Immunolabeling type II collagen in the basilar membrane, a pre-embedding approach

This paper describes the development of a protocol that can be used to detect collagen II in the healthy adult basilar membrane (BM) at the electron microscopic level. This protocol required aggressive epitope exposure techniques to break the crosslinks that bind the collagen molecules tightly into fibrils and to remove a dense mat of ground substance that surrounds the fibrils. On the other hand, the steps had to be carefully controlled to preserve BM ultrastructure and the collagen II epitopes that are typically labile. These requirements were satisfied by introducing a targeted crosslink breakage method and by regulating the duration of epitope exposure based on changes in tissue appearance observed with differential interference contrast microscopy. High levels of immunolabeling were achieved by substituting tissue preservation techniques for most or all of fixation; this was important because fixation reduces antigenicity directly and impedes epitope exposure. When these techniques were combined with more traditional trypsin and pepsin treatments, the result was dense immunolabeling and preservation of ultrastructure that allowed accurate localization of the immunolabeling. This pre-embedding immunoelectron microscopic method is the first to be carried out on the BM and may be adaptable to future studies of the BM as well as other tissues with similar molecular composition.