Fine structure of granulated cells in the posterior cardinal and renal veins of Amia calva L.

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Abstract
Granulated cells located in the walls of the posterior cardinal and renal veins in the holostean fish Amia calva resemble cells of the adrenal medulla of higher vertebrates. The cells all contain similar electron-dense secretory granules, apparently originating from the Golgi apparatus and are innervated by nerve terminals. The cells are probably involved in the production and release of catecholamines. The relationship of the cells to the endothelium of the large veins suggests that the secretory products of the cells reach the circulation by traversing a complex subendothelial space and passing through the fenestrae of the endothelium. This appears to be an efficient means for the immediate release and distribution of catecholamines in this species of fish.