Histological comparison of malignant epithelioid mesothelioma in young and adult cattle

Abstract

The histology and immunohistochemistry of pleomorphic and conventional epithelioid mesotheliomas were examined. The former was detected in two young calves aged 2 and 4 months and was characterized by pleomorphic and atypical cells with decreased expression of cytokeratin 7 (CK7). In contrast, the latter was found in a 31-month-old heifer, consisting of tumor cells uniform in size and shape with CK7 expression in nearly all cells. Production of collagen by tumor cells was demonstrated in both histological types, and was considered to be characteristic of bovine epithelioid mesothelioma. Pleomorphic mesothelioma is far more pleomorphic and mitotically active than conventional mesothelioma, and its normal counterpart may be immature mesothelial cells with high proliferation potential, which exist in fetal life and early calfhood.

Keywords: cattle; collagen production; cytokeratin 7; epithelioid mesothelioma; pleomorphic mesothelioma.

Fig. 1.

Histology. (A) Case 1. Pleomorphic cells and more differentiated columnar cells (lower right) are visible. Hematoxylin and eosin (HE). Bar=100 µm. (B) Case 1. Pleomorphic cells are different in nuclear and nucleolar size from well-differentiated neoplastic cells forming tubules (lower left). HE. Bar=10 µm. (C) Case 1. Pleomorphic cells possess various amounts of intravacuolar collagen fibrils in the cytoplasm. Trichrome. Bar=5 µm. (D) Case 1. A giant epithelioid cell has intracytoplasmic lumina. Some of them are enlarged, and others are lined by microvilli and contain collagen fibrils (arrow). Collagenous cores are also visible in other cells, and a larger one is fibrous and contains cytoplasmic fragments (lower left). Trichrome. Bar=5 µm. (E) Case 1. Unlike in the connective tissue (lower right), collagen deposits in the neoplastic tissue are homogeneous and contain only a few blood vessels as cellular components. Trichrome. Bar=50 µm. (F) Case 2. Neoplastic cells with intravacuolar collagen fibrils (top) are much larger in size than surrounding neoplastic cells. Trichrome. Bar=10 µm. (G) Case 3. Acellular collagenous cores are relatively homogeneous and lie within neoplastic tubular structures. Trichrome. Bar=50 µm. (H) Case 3. A collagenous core, which appears faintly concentric, is encircled by double layered tubules. Trichrome. Bar=5 µm. (I) Case 3. A collagenous core is continuous with oriented connective tissue stroma at the base of a papillary projection (arrow). Trichrome. Bar=10 µm.

Fig. 2.

Immunohistochemistry. (A) Case 1. Both well-differentiated cuboidal cells and pleomorphic cells show positivity for cytokeratin 20 (CK20), with the former staining more intensely in this field. Streptavidin-biotin complex/horseradish peroxidase (SAB) method. Bar=50 µm. (B) Case 1. Well-differentiated cuboidal cells show distinct positivity for CK7, whereas pleomorphic cells are faintly positive or negative. SAB. Bar=5 µm. (C) Case 2. Papillae or tubules of columnar cells are positive (arrow) or negative (arrowheads) for CK7. (D) Case 1. Arrows indicate vimentin-positive pleomorphic cells, whereas other pleomorphic cells are negative. Connective tissue stromal cells are also positive for this marker (lower left). SAB. Bar=10 µm. (E) Case 1. Adjacent section shows the same cells as those depicted in D. Contrary to the vimentin staining, only pleomorphic cells indicated by arrows and stromal cells (lower left) are CK18 negative. SAB. Bar=10 µm. (F) Case 2. Some of moderately differentiated columnar cells show positivity for vimentin (arrow). Smaller positive cells are blood capillary cells (arrowheads). (G) Case 3. CK7 is expressed in well-differentiated epithelioid cells. SAB. Bar=50 µm.