Emerging view of cell surface differentiation

The wealth of information coming from serologic studies of cell surface antigen expression in different species and in diverse cell and tissue types has led to the demise of earlier concepts about how the surface of cells is constructed. Chief among these is the notion that developmental cell lineage patterns provide the general organizing principles for differen-tiation-related cell surface diversity. The expectation was that distinct cell lineages in developing embryonic and fetal tissues and in self-renewing adult tissues would be marked by unique surface antigens (e.g. lymphoid or neuroectodermal lineage markers) and that discrete phases in these lineages would be marked by differential expression of additional antigens (phase markers). These molecules would serve cell functions unique to the respective lineages and phases and provide convenient histogenetic markers for the study of embryonic development, tumor classification and other cell type analyses. One early exception to this concept was the mouse Thy-1 antigen, which was found not only on thymocytes and peripheral T cells but also on fibroblasts, in brain tissue, and on a few additional cell types. Thus, Thy-1 shows the restricted expression of a differentiation antigen, yet its tissue distribution is complex, overlapping several distinct embryological lineages and showing differences in developmental regulation within these lineages. Subsequent studies have shown that lineage-restricted antigens are the exception and antigens like Thy-1, whose distribution docs not conform to embyronic derivation or lineage are the ones commonly found. The reluctance of differentiation antigens to fit into established cell lineage patterns means that comprehensive maps of antigen distribution are required before conclusions can be reached with regard to cell type, tissue type or disease-related specificity.

Apart from normal differentiation antigens, tumor cell surface antigens have been the subject of much research, and the question of their normal tissue distribution has been of key importance. One suggestion has been that tumors re-express molecular and biological traits of fetal cells that are absent from normal adult cells. This theory has not held up for

(A) General cell surface distribution

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