Comparison and definition of spleen and lymph node: a phylogenetic analysis

The hitherto largely unsolved problem with a biological definition of spleen versus lymph node seems possible to solve from a phylogenetic point of view. Thus, it is suggested that the spleen be defined as a hemopoietic organ which is able to filter blood with sinusoids. In contradistinction, a lymph node is defined as a hemopoietic organ which is able to filter lymph with sinusoids. Comparative anatomical studies show that the spleen appears as a condensation of the lymphomyeloid complex in the spiral fold of the gut in cyclostomes. The spiral fold spleen vanishes with the bony fishes, while in cartilaginous fishes a similar spleen appears in the dorsal mesentery. The dorsal spleen remains in a retroperitoneal position in higher vertebrates and is regarded as a specialized blood vessel compartment closely connected with the blood stream. In "higher" vertebrates the spleen is a stagnated organ because splenic functions are gradually transferred to other sites. The bone marrow takes over the erythro-, thrombo- and granulocytopoiesis while the lymph nodes take over the lymphocytopoiesis. This transfer of the splenic functions is first seen in anurans and seems to be a marvelous adaptation to life on land where the need for local defence against a large number of antigens is necessary before spread of the antigens to central parts of the body. In higher vertebrates, the great number of lymph nodes at peripheral positions, derived from the lymphatic vessels, are able to do so. It is demonstrated that the definitions of spleen and lymph nodes as hemopoietic organs which by their sinusoids are able to filter blood and lymph, respectively, are not only of semantic interest but also useful in regard the immunohematological system as an entity.