Ontogenesis and functional metamorphosis of the head-kidney in bottomspawner and mouthbrooder cichlid fishes (Cichlidae, Teleostei)

Histological and electron-microscopical studies of the head-kidney ontogenesis in a group of bottomspawner and mouthbrooder cichlid fishes reveal that this organ undergoes three functional and structural stages. At the initial stage, 2-12 days after fertilization, the head-kidney is a filtering organ of two glomeruli and adjacent renal tubules; the second stage, beginning 12-30 days after fertilization, is a functionally mixed one, with the nephron and developing hemopoietic and endocrine, chromaffin, and interrenal cells; during the third stage, which begins 30-60 days after fertilization, the nephron degenerates, and the head-kidney starts functioning solely as a lympho-adrenal organ. With age the interrenal tissue expands, forming fields, especially along the blood vessels. The number of melano-macrophage centers progressively increases within the hemopoietic parts. Data are given on the various cell types, the reticuloendothelial stroma, and the renal part of this organ in various species. The earlier ontogenesis and function of the head-kidney, as thymus and other organs, in bottomspawner larvae compared to the more protected mouthbrooders provide support for earlier free functioning in a dangerous environment.