The fine structure of the follicle-forming cells in the adenohypophysis of the ironfish, a natural hybrid between the Funa and the goldfish (Teleostei, Cyprinidae).

The ironfish considered to be the natural hybrid between the Funa (Carassius carassius) and goldfish (C. auratus), has several peculiarities in its morphology, karyology and reproductive biology. Its adenohypophysis also shows follicular formations which are never found in either parent species. The follicles of the ironfish adenohypophysis consist of both prolactin cells and stellate (agranular) cells. The prolactin cells have a tall pyramidal form, and their secretory granules are seen more often in the basal cytoplasm than in the apical region. The electron-dense content of the granule is released into the perifollicular space by exocytosis. The apices of the cells bearing microvilli and a cilium are exposed to the follicular lumen. Many of the cilia, however, protrude from the lateral surface of the apical region into the intercellular cleft. The apical processes of the stellate cells extend among the prolactin cells and reach the follicular lumen. The end of the process becomes broad with microvilli and a cilium, and is joined together with the apical processes of other stellate or prolactin cells and sometimes lies in close proximity to the corticotrophs near the follicle. Nerve terminals in the perifollicular region contain small clear vesicles and large granular vesicles. They are occasionally in direct contact with prolactin cells and corticotrophs. By fluorescence histochemistry for catecholamines, blue-green fluorescent fibers are frequently detected in the RPD but rarely near the follicle. These findings suggest that the follicles seen in the ironfish are derived from the regional rudiments of the Rathke's pouch.