Biomedical Engineering Reference
In-Depth Information
quantitative change long after closure. These quantitative changes in the configura-
tion of the final state after closure is complete must be taken into account in future
studies, as long-term data become available.
5.1.3
Assays of Configuration of the Final State
A very wide variety of assays have been employed to measure healing outcomes,
including, for example, the time for defect closure by epithelialization, percentage
“take” of experimental grafts, ability to cross over major histocompatibility bar-
riers and regeneration of a dermoepidermal junction. Most of these observations
do not allow the reader to draw a firm conclusion on whether the dermis, the key
non-regenerative tissue in skin, have been even partly synthesized or whether spon-
taneous scar formation has at all been inhibited or not. In spite of such divergence
in objectives, studies were included in the review of this chapter provided that they
were conducted under the same initial conditions (i.e., in a dermis-free defect) and
the outcome was described sufficiently well to conclude on the incidence of regen-
eration versus scar formation. Clearly, most of the studies of skin defect healing
were not at all designed as experiments in induced regeneration but as efforts to
understand or modify (typically, accelerate) the healing process. The emphasis in
the healing outcomes reported here is, however, on the identity of tissues at the end
of the study. The author's familiarity with chemical synthetic processes accounts
for the frequent use of the term “synthesis” when describing outcomes of healing
processes.
The major questions that must be answered in order to provide a complete de-
scription of the outcome: Which types of tissue(s) were synthesized? What fraction
of the initial defect area were eventually closed by regeneration? The first question
is qualitative; it is answered by reference to the morphological characterization of
the tissues inside the defect in the final state and the degree of their resemblance to
intact skin, the physiological “standard.” The second question is quantitative; it is
answered by data that are expressed in terms of the defect closure rule.
5.2
Synthesis of an Epidermis and a Basement Membrane
A skin defect can be considered to have closed when its initial surface area has been
fully covered by confluent, mature epithelium (epidermis). Let us describe in some
detail the epidermis, the critical marker of closure in a skin defect. Such a detailed
description is an indispensable aid in following the outcomes of various protocols.
5.2.1
Morphology and Function of the Epidermis
The epidermis is the external tissue layer of skin. It protects the organism against
dehydration and acts as a physical barrier against invasion by microorganisms;
