Another vignette, given by Dr. Margarita Gruebbel (Experimental Pathology Laboratories, Inc., Research Triangle Park, NC, USA), presented two rare neoplasms that had challenging and/or unusual morphology.
The first case was a pancreatic lesion in a control male F344/N rat necropsied on day 731 at the end of a NTP 2-year carcinogenicity bioassay. The small, encapsulated, slightly compressive tumor consisted of approximately equal proportions of islet cells and acinar cells intermingled in a mosaic-like pattern (). In some areas the neoplastic islet cells were arranged in small, tightly packed nests, but in others they formed invasive sheets () (Riley et al. 1990
). Neoplastic islet cells had indistinct membranes; abundant, pale eosinophlic cytoplasm; variably sized but usually large, oval to round nuclei with finely stippled chromatin and central nucleoli; and occasional mitotic figures (). Clusters of acinar cells usually formed acini with narrow lumens that were bounded by basal lamina, which separated them from the surrounding islet cells. Compared to normal exocrine cells, the neoplastic acinar cells were generally large; had more abundant pale blue cytoplasm but fewer pale pink zymogen granules (); and possessed large, oval to round, vesicular nuclei with marginated chromatin and prominent, sometimes multiple nucleoli (). Mitotic figures were occasionally present (). Scattered among the acinar and islet elements were occasional ductules lined by plump epithelial cells with pale pink cytoplasm and large, vesicular nuclei ().
Figure 3 Mixed tumors in the rat. (A) Pancreatic malignant mixed tumor from a male F344/N rat from a 2-year bioassay. (B) Higher magnification of the neoplasm in (A) showing intermingled but distinct acinar cell (AC) and islet cell (IC) components. (C) Extension (more ...)
The voting choices for this first lesion were (1) islet cell tumor, benign; (2) islet cell tumor, malignant; (3) mixed tumor, benign; (4) mixed tumor, malignant; (5) mixed acinar-islet cell tumor, malignant; and (6) acinar-islet cell carcinoma. The top choice was malignant mixed acinar-islet cell tumor (44%), with fewer votes for malignant islet cell tumor (16%), benign mixed tumor (14%), acinar-islet cell carcinoma (12%), malignant mixed tumor (11%), and benign islet cell tumor (3%). This case—which has undergone the NTP peer-review process—is recorded in the NTP database (1982–2010) as a pancreatic malignant mixed tumor. Thus, although the preferred diagnostic terms differed, there was general concurrence for a term denoting a malignant neoplasm with proliferative acinar and islet cell populations (81% of the votes).
During subsequent discussion, the possibility was raised that the intra-tumor acinar cells were non-neoplastic, and simply entrapped within an islet-cell tumor. However, in this case, the morphology of the acinar cells in the neoplasm was quite distinct from that of normal acinar cells, and was characterized by multiple traits consistent with a proliferative process: variable arrangement (solid nests or indistinct acini); more abundant, pale basophilic cytoplasm with fewer zymogen granules; pleomorphic nuclei; prominent, often multiple nucleoli; and occasional mitotic figures. The possibility that this case represented a “collision” of two separate neoplasms was also raised. However, rather than being restricted to particular zones with little intermingling (as might be expected in a collision situation), the acinar and islet cell populations were intimately interdigitated throughout the mass. It was also noted that neither this case nor any of the other pancreatic malignant mixed tumors in the NTP database (1982–2010) exhibited invasion or distant metastasis.
Pancreatic mixed tumors in rats have been described as having “uniformly dispersed islet and acinar cells,” both of which exhibit “some degree of cytologic alteration” (Eustis, Boorman, and Hyashi 1990
). In rodents, these neoplasms have been referred to as pancreatic “mixed acinar-islet cell” neoplasms (Eustis, Boorman, and Hyashi 1990
); “mixed acinar-islet cell tumors” (Riley et al. 1990
); and “acinar-islet cell” neoplasms (Mohr 1994
). They are rare, with only 46 cases in F344/N rats listed in the 1982 to 2010 NTP database.
The NTP cases occurred sporadically in control and treated rats, and all were considered incidental. Of these cases, 42 (91%) were adenomas and 4 (9%) were carcinomas. The great majority occurred in males (44 [96%]), with only 2 (both adenomas) in females. Corresponding mixed pancreatic neoplasms in humans are composed of morphologically and immunohistochemically (IHC) distinct acinar, islet, and/or ductular cell types (Klimstra, Rosai, and Heffess 1994
; Marrache et al. 2005
; Newman et al. 2009
), while others are composed wholly or predominantly of a single “amphicrine” cell type exhibiting concurrent morphologic and IHC markers of at least two of these cell types (Klimstra, Rosai, and Heffess 1994
; Terada et al. 1999
). Proposed pathogeneses of these “mixed” pancreatic neoplasms in humans include bilinear or trilinear development from a putative multipotent stem cell (Klimstra, Rosai, and Heffess 1994
; Newman et al. 2009
), transdifferentiation of one neoplastic cell type into another type (Terada et al. 1999
), or modulation of acinar cell proliferation via peptides secreted by neoplastic islet cells (Marrache et al. 2005
). Whether any of these proposed mechanisms are also pertinent to the development of the pancreatic mixed acinar-islet cell tumors in rats remains undetermined.
Dr. Gruebbel's second case was an incidental preputial gland neoplasm from a mid-dose male F344/N rat from a NTP 2-year carcinogenicity bioassay. This animal was found dead on day 722 due to mononuclear cell leukemia. At necropsy, the left preputial gland was grossly dark and enlarged. Microscopically, the normal gland architecture was compressed and partly replaced by proliferative mesenchymal tissue () consisting of coalescing, hyaline-like cartilage nodules alternating with sheets and interlacing bands of fibrous connective tissue (). Lateral duct branches intercalated between the cartilaginous/fibrous masses were lined by poorly differentiated squamous epithelial cells () arranged in disorderly layers. Downward extension of branching cords and “dropping off” of epithelial cell nests and individual cells was evident in the underlying fibrous tissue (). Occasional solid nests or gland-like structures composed of crowded, poorly differentiated acinar cells (). The fibrous and epithelial areas exhibited focal necrosis and variable but often abundant mixed infiltrates of inflammatory cells (). The central duct and its remaining larger lateral branches were focally ulcerated and distended by inspissated secretory material, sloughed keratin, inflammatory cells, and necrotic debris ().
The voting choices were (1) chondrosarcoma; (2) chondrofibrosarcoma; (3) carcinoma (with cartilaginous metaplasia); (4) carcinosarcoma; and (5) mixed tumor, malignant. The audience tally was carcinoma with cartilaginous metaplasia (28%), mixed tumor (26%), chondrofibrosarcoma (19%), chondrosarcoma (15%), and carcinosarcoma (13%). Thus, the voting indicated a wide range of opinion regarding the general nature of the tumor, 34% favoring a mesenchymal origin (chondrosarcoma, chondrofibrosarcoma), 28% for an epithelial origin (with cartilaginous metaplasia), and 38% for a neoplasm with both epithelial and mesenchymal neoplastic components (carcinosarcoma, mixed tumor). This lack of consensus may reflect the unusual morphology and rarity of this case. It was noted during the discussion that this case has not yet completed the NTP peer-review process, so the presenter's diagnostic choice (preputial gland mixed tumor) was not definitive.
The preputial and clitoral glands are homologous, specialized, paired sebaceous glands found, respectively, in male and female mice and rats. Their secretions contain pheromones attractive to the opposite sex (Orsulak and Gawienowski 1972
). The glands (8–20 mm long and up to 5 mm wide) are located in the subcutaneous tissue on either side of the base of the external genitalia (Beaver 1960
; Hebel and Stromberg 1976
; Reznik and Ward 1981
). Histologically, the preputial and clitoral glands are lightly encapsulated, holocrine, branched, tubuloalveolar glands composed of acini supported by a scanty fibrous stroma and lined by flattened, peripheral, basal cells and more central, plump, sebaceous-type cells packed with numerous lipid droplets and bright eosinophilic secretory granules (lysosomes containing acid hydrolases, phospholipids, glycoproteins, and α2μ
-globulin) (Beaver 1960
; Mancini et al. 1989
; Mesquita-Guimarães and Coimbra 1974
). The acini empty into branching lateral ducts, which in turn empty into a large central duct, and then into an extraglandular excretory duct. The preputial excretory ducts terminate at the junction of the parietal prepuce and skin, while the clitoral excretory ducts open near the urethra near the distal end of the clitoris (Hebel and Stromberg 1976
). Both ducts are lined by stratified squamous epithelium, with keratinization usually only in the excretory duct (Reznik and Ward 1981
Epithelial neoplasms of the preputial/clitoral glands are uncommon spontaneous lesions in F344/N rats. For example, the incidences of combined adenomas and carcinomas for control F344/N rats in the current (2010) NTP historical control data (HCD) of 1,295 male and 1,247 female F344/N rats (http://ntp.niehs.nih.gov/
[accessed August 2010]) are 5.3% for preputial glands and 12.7% for clitoral glands. Other epithelial neoplasms, such as squamous cell papillomas and carcinomas, are less common in these glands as there are no cases of either in the current NTP HCD. Increased incidences of preputial and/or clitoral gland epithelial neoplasms in male and/or female rats have been considered clear evidence of carcinogenicity in several NTP 2-year carcinogenicity bioassays (NTP 1989
). In most of these studies, the neoplasms were components of multiorgan neoplastic responses caused by potent carcinogens. Mesenchymal-origin tumors of the preputial/clitoral glands are rare spontaneous lesions in F344/N rats, and have not been identified as treatment effects in any NTP studies. In the current (2010) NTP HCD, the only preputial/clitoral gland mesenchymal neoplasms are one case each of fibrosarcoma and sarcoma in female F344/N rats (0.08% incidence for each). An earlier survey of preputial/clitoral gland neoplasia in treated and control F344/N rats noted only 1 preputial gland fibroma (in a treated male) out of 359 clitoral/preputial gland neoplasms examined (Reznik and Ward 1981
). Thus, the present case is unusual not only for its mixture of presumably neoplastic mesenchymal (cartilaginous/fibrous) and epithelial (ductal/acinar) elements, but also for the extensive cartilaginous differentiation of the mesenchymal component.