Mouse mammary tumor metastases in lung: an electron microscopic study.

The ultrastructure of mammary carcinoma metastatic to the lungs of old, female (C57L X A/He)F, hybrid and C3H/Crgl mice is described and illustrated. Metastatic mammary tumor nodules in lung were usually separated from lung tissue by a thickened basement membrane and loose connective tissue. Connective tissue reaction to the metastatic nodules was minimal and in some places com pletely absent. Where mammary tumor cells were seen within alveolar air spaces, the tumor cells shared the same basement membrane with extant alveolar lining cells. Type A alveolar cells could not be identified within tumor cell-filled alveoli, but surviving type B alveolar cells were observed, although most appeared to be degenerating. A narrow, compact zone of lung tissue surrounded the metastatic nodules. This zone was characterized by the presence of hyperplastic type B alveolar cells. The hyperplasia took the form of small acinar structures lined by type B cells. Metastatic mammary tumor cells grew in multiple layers within lung. The stroma of the outermost layers consisted, at least in part, of existent alveolar septal elements. Cells of the innermost layers appeared to grow around small, cyst-like spaces characterized by the presence of multiple layers of membranes resembling basal lamina.