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The availability of antibodies to and the electrophoretic characterization of components of the cellular cytoskeleton were extended with great success to cultivated AFC types. For example, the close relationship between AF and E cells received support from immunofluorescence studies using antibodies against epidermal keratins.^{583, 584} Such immunofluorescent staining of keratin filaments also confirmed the epithelioid nature of most cells in AFC cultures.⁵⁸⁵ However, AF cells (labeled E1 by Virtanen; see Table 3.7) appeared to express intermediate‐type filamentous structures that reacted with both prekeratin and vimentin antibodies. The conclusions from these early studies must be viewed in the context of the limited specificity (mostly to epidermal keratins) of the antibodies then available. Later, Moll et al.⁵⁸⁶ provided a comprehensive catalog of well‐characterized prekeratin peptides. This new knowledge was then applied to the identification of AFC clones.

Ochs et al.⁵⁸⁷ found that both AF and F cells coexpress prekeratin and vimentin filaments, and the cytoplasmic margins of a singular cell type lit up strongly with desmoplakin‐specific antibodies (Figure 3.7). These large, polygonal cells, labeled ED cells, have a distinctive cobblestone pattern, a low growth rate, and resistance to trypsin (see Table 3.7). They were referred to as sheath‐like cells by Hoehn et al.⁵⁵⁶ Coexpression of cytokeratin and vimentin filaments appears to be promoted by serial culture in many cell types of epithelioid origin. Ochs et al.⁵⁸⁷ referred to the ED cell as archetype E cell, as it retains close cell‐to‐cell contacts by virtue of an abundant number of desmosomes. All other AFC E cell types, and notably AF and F cells, have lost their desmosomes, together with a number of prokeratin peptides. They display only a remnant pattern of cytoskeletal structures.⁵⁸⁶

Figure 3.7 Immunofluorescence staining of ED‐type amniotic fluid cells using antibodies against desmoplakin. Bar = 0.05 mm. Note the exclusive reaction with cell boundaries (desmosomes). Source: Ochs et al. 1983⁵⁸⁷. Reproduced with permission from Elsevier.

F‐type AFCs share many properties with classic fibroblast‐like cells from postnatal skin or foreskin: shape, whorl clone pattern, production of collagenous matrixes, failure to produce hCG, ultrastructure, types of surface glycoproteins and remarkable longevity. Figure 3.8 shows that serially propagated derivative cultures of individual F, E, and AF clones show major differences in their longevities.⁵⁸⁸

Figure 3.8 Serial propagation and longevity of mass culture progeny of F‐, E‐ and AF‐type amniotic fluid cell clones isolated individually from 20 consecutive amniotic fluid specimens (18 weeks gestational age). The number of primary isolates of each clone type is given in parentheses. Note the relative paucity of F‐type isolates. The progeny of F‐type clones, however, reached the greatest number of cumulative population doublings. In contrast, all E‐type isolates were short‐lived, whereas AF‐type isolates display a wide range of longevities. Source: Based on Hoehn et al.⁵⁵⁶