The most common monodermal teratoma is the carcinoid tumour of the testis.
Microscopically,there are islands of cells forming small acini and cords forming rosettes or sheets. The cells have granular eosinophilic cytoplasm, round nuclei with granular chromatin. Usually there is no intratubular germ cell neoplasia, no/minimal mitotic activity and usually minimal atypia or necrosis
The features of the primary tumour and those that distinguish them from metastatic carcinoid are well known- ( metastatic carcinoid: usually bilateral ; multifocal ; vascular invasion)
In contrast, it has been noted that primitive neuroectodermal components in testicular germ cell tumours may in some instances be conspicuous. It may even dominate the microscopic picture, rarely representing most of or the entire neoplasm. These foci of malignant small cell neoplasia with varyingly prominent differentiation of neuroectodermal type are usually recognized when there is an associated teratomatous component from which they arise, but when the latter are inconspicuous, problems in differential diagnosis may result.
Foci resembling various forms of central nervous system tumours, such as medulloepithelioma or ependymal neoplasms or neuroblastoma, may be noted.
Neuroendocrine carcinomas (carcinoid tumor) of the testis. A clinicopathologic and immunohistochemical study of ten cases. Am J Clin Pathol.2003 Aug;120(2):182-7.
We studied 10 cases of primary pure testicular neuroendocrine carcinoma. Patients were between 16 and 48 years old and had testicular swelling with pain or a painless testicular mass and no history of neuroendocrine carcinoma or other malignant neoplasm. All underwent orchiectomy. The tumors were low (n = 9) and intermediate (n = 1) grades with a variegated histologic appearance characterized by a nesting pattern, cords of neoplastic cells with rosettes, or sheets of neoplastic cells. Mitotic activity was lacking in 9 cases. In 1 case, mitotic figures ranged from 7 to 8 per 10 high-power fields, and cellular atypia and comedo-like necrosis were present. Immunohistochemical studies using a keratin cocktail, chromogranin, synaptophysin, epidermal growth factor, p53, placental-like alkaline phosphatase, and CD117 (c-kit) were performed in all cases. Keratin, chromogranin, and synaptophysin were positive in all tumors. Clinical follow-up information was obtained for 6 patients (range, 12-60 months): 5 with low-grade tumors were alive 24 to 60 months after diagnosis; 1 with an intermediate-grade tumor died of tumor 12 months after initial diagnosis. The behavior of these tumors, while in the testicular region, correlates well with the histologic grade. We propose replacing the term testicular carcinoid with neuroendocrine carcinoma, which better reflects the nature of these neoplasms.
Primary malignant carcinoid of the testis. Arch Pathol Lab Med. 1981 Oct;105(10):515-7.
A primary malignant carcinoid in the testis of a 76-year-old man was studied. The clinical indication was the appearance of extensive, papular cutaneous metastases. A markedly enlarged, indurated left testis was discovered during evaluation of the skin lesions. There were no clinical features of carcinoid syndrome. Histochemically, the tumor cells were argyrophilic but nonreactive to an argentaffin stain. Electron microscopy revealed numerous round, neurosecretory-type granules that ranged from 70 to 300 nm. At the time of autopsy, metastases were found in the heart, lungs, and the opposite testis. Review of the literature failed to uncover any previous reports of a primary malignant carcinoid in the testis.