The spleen of Physalaemus nattereri (Amphibia: Anura): morphology, melanomacrophage pigment compounds and responses to α-melanocyte stimulating hormone

The spleen is a lymphoid organ associated with defense mechanisms in anurans, and also it has pigmented cells. Melanomacrophages (MMs) are melanin-containing cells, which originated from hematopoietic stem cells and are found in hematopoietic organs. Melanin has bactericide and cytoprotective actions against free radicals. Furthermore, the α-melanocyte stimulating hormone (MSH) induces dispersion of melanosomes in melanocytes, besides having an anti-inflammatory action. Here, we describe the spleen morphology of Physalaemus nattereri and the response of splenic melanomacrophages to the α-MSH hormone. Animals were treated with α-MSH for 2, 6, 12, 24 and 48 h. Then, we measured the amount of melanin, lipofuscin and hemosiderin in each treatment. The spleen of P. nattereri is ovoid, and a connective tissue capsule covers the organ externally. There are no septa and the stroma is reduced. The parenchyma has two regions slightly separated from each other: white and red pulps. MMs occur in the red pulp. The α-MSH increased the volume of lipofuscin and hemosiderin in MMs after 6 h, but not melanin. Thus, the α-MSH altered only metabolic substances in splenic melanomacrophages, but not melanin, which usually is responsive to this hormone in hepatic MMs. Also, the description of the spleen morphology can help future comparative morphological and evolutionary studies on spleen morphology of vertebrates.