Changes in glomerular structure after sexual maturation and seawater adaptation in males of the euryhaline teleost Gasterosteus aculeatus L.

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Abstract
In sexually mature male sticklebacks, the renal tubular cells are transformed from ion reabsorbing to mucus secreting cells and in these fish concomitant changes take place in the glomeruli. The present study compares glomerular structure of immature males in fresh water (controls) to those of mature males in fresh water and to immature male sticklebacks in seawater. Glomerular structure is markedly altered in the latter two groups and the changes are similar to a large extent. In these two groups the renal capsules and glomeruli are smaller and the lumina of the glomerular capillaries decrease in diameter, while the number and size of the endothelial fenestrations are reduced. Mesangial cells proliferate and the mesangial matrix greatly expands in both the centrolobular region and the subendothelial space around the capillaries. The secretory activity of the podocytes is enhanced and is responsible for the observed increase in thickness of the outer layer of the basal lamina, the lamina rara externa. The area covered by the filtration slit membranes is reduced, probably as a consequence of fusion of the pedicels of the podocytes. The permeability characteristics of the glomerular filtration barrier for macromolecules, as studied with ferritin injections, remain unaltered. However, the observed differences point to a reduction of the glomerular filtration rate (GFR) during maturation in male sticklebacks, as well as during adaptation of sticklebacks to seawater. This conclusion is in line with physiological evidence.