Ovarian carcinoid. Light micrograph of a primary ovarian carcinoid showing insular architecture consisting of a uniform population of tumour cells arranged in nests and islands separated by fibrous stroma. Ovarian carcinoids can be primary or metastatic. Primary ovarian carcinoid is a germ cell tumour that resembles well-differentiated neuroendocrine tumours arising in the gastrointestinal tract. Most cases occur as a component of mature cystic teratoma and develop from the neuroendocrine cells within intestinal-type or respiratory epithelium of the teratoma. Pure ovarian carcinoids are a form of monodermal teratoma and contain no other teratomatous elements. Primary ovarian carcinoids are rare and comprise less than 0.1% of all ovarian neoplasms. Metastatic ovarian carcinoids originate from the primary tumours located in the gastrointestinal tract, lung, or other sites. Haematoxylin and eosin stained section. Magnification: x200 when printed at 10 centimetres wide.