Fig.1 
Fig. 2. A 22-year-old-male, complaining of dyspnea and vague chest pains of two months duaration, was shown to have an anterior mediastinal mass by routine chest x-ray. Clinically, because of his age, a germ cell tumor or nonHodgkin's lymphoma were the main differential diagnoses. He was referred for CT guided transthoracic percutaneous aspiration needle biopsy and adequate core biopsies were obtained.
Cytopathology preparations were positive for malignant cells. These were organized as fragments of cohesive tumor cells with vesicular nuclei (Fig. 1). Immunocytochemistry performed on a cell block preparation was negative with antibodies to cytokeratins and human chorionic gonadotrophin. The cytologic features were reported as consistent with a non-seminomatous germ cell tumor.
Small fragments of the aspirate were fixed in glutaraldehyde and prepared for electron microscopy immediately following the transthoracic biopsy. The tissue contained small glandular formations and loosely arranged tumor cells separated by a myxoid stroma. Ultrastructurally, the glands were formed by columnar cells with microvilli (lacking microfilamentous core rootlets) on the luminal surface. Basal aspects of these glandular cells rested on a somewhat thickened layer of basal lamina (Fig. 2). Within the cytoplasm of some tumor cells were relatively large dark-staining inclusions (Fig. 3). Surrounding the glands were small, more primitive looking cells that were separated by glycosaminoglycans and basal lamina-like materials. In some areas, these cells formed more cohesive groups joined by desmosomes and forming small intercellular lumens often filled by microvilli. Between the cells, small intercellular spaces lined by basal lamina developed (Fig.4).
Fig.1
Fig. 2.
Fig.3 
Fig. 4
This case illustrates the value of electron microscopy in not only confirming the diagnosis of a germ cell tumor suggested by cytologic preparations (1,2), but in enabling the diagnosis of a specific subset of these tumors in the anterior mediastinum, an endodermal sinus tumor. The minibiopsy-like nature of fragments of aspirated tissue can retain the architectural arrangement of the tumor (3-5). Definition of features such as glandular differentiation, even when lumen formation may be submicroscopic, and the distribution and amounts of basal lamina were instrumental in recognizing this neoplasm as an endodermal sinus tumor (6-9). The dark-staining cytoplasmic inclusions are the counterpart of hyaline (PAS-positive) globules that are often seen by light microscopy in routine tissue sections and, if they are present, can assist in establishing the diagnosis in smear preparations (7,10). Ultrastructural features of malignant teratoma and endodermal sinus tumor of the anterior mediastinum diagnosed by fine-needle aspiration biopsy have been previously reported (8,9). Since endodermal sinus tumor is also referred to as yolk sac tumor, scanning and transmission electron microscopy of the human yolk sac may also be of interest. (11). A review of clinical aspects of germ cell tumors of the anterior mediastinum is also available (12).
Endodermal sinus (yolk sac) tumor - Anterior mediastinum.
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