Malignant stromal cell tumor of the spleen in a WBN/Kob rat

Abstract

Primary splenic stromal tumors have rarely been reported in rodents. We report the case of a 90-week-old male WBN/Kob rat with a nodular demarcated mass in the spleen, which was kept as a non-treated animal in a long-term animal study. Histopathology revealed round to short spindle-shaped tumor cells arranged in a solid growth pattern. Invasive growth, anisokaryosis, and high mitotic activity (46 per 10 high-power fields [2.37 mm²]) were observed to be multifocal, but most tumor cells showed mild nuclear pleomorphism. The pattern of silver impregnation corresponded to that of the marginal zone of the red pulp. Immunohistochemistry revealed that the tumor cells were double positive for fascin and desmin and focally positive for Iba-1 and OX-6 expression. These characteristics were similar to those observed in fibroblastic reticular cells and dendritic cells in the marginal zone of the red pulp. These findings suggest that the malignant stromal cell tumor of the spleen in this case had characteristics of both fibroblastic reticular cells and dendritic cells.

Keywords: dendritic cell; fibroblastic reticular cell; rat; spleen; stromal cell tumor.

Fig. 1.

Gross appearance of the splenic mass. The cut surface was solid, and peach in color. Scale bar: 2.5 cm.

Fig. 2.

(A) The border between tumor and the spleen parenchyma (asterisk) was almost clearly delineated by the capsule structure (arrows) (HE stain. Scale bar: 1 mm). (B) The capsule structure (arrows) partially disappeared, and the tumor cells infiltrated into the existing spleen tissue (asterisk) (HE stain. Scale bar: 100 µm). (C) The nuclei shape in the tumor cells were round to short spindle, with small nucleoli and finely stippled chromatin (HE stain. Scale bar: 50 µm). (D) Increased anisokaryosis and high mitotic activity were observed (HE stain. Scale bar: 50 µm). (E) The sinus structure of the blood vessel resembled the normal splenic sinus of red pulp (HE stain. Scale bar: 500 µm).

Fig. 3.

(A) Black-stained reticular fibers surrounded individual tumor cells or tumor cell nests (silver impregnation. Scale bar: 100 µm). (B) In the normal spleen, reticular fibers were arranged in a stratiform pattern in the periarterial lymphoid sheath (PALS), were sparse in the lymphoid follicle (LF), and arranged in a mech structure in the marginal zone (MZ) and red pulp (RP) (silver impregnation. Scale bar: 100 µm). (C) The majority of the tumor cells stained positive for fascin (scale bar: 100 µm). (D) The tumor cells stained positive for desmin (scale bar: 100 µm). (E) Many non-tumor stellate-shaped cells were positive for Iba-1 (scale bar: 100 µm). (F) Double staining for fascin (red) and desmin (green). Numerous double positive tumor cells (yellow) were observed.