Functional morphology of the glomerular filtration barrier of Gallus gallus

The anionic charge barrier and the endothelial and epithelial pore sizes on the glomerular filtration barrier (GFB) were examined in white leghorn chickens (Gallus gallus). Ruthenium red was used to stain anionic charge sites on the GFB. The tissue was treated by normal dehydration and freeze substitution dehydration for transmission electron microscopy (TEM). In addition, the basal lamina was isolated for study. The results of our study indicate that G. gallus possess a thick, negatively charged glycocalyx surrounding the podocytes and slit diaphragm and on the endothelium. However, in all cases, little anionic charge is present in the basal lamina. The pores on the endothelium are elliptical and have mean dimensions of 148 x 110 nm. This is in contrast to mammals, which have smaller, round pores. The epithelial pores in G. gallus measure approximately 35 nm in length, approximately 4 times larger than those found in mammals. These results indicate that the avian glomerulus may allow the filtration of larger molecules from the plasma than occurs in mammals and that the charge on the molecule may not be as restrictive a filtration characteristic as in mammals.