Twenty-three fishes were used to study the structure and ultrastructure of interrenal tissue,
chromaffin cells and corpuscles of Stannius of Nile tilapia. The interrenal tissue and chromaffin cells are present within the head kidney. The interrenal tissue is arranged in the form of
highly convoluted cords, bordered by the lining endothelium of the adjacent sinusoids. It has
no connective tissue capsule. The cytoplasm of the interrenal cells contains abundance of
mitochondria, vacuoles and smooth endoplasmic reticulum, characterizing of steroidproducing tissues. Two types of chromaffin cells; noradrenaline (NA) cells and adrenaline
cells (A) could be recognized by light microscope using chromaffin reaction, as well as by
electron microscope they could be distinguished depending on the size and electron density
of their granules. The corpuscles of Stannius are two in number and located on the dorsal
aspect of the tail kidney. Each corpuscle is surrounded by thick connective tissue capsule.
The parenchyma is divided into lobules, each of which is surrounded by distinct basal lamina
and has a pseudo lumen. Depending on the presence of secretory granules and the relative
abundance of cell organelles, three cell types could be recognized; granular cell, agranular
cell (Type I) and agranular cell (Type II). In conclusion, the morphological and ultrastructural
analysis of the endocrine tissues of the kidney of Nile tilapia has revealed only one type of
interrenal cells, two types of chromaffin cells and three staged-cells of Stannius corpuscles.