The Leydig organ of elasmobranchs: Shed light on an active and mysterious defense center with immunological characterization of its cells.

In elasmobranchs, blood cells originate from the thymus, spleen, and two organs associated with the gonads and esophagus: the epigonal and Leydig organs (LO), respectively. In the LO parenchyma, various granulocyte types, thrombocytes, and lymphocytes were observed, with notable species-specific diversity. Cells of the LO in the catshark Galeus melastomus were characterized using immunohistochemistry, immunofluorescence, and electron microscopy. Image analysis of sections treated with the anti-proliferative cell nuclear antigen antibody revealed that approximately 40 % of LO cells were in mitosis and approximately 15 % were myeloid progenitors positive for the anti-c-kit antibody. Additionally, a panel of 12 immunological markers revealed that 18.4 %, 17.8 %, 13.7 %, and 14.1 % of the LO section area was occupied by cells positive for anti-serotonin, interleukin-6, histamine, and immunoglobulin E-like receptor antibodies, respectively. Two main reactive cell types, mast cell and macrophage lineage, were revealed by co-localization image analysis using confocal microscopy. Ultrastructural observation identified granulocytes and macrophages as the main cell types. To our knowledge, this is the first report on the distinct patterns of LO cell populations and their morphological features, confirming granulopoietic role of the elasmobranch LO.