Storage of Blood in the Mammalian Spleen: an Evolutionary Perspective

Splenic blood storage is usually considered a characteristic restricted to Carnivora, Perissodactyla, and Ruminantia. In these mammals, the red pulp comprises the major part of the organ and - within it - the cords show a vast extension, allowing the storage of a great quantity of erythrocytes. Moreover, well-muscularized capsule and trabeculae permit a strong contractility. In this way, a significant quantity of erythrocytes can be poured into the circulation in response to an increased demand of oxygen. These spleens are usually classified as the storing type, in contrast to the defensive type, characterized by negligible quantity of blood stored. Past evolutionary interpretations have seen the former type as derived from the latter one. This explanation was based on some ontogenetic observations, thus deriving from the approach of Haeckel's biogenetic law, and caused some contradictions that remained unsolved. An in-depth examination of the literature reveals that almost all mammalian species can store erythrocytes in their spleen. What changes is the quantity of this storage and the efficiency of its release. This means that the dichotomy between storage and defensive types is just an approximation - and not very useful. Splenic storage, indeed, is not a recently acquired character by some mammals, rather a primitive one, that underwent different quantitative and qualitative changes during the evolution of the different mammalian lineages. This review offers a new hypothesis on the evolution of this function (viewing the presence of splenic erythropoiesis as a biological constraint) and proposes it as a case of exaptation.