The head-kidney (HK) is a unique immune, hemopoietic, and endocrine organ in fish.
Twenty fish were used to study the structure, ultrastructure, and
immunohistochemistry of the HK of Grass carp. The HK of grass carp was consisted
mainly of a network of hematopoietic cells with a few kidney tubules. The HK
contained all the developmental stages of erythrocytes and leucocytes, in addition to
the presence of rodlet cells, dendritic cells, and epithelial reticular cells. Two
populations of lymphocytes could be identified in HK; B-lymphocytes expressed
CD20 and T cells expressed CD3. The cytotoxic T lymphocytes (CTLs) expressed
CD8, while polymorphic granulocytes expressed iNOS2. Neutrophils showed
myeloperoxidase immunoreactivity, as well as eosinophils expressed CCR3.
Furthermore, macrophages were the most frequent cells found in the anterior kidney,
which aggregated into structures called melanomacrophage centers (MMCs).
Macrophages and monocytes showed immunoreactivity for CD68, CD163, and CD64.
On the other hand, the interrenal cells (ICs) and chromaffin cells were mixed and
arranged in large anastomosing cords intermingled with numerous capillary vessels
that that lined the posterior cardinal veins of the head kidney. The cytoplasm of the
interrenal cells showed the characteristic features of steroid-producing tissues. Three
types of chromaffin cells; noradrenaline, adrenaline, and small granule containing
cells could be distinguished in HK of grass carp. GFAP-positive sustentacular cells
were identified for the first time among the chromaffin cells. The present study
showed the presence of hemopoietic cells, MMCs, rodlet cells, immune cells, as well
as one type of interrenal cells and three types of chromaffin cells in the HK of grass
carp.