Type VII collagen associated with the basement membrane of amniotic epithelium forms giant anchoring rivets which penetrate a massive lamina reticularis

In human amnion a simple cuboidal epithelium and underlying fibroblast layer are separated by an almost acellular compact layer rich in collagen types I and III. This (>10 mu m) layer, which may be a thick lamina reticularis, apparently presents an unusual set of conditions. Integration of the multilaminous tissue across it is apparently achieved by waisted structures which we have observed with the light microscope in frozen, paraffin-wax and semi-thin resin sections. We have also captured transmission and scanning electron micrographs of the structures. These structures which cross the compact layer we call rivets. The composition of these rivets has been examined immunocytochemically and in three dimensions using the confocal laser scanning epi-fluorescence microscope. The rivets contain type VII collagen and an alpha 6 integrin. They associate with type IV collagen containing structures (basement membrane lamina densa and spongy coils) and a special population of fibroblasts which may generate, maintain or anchor rivets to the underlying mesenchymal layer. Although type VII collagen is well known to anchor basal lamina to underlying mesodermal collagen fibres these rivets are an order of magnitude larger than any previously described type VII collagen containing anchoring structures. Intriguing possible functions of these features include nodes for growth of fibrous collagen sheets and sites of possible enzymatic degradation during regulated amnion weakening approaching term. If these sites are confirmed to be involved in amnion degradation and growth they may represent important targets for therapeutic agents that are designed to delay preterm premature rupture of the membranes a major cause of fetal morbidity and mortality. (C) 2013 Elsevier Ltd. All rights reserved.