In contrast to germinomas, yolk sac tumors rarely occur in pure form. More commonly, they are a component of a mixed germ cell tumor. Cytologically, neoplastic cells are large and polygonal in shape with faint eosinophilic or clear cytoplasm and well-defined cytoplasmic borders. Nuclei are moderately atypical, but generally lack marked pleomorphism (Fig. 11.4). Tufts of malignant cuboidal-to-columnar tumor cells surrounding central blood vessels, known as Schiller-Duval bodies, are common though not universal, and not necessary for the diagnosis. The characteristic intercellular and extracellular PAS-positive/diastase-resistant eosinophilic hyaline globules and intercellular longitudinal bands of eosinophilic basement membrane material offer additional diagnostic clues.

Perhaps the most consistent finding in yolk sac tumors is the variety of different morphologic patterns often encountered within the same tumor; a mixture of patterns is the rule (Figs. 11.4 and 11.5). The most common among these is the reticular or microcystic pattern consisting of cysts lined by a loose network of flattened-to-cuboidal cells. A macrocystic pattern is seen when the microcysts coalesce. The polyvesicular vitelline pattern displays larger vesicles lined by flat-to-columnar epithelial cells. The endodermal sinus pattern shows a predominance of Schiller-Duval bodies and the papillary pattern shows papillae rimmed with tumor cells. Clusters of liver-like tumor cells are seen in the hepatoid pattern and neoplastic cells embedded in a matrix of basement membrane-rich material are evidence of parietal differentiation. Glandular, myxomatous, solid, and sarcomatoid patterns are often found in association with the aforementioned patterns.

Tumor cells are highly atypical and are generally larger, more pleomorphic, and more carcinoma-like than those of germinoma (Fig. 11.6). Nuclei are oval-to-round and hyperchromatic with irregular nuclear contours and large single or multiple nucleoli. Their cytoplasm is fairly abundant, somewhat granular, and variably staining. Cytoplasmic borders are not well defined, accounting for the syncytial appearance of the tumor (Fig. 11.7). The malignant cells can be arranged in solid sheets, cords, papillae, or gland-like patterns. The so-called appliqué pattern, characterized by smudged, degenerate-appearing cells seen towards the periphery of tumor nests, imparts a superficial resemblance to choriocarcinoma. Necrosis is common and the mitotic rate is typically high. As with germinomas, syncytiotrophoblastic giant cells are not an infrequent finding. Small foci of neoplastic poorly differentiated stroma, considered by some investigators to be teratomatous in nature, may accompany embryonal carcinoma and account for chemotherapy failure.

Akin to their gonadal counterparts, teratomas of the CNS can comprise both mature and immature elements. Pure mature teratomas tend to behave in an indolent fashion whereas teratomas occurring as part of a mixed GCT are more aggressive regardless of their degree of maturity. Their biologic behavior, thus, is likened to that of ovarian teratomas. Congenital teratomas, on the other hand, have a universally poor prognosis regardless of their composition.

Choriocarcinomas are the most malignant GCTs but are, fortuitously, the least common among the primary tumors. They are extensively hemorrhagic and highly necrotic tumors comprising of two cell types: syncytiotrophoblasts and cytotrophoblasts (Fig. 11.10). Syncytiotrophoblasts are easily recognized as large multinucleated cells with smudged vesicular nuclei and dark eosinophilic-to-amphophilic cytoplasm. Cytotrophoblasts are more uniform and have single bland nuclei with vesicular chromatin and pale-to-amphophilic cytoplasm. Cytoplasmic borders are distinct. Syncytiotrophoblasts and cytotrophoblasts are arranged in a biphasic plexiform pattern similar to that seen in chorionic villi where sheets of cytotrophoblasts are surrounded (caped) by syncytiotrophoblasts. Mitoses are easily identifiable in the cytotrophoblastic component, but occur only exceptionally in syncytiotrophoblasts. Rarely, the cytotrophoblastic component dominates. This monomorphic pattern, which can be seen following chemotherapy, may be difficult to recognize and often requires immunohistochemical confirmation. As discussed earlier, it is important to recognize that scattered syncytiotrophoblasts are not uncommonly encountered in other GCTs and their presence alone is not diagnostic of choriocarcinoma.

As previously mentioned, mixed germ cell tumors comprise two or more of the previously described histologic variants. The relative percentage of each component should be reported. It is recommended that the tissue be, at minimum, extensively sampled if not entirely embedded to avoid underreporting of a GCT component.

Routine use of ancillary immunohistochemical studies is standard of practice in the work-up of primary CNS GCTs. The most historically utilized immunostain in GCTs, placental alkaline phosphatase (PLAP), is now obsolete and has been replaced by superior, more specific and sensitive transcription markers, such as OCT4 [10]. OCT4 preferentially highlights germinomas and embryonal carcinomas and has the added advantage of being a nuclear marker allowing for easier interpretation (Fig. 11.11). CD30 shows strong membranous staining in embryonal carcinomas while other GCTs, including germinomas, are negative (Fig. 11.12). C-kit can also be exploited in the differential diagnosis of embryonal carcinoma versus germinoma as it shows strong and diffuse membranous staining in germinoma but focal or weak cytoplasmic staining in embryonal carcinomas [11–13].

SALL4 appears to be a fairly sensitive and specific pan-germ cell marker and can be used as a screening marker. A recent study by Mei et al. demonstrated 100 % sensitivity for germinomas, embryonal carcinomas, and yolk sac tumors (Fig. 11.13) [14]. Additionally, positive staining was observed in approximately two-thirds of teratomas and choriocarcinomas; all non-GCTs were negative for SALL4 staining [14].

Alpha-fetoprotein (AFP) has historically served as the marker of choice for yolk sac tumors. Staining, however, is often focal and patchy and generally varies among the different patterns of tumors. Additionally, abundant background staining is often observed [14]. Glypican-3 is touted as a more superior marker in diagnosing yolk sac tumors of the ovaries and testes. Glypican-3 offers more precise and easy to interpret staining characteristics as well as improved sensitivity (Fig. 11.14) [15]. Studies evaluating glypican-3 staining in CNS yolk sac tumors, however, are limited.

βhCG and human placental lactogen (hPL) strongly stain the syncytiotrophoblastic cells of choriocarcinomas as well as those intermixed with other germ cell tumors [16, 17]. Cytotrophoblastic cells often are weakly positive or negative for these markers. Additionally, cytokeratins can also be used to highlight choriocarcinomas.

GCTs are thought to arise from progenitor germ cells, mainly due to the facts that the germinoma component resembles progenitor cells very closely, intracranial GCTs resemble their extracranial counterparts morphologically and immunophenotypically, a single tumor can have multiple components (mixed GCTs) suggestive of differentiation of progenitor cells along various lines (embryonic and extraembryonic), and because none of the progenitor cells in the brain share any morphologic features with CNS GCTs. It is hypothesized that aberrant migration of the germ cell progenitors ventrally along the midline is responsible for the predominant midline location of these tumors throughout the body. Both testicular and CNS GCTs have been shown to have overexpression of wild type p53 and MDM2 proteins with a low incidence of TP53 gene mutation and a moderate incidence of MDM2 gene amplification pointing towards a common origin of these tumors [18]. Since p14^ARF, a protein coded by the INK4a/ARF gene locus, functions as a tumor suppressor and regulates the interaction between the MDM2 and p53 proteins by stimulating degradation of MDM2, Iwato et al. further tested for gene mutations in the INK4a/ARF gene in 21 CNS GCTs. They found that 71 % of tumors (90 % of germinomas and 55 % of NGGCTs) had either a homozygous deletion (14/15) or a frameshift mutation (1/15) in this gene pointing towards a more central role for this protein in the development of CNS GCTs [19]. More evidence linking germ cell progenitors to CNS GCTs is the lack of methylation seen in gonadal and extragonadal GCTs, since the progenitor cells transiently lose methylation of imprinted genes during migration. Small nuclear ribonucleoprotein polypeptide N (SNRPN) is an imprinted gene with complete lack of methylation, which is common to GCTs and progenitor cells. However, Lee et al. showed that lack of methylation of SNRPN and other imprinted genes is also seen in neural stem cells in the brain providing an alternate hypothesis about the origin of CNS GCTs [20].