufficient
tissue mass to provide ample opportunity for a definitive analysis. An important
upper limit of wound size is usually set by the need to maintain the experimental
animal model as free of morbidity as possible.
In an effort to identify the lower limit one may well revisit the question, posed
qualitatively in Chap. 1, whether there is a wound of irreducible size, a critically
sized defect, above which the response to injury changes from spontaneous regen-
eration to repair. If a limit of that type exists, it makes sense to make sure that it is
exceeded in order to study induced regeneration in a wound that is as free of sponta-
neous regeneration as possible. For example, it has been mentioned earlier that skin
wounds smaller than 2 mm diameter in the lamb fetus, including incisional wounds,
healed without scar while larger wounds healed with scar (Cass et al. 1997a, b).
Control of the skin wound size is, therefore, a critical experiment variable that re-
quires control. Studies with larger skin wounds showed quite different results. For
example, the kinetics of contraction of a spontaneously healing full-thickness dorsal
wound in the rabbit were not significantly affected as the initial wound area was re-
duced to 50 % or even to 25 % of its baseline value (Billingham and Russell 1956).
In another study, the kinetics of skin wound contraction in the rat model were not
affected when the size of the wound was increased by a factor of 1.8 (Kennedy and
Cliff 1979) nor was the incidence of induced regeneration affected by a change in
skin wound size by a factor of 3.6 (Orgill 1983). If the range in wound size studied
is increased considerably more, however, one would expect to observe quantitative
effects potentially arising from heterogeneity in skin tethering, skin thickness, or
encounter problems in anatomical locations that seriously affect wound care and in-
crease morbidity (e.g., wounds in joints). Increase in skin wound size was observed
to delay closure in humans (Ubbink et al. 2013).
Evidence for existence of a critically sized defect (CSD) appears persistently
in the literature of peripheral nerve regeneration. For example, in the absence of a
conduit (tubulation) connecting the two nerve stumps produced by transection, it
was observed that a 2-mm gap in the mouse sciatic nerve led to a 20 % frequency
of spontaneous reinnervation while a gap of 4 mm or longer consistently yielded
a reinnervation frequency of zero (Butí et al. 1996). In the rat sciatic nerve, an
untubulated 5-mm gap was bridged spontaneously by a nerve trunk (Thomas and
Jones 1967) while a 15-mm gap resulted in no regeneration at all (Lundborg et al.
1982a). (A detailed review of studies of peripheral nerve regeneration is discussed
in Chap. 6). The CSD concept also appears in the literature of bone healing. There
is an evidence that defects in bone that exceed several millimeters in characteristic
size do not spontaneously heal to form a mechanically stable, osseous tissue but
result, instead, in formation of a “nonunion” (Wornom and Buchman 1992). For
example, an increase in size of a defect in the rat skull (calvaria) from 4 to 8 mm
led to a sharp drop in the bone content of the newly formed tissue that bridged the
