Histogenetic classification of lung carcinomas. small cell carcinomas studied by light and electron microscopy

Forty-six small cell carcinomas of the lung identified by light microscopy were also studied by electron microscopy in an attempt to subclassify them histogenetically. Thirty were subclassifiable into neuroendocrine secretory, mucus secretory, or epidermoid categories on the basis of their differentiated structural features; 16 were too poorly differentiated to assign to any specific cell type and were designated null. In addition to histogenetic separation, electron microscopy revealed a spectrum of average cell differentiation within each subclass except null. The widest spectrum occurred in the most frequent subclass, the neuroendocrine group. This study clearly demonstrated that the light microscopic diagnosis of small cell carcinoma does not identify a morphologically homogeneous process. Whether histogenetic separation and/or some assessment of the relative degree of differentiation have any consistent clinical value remains to be shown; however, combined light and electron microscopy should improve the precision of the initial diagnosis over that of light microscopy alone.