Experimental Methods II. The Defect Closure Rule - Tissue and Organ Regeneration in Adults - page 77

4.3.4 Closure by Regeneration

Spontaneous regeneration of skin in adults has been observed to occur in the perfo-
rated ear of the rabbit and other lagomorphs, including hares and pikas; it included
recovery of hair follicles and sebaceous glands (Joseph and Dyson 1966; Goss and
Grimes 1972, 1975). Furthermore, deer antlers, shed and regrown annually, are also
spontaneously regenerated (Goss 1980, 1987). Spontaneous regeneration of tran-
sected peripheral nerves in adults across a distance of very few millimeters has been
observed in the mouse (Butí et al. 1996).
Induced regeneration of skin in adults using a collagen scaffold with highly spe-
cific structure has been observed in a number of instances that are described in
detail in Chap. 5. Peripheral nerve trunks have been induced to regenerate in several
instances; these are reviewed in Chap. 6. Regeneration of the conjunctival stroma
has also been induced (Hsu et al. 2000) and is described further in Chap. 8.
The basic assay in studies of induced regeneration comprises determination of
the extent of restoration of the normal structure and function. These are compared
with the corresponding value in the normal organ, preferably under adequately
controlled conditions of maturation, especially of the regenerated tissues. Several
examples of such assays are described in detail in Chaps. 5 (skin) and 6 (periph-
eral nerve). An especially simple metric is the percent fidelity of regeneration, a
measure of the fractional extent to which a selected morphological or functional
property of the regenerate approaches the value of that property in normal tissue.
Organs are highly complex structures. The detailed comparison of structural and
functional properties of the regenerated skin and normal skin presented in Chap. 5
makes the point that an evaluation of the degree of regeneration is especially use-
ful when it includes not one or two but a large number of structural and functional
characteristics of the reference organ. We stress that, in the emerging field of organ
regeneration, there are very few instances where complete (perfect) regeneration
has been achieved.

4.4 The Defect Closure Rule

The preceding sections included descriptions of the three modes by which a defect
in skin and peripheral nerves closes during healing, as well as the methodology em-
ployed to measure the contribution of each mode. We consider each of these three
modes to be operationally well defined. The configuration of the final state can be
described now simply in terms of the relative contribution made by each closure
mode. This approach reduces the description of configuration of a healed defect to
just three numbers. An illustration of the use of the defect closure rule to describe
closed skin wound by repair and by regeneration is shown in Fig. 4.9 .
In skin wounds, each mode of closure participates by covering a certain fraction
of the initial surface area of the defect. Using percentage values for contributions