Yolk sac tumors are the most frequent kind of malignant pediatric germ cell tumor and may have a fundamentally different pathogenesis than adult germ cell tumors. Since few cytogenetic studies have been performed so far, in situ hybridization was applied to interphase cell nuclei of seven gonadal yolk sac tumors of childhood in routine paraffin-embedded tissue sections. The panel of chromosome-specific DNA probes was selected on the basis of their relevance in adult germ cell tumors and consisted of five DNA probes specific for the (peri)centromeric regions of chromosomes 1, 8, 12, 17 and/or X and/or one DNA probe specific for the subtelomeric region of chromosome 1 (p36.3). As in adult germ cell tumors, all pediatric gonadal yolk sac tumors had an increased incidence of numerical chromosome aberrations. All tumors showed an overrepresentation of at least three chromosomes. Gains of chromosome 12, which is highly specific in adult germ cell tumors, were diagnosed in six pediatric gonadal yolk sac tumors. The DNA indices determined in the paraffin-embedded tumor material correlated well with the in situ hybridization findings. A chromosome was either over- or underrepresented, compared with the corresponding DNA indices, in only a few cases. The short arm of chromosome 1 in adult germ cell tumors is often involved in structural aberrations. In pediatric germ cell tumors, the short arm of chromosome 1 is also a nonrandom site of structural aberrations. Moreover, the presence of a deletion at 1p36.3 in four out of five tumors suggests that the loss of gene(s) in this region is an important event in the pathogenesis of gonadal yolk sac tumors of childhood.

Germ cell tumours in children are more often extragonadal than in adults and the most frequent type is the yolk sac tumour. Limited cytogenetic data exist on extragonadal yolk sac tumours in children. We applied in situ hybridization (ISH) to interphase cell nuclei of four paediatric extragonadal pure yolk sac tumours and one yolk sac tumour component of a mixed germ cell tumour using paraffin-embedded tissue sections. The panel of chromosome-specific DNA probes was selected on the basis of their relevance in adult germ cell tumours and consisted of five DNA probes specific for the (peri)centromeric regions of chromosomes 1, 8, 12, and/or 17, X and/or one DNA probe specific for the subtelomeric region of chromosome 1 (p36.3). Only one tumour failed to show numerical and structural chromosome aberrations with the DNA probes used. The other four had an increased incidence of numerical chromosome aberrations with an over-representation of at least one chromosome. The DNA indices determined in the paraffin-embedded tumour material correlated well with the in situ hybridization findings. In only a few cases were chromosomes over-represented, when compared with the corresponding DNA indices. Recently, we have shown that the short arm of chromosome 1 is a non-random site of deletion in paediatric gonadal pure yolk sac tumours. The occurrence of similar deletions in one extragonadal pure yolk sac tumour and in one yolk sac tumour component, in conjunction with two further ISH reports, suggests that the loss of gene(s) in this region is an important event in the pathogenesis of paediatric malignant germ cell tumours of nearly all sites.

Five cases of intraabdominal small-cell tumor with divergent differentiation are reported. All patients were of male sex. They were 10, 15, 20, 21, and 30 years of age at time of diagnosis, respectively. By light microscopy, the tumors consisted of small cells arranged in groups, nests, and clusters separated by a collagen-rich desmoplastic stroma. Immunohistochemical studies revealed the coexpression of mesenchymal, epithelial, and neural markers. Notably, all tumors coexpressed vimentin, cytokeratin, and desmin, the latter in a remarkable paranuclear dot-like fashion. In contrast to other authors, we did not find chromogranin. DNA image cytometry on four cases demonstrated two diploid and two aneuploid (hyperdiploid) cases. No correlation was found between ploidy and prognosis. One patient died from disease, another died from veno-occlusive disease after bone marrow transplantation, and the remaining patients are alive, but have progressive intraabdominal disease. Thus, our findings support the poor prognosis in this type of tumor.

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