149 Elastin in pulmonary pathology Introduction In pulmonary pathology, elastin staining is used to variable extents in different countries. In some settings, it is used to distinguish collapsed alveoli from papillary adenocarcinoma. This practice is not uniformly accepted, and, as a result, this has not been incorporated into the World Health Organization (WHO) definitions of lepidic and papillary adenocarcinoma. The theoretical basis for the use of elastin staining is that normal lung tissue and pleura have elastic tissue that helps to delineate the histology of these lung structures. As a result, pathological conditions involving the lung and pleura can alter this architecture, and this alteration can be exploited diagnostically. However, there are differences in opinion on the value of the presence or absence of elastin once normal lung tissue has been altered by disease. In pulmonary adenocarcinoma, some use the presence of elastin as an argument for pre-existing structure [implying the diagnosis of adenocarcinoma in situ (AIS) or a lepidic pattern as part of a minimally invasive adenocarcinoma or lepidic-predominant adenocarcinoma (LPA)] to highlight the retention of existing architecture. Others emphasise the lack of elastin as an argument for papillary carcinoma, whether by destruction of elastic tissue or the lack of it in a new proliferation; in either event, this pattern would indicate invasion. These considerations are critical, as a collapsed lepidic pattern would not be included in the T stage assessment of invasive size. To complicate matters further, the term ‘collapse’ is used in different ways in the literature. This review aims to describe the morphological structure and function of elastin in normal and diseased lung, and apply this to unravel interpretation issues mentioned above in pulmonary adenocarcinomas. Elastin components and structure The human lung is an intricate organ whose architecture includes the vasculature, conducting airways, and terminal airspace compartments, which need an elastic and balanced extracellular matrix to support repeated movements of extension and recoil throughout life253. Elastin and collagen are the main components of the lung connective tissue network, and together provide the lung with elasticity and tensile strength254. The extracellular matrix has been defined as the structural network of collagens, elastin, glycoproteins and proteoglycans surrounding stromal cells and underlying endothelial and epithelial cells. In addition to having structural properties, the extracellular matrix functions as a dynamic modulator of various biological processes1. This is accomplished through the selective binding and subsequent release of growth factors and cytokines, and through its interaction with cell surface receptors255 256. Although collagen and elastic fibers are the major constituents of the extracellular matrix, the overall function is defined by the interrelationships between all of the various components. 12
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