PMC9453206_01
Feline embryonal tumor located rostrally to the cerebellum, sub-gross view. Sagittal para-medial section of the mass, displacing the rostral cerebellar lobe and occupying the rostral fourth ventricle. The neoplasm infiltrates the rostral medullary velum (arrow), the cerebellar leptomeninges, and focally the cerebellar folia. Densely cellular tumor foci are intermingled with paucicellular eosinophilic areas and focally extensive cystic degeneration (asterisk). The lines indicate the region of acquisition of Figure 2 images. Hematoxylin and eosin (H&E) stain.
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F1
It was macroscopically well-demarcated although it compressed and infiltrated the rostral cerebellar lobe, the rostral medullary velum, the floor of the fourth ventricle, and focally the caudal colliculus (Figure 1). At sub-gross examination, the neoplasm was composed of areas with different cellularity with focally extensive cystic degeneration (Figure 1).
Feline embryonal tumor with multilayered rosettes, neuropil-like areas and foci of neurocytic differentiation; histological aspects. (A) The tumor is composed by dense clusters of small-to-medium undifferentiated cells, paucicellular regions with fibrillary eosinophilic (neuropil-like) appearance and multilayered rosettes with slit-like or round lumens. (B) Rosettes have multiple peripheral nuclear layers and a central nuclei-free zone with frequent mitotic figures; central lumens are bounded by distinct cell membranes with eosinophilic contouring and debris (insert). (C) Irregular chords of undifferentiated cells have scant indistinct cytoplasm and form perivascular pseudorosettes (arrow). (D) In neuropil-like areas, rare scattered cells exhibit neurocytic differentiation (arrows). (E) Focal necrosis with macrophage infiltration in a densely cellular and highly proliferating area. (F) Part of the mass limiting the area of cystic degeneration, characterized by optically empty lumen with sparse eosinophilic debris. H&E stain.
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F2
Microscopically, dense irregular clusters, bundles and cords of small-to-medium undifferentiated cells with scant cytoplasm and indistinct cell borders were intermingled with paucicellular regions with fibrillary eosinophilic (neuropil-like) appearance (Figure 2A). Numerous multilayered true rosettes were found in both the highly cellular and neuropil-like areas (Figure 2B). Lumens were empty but frequently contained fine irregular granules and/or exhibited delicate eosinophilic contouring of the membrane surface (Figure 2B insert). Frequently, neoplastic cells also formed palisades around capillaries (perivascular pseudorosettes; Figure 2C). In neuropil-like areas, foci of desmoplastic stromal response and rare neurocytic cells were present (Figure 2D). Few small necrotic foci were scattered through the neoplasia (Figure 2E), mostly near cystic degeneration area, containing sparse necrotic debris (Figure 2F).
Feline embryonal tumor with multilayered rosettes, neuropil-like areas and foci of neurocytic differentiation; immunohistochemistry. (A) Diffuse staining with vimentin both in rosettes and in undifferentiated neoplastic cells. (B) Strong synaptophysin expression in neuropil-like areas with negative rosettes. (C) Multifocal NSE expression by undifferentiated cells and cells with long fibrillar cytoplasmic projections. (D) Scattered cells with strong nuclear NeuN staining, less intense in the cytoplasm of rare cells. (E) Patchy cytoplasmic and nuclear S-100 protein expression in neoplastic cells with variable staining intensity. (F) Frequent Ki67 nuclear expression in rosette-forming and undifferentiated cells. Diaminobenzidine (DAB) chromogen; Mayer hematoxylin counterstain.
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F3
Neoplastic cells were diffusely and strongly positive for vimentin, with exception of few scattered cells, mostly with neurocytic differentiation (Figure 3A). Neuropil-like areas exhibited diffuse and intense expression of synaptophysin and NSE (Figure 3B). Both markers were also multifocally expressed by clusters of small undifferentiated and neurocytic cells, with variable stain intensity, while multilayered rosettes were negative (Figure 3C). Rare, scattered nuclei of neurocytes and undifferentiated cells were positive for NeuN, generally in paucicellular areas (Figure 3D). Patchy cytoplasmic and nuclear S-100 protein expression was observed in cellular clusters, both in highly cellular and neuropil-like areas (Figure 3E). Ki67 nuclear expression was intense both in multilayered rosettes and in small cell areas, with a medium proliferation index of about 70% (Figure 3F).