Biomedical Engineering Reference
In-Depth Information
reactants are required to be added (necessary) in order to synthesize a given tissue
or organ? Discussion of mechanism, appearing in later chapters can then focus on
the simplest known synthetic route.
7.1.2
Approximations Underlying the Use of Reaction Diagrams
Although reaction diagrams greatly simplify the description of complex processes
involving tissues and organs, they are rough approximations of reality and must be
used with caution, as discussed below.
One of these approximations is the use of a single symbol to represent a reactant
or a product, suggesting the existence of a unique state for each. This convention is
normally used in the representation of chemical compounds. Yet, the normal mor-
phology of a tissue may change significantly from one anatomical site to the next
in the same organ, even though its name does not. Furthermore, as maturation (or
remodeling) proceeds, a tissue in an organ can be present at various levels of dif-
ferentiation. Assignment of a single symbol to a tissue does not take into account
many of these variations in morphology or functional state with anatomical location
or maturation time.
A few examples illustrate the degree of approximation involved in describing a
tissue by a single name. The epidermis (E) synthesized by culturing keratinocytes in
vitro with fibroblasts and a collagen gel (COG) is a product that can be prepared at
various identifiable levels of differentiation, depending on timing and other reaction
conditions (Parenteau et al. 1992, 1996). The same caution applies when cells are
used as reactants in a process. The symbol KC is used below to represent keratino-
cytes in culture medium. Implicit in the use of a single symbol is the assumption
that keratinocytes exist in a single state of differentiation. In a number of studies,
however, keratinocytes have been isolated from a skin biopsy and have either been
used to induce synthetic processes without further culture or else have been exten-
sively cultured before being used. The uncultured keratinocytes typically comprise
cells from all epidermal layers, representing various levels of differentiation, while
cultured cells have typically been converted to a higher level of differentiation
(Wille et al. 1984). The details of a synthetic process can be affected, often very
significantly, by the precise state of differentiation of a cell that is used as a reactant.
In the example of KC seeded into dermis regeneration template (DRT), a nondiffus-
ible regulator, the skin synthesized in the process had a significantly higher number
of rete ridges when the keratinocytes had been cultured, than when the cells were
freshly dissociated but not cultured, prior to seeding (Butler et al. 1999a). In a fur-
ther example, a study of synthesis of BM in vitro in the presence of keratinocytes,
fibroblasts, and a COG led to the conclusion that BM was synthesized only when
keratinocytes were added to the COG that had already been cultured for a period
of time with fibroblasts; a BM was not formed when keratinocytes were added to
fibroblasts and the COG without first culturing the latter the two cell types together
(Chamson et al. 1989).
