Experimental Methods II. The Defect Closure Rule - Tissue and Organ Regeneration in Adults - page 84

Table 4.4 Modes of closure at different developmental stages. Dermis-free defects in an
amphibian (North American bulfrog (Rana catesbeiana). Data from Yannas et al. 1996)
Developmental stage a % Contraction b % Scar c % Regeneration d
Larva, premetamorphic stage 40.8 ± 6.8 0 59.2 ± 6.8
Larva, early prometamorphic stage 62.1 ± 3.0 0 37.9 ± 3.0
Larva, mid prometamorphic stage 66.3 ± 8.1 0 33.7 ± 8.1
Larva, late prometamorphic stage 90.1 ± 2.3 0 9.9 ± 2.3
Adult frog 94 ± 4 6 ± 4 0
a Developmental staging was based on classic staging criteria for Rana pipiens relating chrono-
logical age and total body length (Taylor and Kolross 1946)
b Percentage of original defect area closed by contraction. Measured by direct photography of
defect after contraction had stopped. Defect boundaries were observed directly and values were
confirmed by matching with histologically identified discontinuities in morphological features at
the defect edges. Original defect area was generated by excising down to muscle (full-thickness
dermis-free defect) an area measuring 5 mm × 5 mm, approximately 1 cm caudal to the eyes.
Error was determined directly from data
c Percentage of original defect area closed by epithelialized scar. Scar inside the defect was
identified by observing distinct histological and functional differences between the subepider-
mal connective tissue inside and directly outside the defect boundary. Table entry calculated as
[100-% contraction], after ensuring, by histological observation, that no significant fraction of
the initial defect area had closed by regeneration. Error of each table entry assumed identical to
that of corresponding entry for % contraction
d Percentage of original defect area closed by regenerated skin. Regeneration inside defect
boundary was identified by observing lack of significant difference in histological and func-
tional features inside and directly outside defect boundary (intact skin). Table entry calculated
as [100-% contraction], after ensuring, by histological observation, that no significant fraction
of the initial defect area had closed by scar. Error of table entry assumed identical to that of cor-
responding entry for % contraction

4.7 Summary

The regenerative activity of an unknown reactant can be studied by the total change
caused by its presence in the defect between the initial and final states in the healing
process. An initial state (newly generated defect) and a final state (recently closed
defect) of the experimental healing process were defined in terms of the start of
exudate flow from a freshly injured defect and the cessation of exudate flow, respec-
tively. The final state of the healing process is not considered to be a biologically
static state but instead undergoes quantitative changes over a long time due to the
underlying processes of organism growth and defect site remodeling.
Defects in organs close by three modes: contraction, scar (or neuroma) formation,
and regeneration. Methodology for quantitative determination of percent contribu-
tion of each mode to defect closure was described. The experimental evidence is
concisely summarized by the defect closure rule, which states that defects close by
just three modes: contraction ( C ), scar ( S ), and regeneration ( R ). The percentages of
initial defect area closed by each of three modes add up to 100, i.e., C + S + R = 100.
The rule describes the configuration of the final state of defect healing by three
numbers only, each corresponding to the relative contribution of a closure mode.