Cell Types in BAC

A. Clara cells: At high magnification, two cell types are seen here. First, most of the cells are Clara cells with nuclei located in the mid portion of the cell, and apical protrusions of cytoplasm above the cell junctions. The arrow marks the location of a cell junction. By EM, the apical cytoplasm contains the Clara cell granules.

B. Type II cells: Here, nuclear vacuoles or inclusions are prominent (arrow, also find some in A). Pink, with an unstained halo, they indicate type II cell differentiation if they stain with antibody to surfactant apoprotein or if they have a branching tubular structure by EM. However, many other types of tumor show similar vacuoles, which represent invaginations of cytoplasm into the nucleus--so-called nuclear-cytoplasmic inclusions. Note the lymphoid infiltrate that is characteristic of nonmucinous BACs.

Finally, compare the cells of BAC (lower right) with the normal pseudostratified, ciliated, columnar cells of the airway at the upper left with respect to:

Cell size

N/C ratio

Nuclear pleomorphism


Clara cells and type II cells can also occur in multicentric BAC, and mucinous cells may occur in localized BAC. Although the type II and Clara cells shown here are found in many lung adenocarcinomas, they cannot be diagnosed with certainty without EM or immunohistochemical stains, which are not widely used. Thus, an extrapulmonary primary site should always be considered before making the diagnosis of a primary lung adenocarcinoma.

Mucin production: Even the nonmucinous BACs may show some mucus production, as shown by the cytoplasmic inclusion (arrow) with a red rim and a central red target separated by a halo from the rim. Note also the mucus in the lumen. PAS-D stain.

Clinical summary Discussion

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Cell size--increased

N/C ratio--increased

Nuclear pleomorphism--more variation in shape than normal