151 Elastin in pulmonary pathology Figure 1. A,B, Two examples of elastin staining in a non-malignant peripheral lung with minimal iatrogenic collapse (perfusion-fixed). A, a 31-year-old male with organizing pneumonia. B, a 39-year-old female with invasive mucinous adenocarcinoma. C, a specimen submerged in fixative with prominent iatrogenic collapse. The diagnosis was made on other sections of the same resection specimen. Circles (blue) encompass elastic fibers. Note: (i) the reduction in the amount of air because of collapse; (ii) that the number of discontinuous elastin fragments is dependent on the number of alveolar cross-sections in the field of view; (iii) that the alveolar cross-sections sometimes result in a pseudopapillary appearance; and (iv) that if one were to replace all pneumocyte type I cells lining the alveolar walls with a monolayer of cylindrical tumour cells, the proper diagnosis would be adenocarcinoma in situ (AIS), whereas, in terms of patterns, this may be perceived as a mixture of a lepidic pattern and a papillary pattern. The use of an elastin stain would reveal the underlying architecture of the preexisting lung, as proven by the inherent distribution of fragmented elastic fibers, including some of the alveolar wall parts without elastin. D, Elastin staining of AIS showing the pre-existing alveolar structure with mild iatrogenic collapse (perfusion-fixed). Note the continuous sheets of elastin (compatible with Noguchi type B10). In elastin-stained histological 3–5-µm sections, thick elastic fibers may easily be recognized, whereas thin elastic fibers cannot readily be discerned. The lightmicroscopic literature on elastin usually deals with thick elastic fibers, which will be subsequently referred to as ‘elastic fibers’. In two-dimensional histological sections of normal alveolar walls, the elastic fibers appear discontinuous curvilinear or dot-like254 266 (Figure 1A–C). It is of note that these seemingly fragmented elastic fibers form part of the three-dimensional elastin network (scaffold); that is, in serial sections they are connected to each other. 12
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