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I n parallel experiments with pigmented retina, desmosomes formed exclusively between corneal cells, yet junctions of the intermediate type occurred between unlike cells. These zonulae or fasciae adhaerentes are readily recognizable in most instances. Occasionally such junctions may have a macular appearance, possibly due to the plane of the section or to their developmental state, but such regions never have a central stratum, clearly defined plaques, or associated 100 A filaments, all of which are characteristic of the mature desmosome or macula adhaerens.

1972), or the reactivity of formed aldehydes is blocked (for example, by D-penicillamine) (Nimni, 1965), cross-links do not occur and collagen is rendered extractable by mild procedures, such as use of 1 M NaCl. , 1969). There is increasing evidence that collagen may be normally associ- 2. T H E EXTRACELLULAR MATRIX 43 TABLE IV CHAINCOMPOSITION A N D MATURE TISSUEDISTRIBUTION OF DIFFERENT COLLAGEN TYPES Collagen Type Type Type Type I I1 111 IV Chain composition [ff 1 (U12ff2 ~cxl(WJ3 [ff 1 (1Wla bl(IV)b Sources, mature tissues Skin, bone, tendon Cartilage Skin, dentine, aorta Some basal laminae such as lens capsule, Descemet’s membrane, renal glomerular and alveolar ated with noncollagenous protein by means of covalent bonds that are also sensitive to these agents (Veis and Perry, 1967; Volpin and Veis, 1973).

When cells are disrupted, such forces would be changed and might entail displacement of the plaques. In the presence of cytochalasin B the plaques are not engulfed yet the tonofilaments are not destroyed, so a causal relation between engulfment of plaques and their tonofilament connections seems unlikely. Engulfment of the plagues presumably results from the same types of surface movement as endocytosis. I n cytochalasin B-treated cells, desmosomes that have been disrupted remain a t the cell surface with their fibrillar connections both in isolated cells and also, apparently, after these cells have been reaggregated.