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The most intriguing aspect of the endocrinology of the lung is the frequency with which its neoplasms display endocrine differentiation and the diverse ways in which this shows itself. The endocrine element in a pulmonary neoplasm may be subtle, such as when histologically typical squamous and adenocarcinomas express substances indicating a degree of endocrine differentiation unapparent from their appearance. This chapter provides an account of the WHO classifications for pulmonary endocrine neoplasms. The spectrum of pulmonary endocrine tumors or pulmonary endocrine spectrum is a biological continuum in which three zones can be recognized, although the borders among them are distinctly blurred. The best differentiated neoplasms, the carcinoid tumors, occupy the zone at the most benign end of the spectrum. The most poorly differentiated, the small cell, small/large cell, and combined carcinomas, occupy the zone at the most malignant end. Spanning the third zone in between these are the atypical carcinoid tumors. The heterogeneity of pulmonary neoplasms and the variety of ways in which endocrine differentiation can manifest itself have important practical implications.

Publisher Summary This chapter reviews an investigation concerning primary pulmonary neoplasms in animals. The terms of reference exclude metastatic neoplasms of the lung and more generalized neoplasms with incidental localization in the lung, such as lymphosarcoma or hemangioendothelioma. Most of the experimental work has been performed with the adenomatous pulmonary tumor of the mouse, and most of the observations and the conclusions perforce must be limited to mice. The chapter discusses that recent reports on the induction of pulmonary tumors in rats and in guinea pigs allow the extension of investigations to other species. Reports on pulmonary tumors in larger animals are limited to descriptions of individual cases and a few, mostly unsuccessful, attempts to induce the tumors. Cancer research, as all biological research, is dependent upon the availability of proper materials and techniques for its work. The chief material has been the mouse, and the chief techniques have been the transplantation and the induction of neoplasms. On this basis, a large portion of the history of cancer research can be divided into three main periods: 1890-1918, morphology-transplantation era; 1918-1935, genetics-tar tumor era; and the modern period since 1935.