FINE STRUCTURE OF DESMOSOMES, HEMIDESMOSOMES, AND AN ADEPIDERMAL GLOBULAR LAYER IN DEVELOPING NEWT EPIDERMIS

The skin of late embryonic, larval, and young postmetamorphic newts, Taricha torosa, has been examined with particular reference to areas of cellular attachment. Stereo electron microscopic techniques and special staining methods for extracellular materials were utilized in addition to conventional avenues of ultrastructural study to investigate the fine architecture of desmosomes, hemidesmosomes, their associated filament systems, and extracellular materials. No evidence has been found that continuity of tonofilaments between adjacent cells exists at desmosomes. Rather, most of the tonofilaments which approach desmosomes (and perhaps also hemidesmosomes) course toward the "attachment plaque" and then loop, either outside the plaque or within it, and return into the main filament tracts of the cell. These facts suggest that the filamentous framework provides intracellular tensile support while adhesion is a product of extracellular materials which accumulate at attachment sites. Evidence is presented that the extracellular material is arranged as pillars or partitions which are continuous with or layered upon the outer unit cell membrane leaflets and adjoined in a discontinuous dense midline of the desmosome. A similar analysis has been made of extracellular materials associated with hemidesmosomes along the basal surface of epidermal cells. An adepidermal globular zone, separating the basal cell boundary from the underlying basal lamina and collagenous lamellae during larval stages, has been interpreted from enzyme and solvent extraction study as a lipid-mucopolysaccharide complex, the function of which remains obscure. These observations are discussed in relation to prevailing theories of cellular adhesion and epidermal differentiation. They appear consistent with the concept that a wide range of adhesive specializations exists in nature, and that the more highly organized of these, such as large desmosomes and hemidesmosomes, serve as strong, highly supported attachment sites, supplemental in function to a more generalized aggregating mechanism.