Biomedical Engineering Reference
In-Depth Information
How well differentiated a product needs to be in order to merit being referred to
by unique symbolism? Most tissues discussed in this chapter have been synthesized
in more than one distinguishable level of differentiation. In contrast to the symbol-
ism of organic chemistry, where the term benzene refers to a unique compound,
investigators have typically not employed standard definitions of the tissues under
study. Although the vast majority of investigators agree that a tissue that displays
a minimum number of well-defined morphological characteristics can be uniquely
identified, a formal process of standardization of tissues based on a necessary and
sufficient set of morphological and functional characteristics has not yet been de-
veloped. As an example, we often find that an investigator defines the product of
a reaction as the “basement membrane” of skin if it comprises at least four dis-
tinct structural characteristics of this tissue, identified immunohistochemically in
terms of the major protein constituent that is uniquely associated with each layer.
In this example, these constituents are the a6b4 integrin, characteristic of hemides-
mosomes; laminin, present in lamina lucida; type IV collagen, a major constituent
of lamina densa; and type VII collagen, the main component of anchoring fibrils.
In addition to this process of tissue identification based on protein components,
an investigator may provide ultrastructural evidence of normal organization of these
macromolecular elements into a whole, functioning tissue. At the other extreme, an-
other investigator may report synthesis of a “basement membrane” based on immu-
nohistochemical identification of just two protein constituents, such as laminin and
type IV collagen, without reporting on the presence of the other proteins commonly
found in BMs or providing any ultrastructural data to document the organization of
the tissue. In this example, both investigators have identified the product of the re-
action as BM but have employed different criteria in assaying for it. In view of this
diversity in use of identifying criteria, I have arbitrarily chosen to report synthesis
of a given tissue if the investigators provided clear evidence that at least one assay
of widely recognized value, such as those described in histology textbooks (Young
et al. 2006; Kierszenbaum and Tres 2012), was employed in its identification. This
nominal approach clearly errs on the side of inclusion of products and probably
leads to irreducible reaction diagrams that are weighted excessively toward simplic-
ity in description of reaction conditions.
To discuss synthesis of individual tissues of an organ in the actual presence of
part of the organ, we will introduce the symbolism used to represent an organ in
terms of its tissue components. The physical connection between two tissues in
the context of an organ is represented as a dot between the symbols of adjacent tis-
sues; use of a dot, rather than a connecting line, prevents confusion with a chemical
bond. As an example, physiological skin (S) is considered below, in abbreviated
fashion, to comprise only of an epidermis, a BM with hemidesmosomes, lamina
lucida, lamina densa, and anchoring fibrils (BM), rete ridges with dermal papillae
(RR), and a thick, vascularized dermis (D) with sensory nerves and appendages
(AP); the latter derive from the epidermis during development but are located in
the dermis. A completely physiological dermis with nerve fibers and appendages
will, accordingly, be referred to as RR
·
D
·
AP while the physiological skin organ is
