LAMINA-LUCIDA OF BASEMENT-MEMBRANE - AN ARTIFACT

In tissues prepared with chemical fixation followed by conventional de hydration, basement membranes have been observed to be laminated struc tures composed of a lamina lucida and lamina densa as well as a poorly limited transitional zone referred to as the pars fibroreticularis. S cattered attempts in the application of new techniques of tissue prepa ration such as cryofixation or freeze substitution for the study of th e basement membrane structure have been made in recent years. From the se studies, the possibility has arisen in which basement membranes are composed of only the lamina densa without a lamina lucida. In recent studies in this laboratory, the attempt was made to determine whether or not this lamina lucida is an artefact, and if so, which step in the conventional method of tissue preparation is responsible for its form ation. Basement membranes from diverse sources in the mouse and rat in cluding the testis, ductus epididymis, eye, thyroid, kidney, and skin, were observed after either cryofixation by slam freezing followed by freeze substitution, or aldehyde fixation followed by freeze substitut ion. The basement membranes after preservation with either of these tw o methods were composed of only the lamina densa with no lamina lucida . It indicates that an artefactual formation of the lamina lucida occu rs during dehydration in conventional tissue preparation rather than d uring chemical fixation. In view of the well known superiority of free ze substitution over conventional dehydration, the lamina lucida of th e basement membrane is likely to be an artefact. Therefore, it is conc luded that the lamina lucida is an artefact formed during conventional tissue preparation, and in its original condition in the living state , the basement membrane is composed of a single layer made up of lamin a densa material. (C) 1994 Wiley-Liss, Inc.