Structure of the respiratory islets of accessory respiratory organs and their relationship with the gills in the climbing perch, Anabas testudineus (Teleostei, Perciformes).

The epithelial and sub-epithelial organization of the accessory respiratory organs of Anabas testudineus has been compared with that of gills by using light and transmission electron microscopy. The details of vascular supply of respiratory islets (RI) and gill filaments and the presence of venous sinusoids in the two systems suggest that the RI have been derived from gill filaments and lamellae. The biserial arrangement of transverse capillaries (TC) in the respiratory islets (RI) is evident under the scanning electron microscope and their homology with the gill filaments and their secondary lamellae has been established. The two sets of transverse capillaries of respiratory islets have been derived either from embryonic transverse or marginal channels of two sets of lamellae of a gill filament. These capillaries with their endothelial septate valves and tongue-like processes offer resistance to blood flow. Gill filaments have two vascular pathways, arterio-arterial and arterio-venous. However, the RI of accessory respiratory organs contain the arterio-venous pathways. This arrangement as well as the septate transverse capillaries may lower the "pulmonary" blood pressure considerably. Two types of mitochondria-rich cells are identified: i) chloride cells with flat microvilli bearing surfaces, devoid of apical pit and (ii) an elongated cell type with sac-like endoplasmic reticulum, with apical pit that comes into close contact with the complex matrix of macrophages, lymphocytes and other loose cells of the epithelium. These cells may be associated with detoxification of the internal media of fish inhabiting foul waters.