Intrapulmonary airways visualized by staining and clearing of whole-lung sections: the transparent human lung

Methods for the study of cartilaginous airways represent technically very laborious and time-consuming procedures, many of these with the inevitable disruption or elimination of the distal bronchi and bronchioles. We describe and illustrate a methodology to demonstrate the cartilaginous support of the most distal intrapulmonary airways in hemisections or slabs of whole-, fixed-lung specimens. By this process, the cartilaginous framework of intrapulmonary air passages is highlighted and their outlines are defined. An important and distinct benefit of our procedure is the preservation of the alveolar parenchyma, vasculature and pleura, serving as an anatomic structural context. This improved methodology is based on procedures used in the past, now applied to entire half-sections or slabs of lungs, stained with toluidine blue, subsequent removal of stain from noncartilaginous elements and finally clearing of the specimen. The procedure takes 7-8 days but with limited technical - manual involvement. The resulting specimens demonstrate a highly complex, variable and at times, even unpredictable distribution of cartilage throughout the bronchial anatomy. This method represents a practical way of studying intact, this vital component of the respiratory tract. It also allows assessment of the potential implication of these critical smaller pathways in pathologic conditions, where thus far they have been under-studied.