Most anchoring fibrils in human skin originate and terminate in the lamina densa

Abstract

Anchoring fibrils (AF) at the dermo-epidermal junction are well characterized as ultrastructural entities. They are composed mainly of collagen VII and play a key role in dermo-epidermal adhesion. Previous studies have suggested that AF originate in the lamina densa (LD), extend perpendicularly into the dermis, and insert into amorphous elements, called "anchoring plaques," in the dermal connective tissue. To elucidate the precise structural organization of the AF network in human skin, we analyzed quantitatively the distribution of different domains of collagen VII in the epidermal basement membrane zone, using various techniques of immunoelectron microscopy with a range of domain-specific antibodies that we prepared. Some electron-dense amorphous structures (ie, anchoring plaques) that were positive with aminoterminal end of collagen VII could be recognized only by pre-embedding en bloc labeling, and not by postembedding section labeling of immunoelectron microscopy. Quantitative analysis of surface labeling with postembedding immunoelectron microscopy demonstrated that most (> 90%) gold particles labeling the epitopes in the aminoterminal (NC-1 domain) of collagen VII were precisely localized to the LD, whereas no specific labeling was observed in the dermis. Most (> 90%) of the gold particles labeling the carboxyterminal end of collagen VII localized at a 160- to 360-nm distance from the LD, and most (> 90%) of the labeling epitopes in the central triple-helical collagenous domain were distributed between the LD and up to 360 nm from it; no specific labelings were observed beyond this area. These results suggest that most (> 90%), if not all, of the AF in human skin do not extend perpendicularly into the dermis, but instead originate and terminate in the LD, forming individual semicircular loops that constitute a network of AF.