Alexander Schermer

Chapter 5 Patterns of Keratin Expression Define Distinct Pathways of Epithelial Development and Differentiation

Publisher Summary The precise localization of particular keratin polypeptides eventually enables to sub-classify both embryonic and adult epithelial cells and tissues with respect to their relative degree of differentiation. This capacity should be exceptionally useful in studying the various aspects of epithelial differentiation. These localization studies are largely dependent upon immunological procedures involving monoclonal antibodies. While a great deal has been learned about the biochemical and immunological properties of keratin polypeptides through the use of broadly cross-reacting monoclonal antibodies such as AE-1 and AE-3, they are of limited usefulness in the tissue localization of individual keratins within various epithelia. Any studies using conventional or monoclonal antibodies that recognize multiple keratin polypeptides must always be performed in conjunction with the immunochemical analysis of the keratins recognized in a given tissue before conclusions may be made about their localization. The current “state of the art” monoclonal antibody studies on keratin localization involve highly selective antibodies (usually recognizing a single keratin polypeptide) that demonstrate an extraordinary degree of keratin specificity. Despite the relative absence of comprehensive studies on the precise tissue localization of individual keratin polypeptides, the apparent ubiquity of the keratin pair concept and the theory of differentiation-specific pairs are so uniformly consistent that the total keratin polypeptide composition of any epithelial tissue, given only the species of origin and its histological characteristics, can be accurately predicted.

Differentiation-Specific Expression of Keratin Pairs

Keratins are by far the most complex and heterogeneous family of polypeptides that comprise a subclass of the intermediate filaments (IF). A survey of various human epithelia has demonstrated the existence of approximately 25 distinct keratin polypeptides that are expressed as subsets of two to ten polypeptides in any given epithelial cell or tissue. With the goal of understanding the biological significance of keratin heterogeneity, many investigators have studied keratins in detail. Consequently, a tremendous volume of information regarding keratins has been generated during the past 10–12 years. Because of this rapid accumulation of data, the field of keratin research has been a difficult one to follow for those not directly involved. This is due not only to the sheer amount but also to the diversity of the types of information. The keratin literature is filled with seemingly incongruous information derived from a combination of immunological, biochemical, biophysical, and clinical studies with each progressing at a rapid pace. However, general trends in the data have now allowed us to summarize most of the available information into a set of patterns or “rules” of keratin expression. These rules are: (1) Keratin expression is a characteristic of and is mostly restricted to epithelial cells (except a few epithelial tissues, e.g., lens and retinal epithelia, that seem to lack keratins) and their derivatives (Franke et. al., 1978, 1979; Sun and Green, 1978b; Sun et. al., 1979) (Figs. 1, 2).

Immunofluorescence staining of keratin filaments in cultured epithelial cells

A procedure is described for the identification of cultured epithelial cells by indirect immunofluorescence staining of keratins. Additionally, some associated techniques such as double staining, keratin antigen unmasking, cytostatic drug treatment and photography are briefly outlined.