OF MAMMALS

ABSTRACT- Mammals present different types of spleen. In the past, attempts have been made to define characteristics for a classification of this organ, but the evolutionary process that has led to the splenic types of modern mammals has been poorly investigated. This review categorizes all mammalian spleens studied so far, synthesizing several earlier classification principles and updating them in the light of recent research. The main qualitative features are the type of circulation (closed, open or both) and the nature of the venous vessels in the splenic cords (venules or sinuses). The main quantitative feature is the proportion between red and white pulp. Moreover, a phylogeny of this organ is proposed: from a ‘primitive type ’ with closed circulation (characteristic of monotremes, insectivores and tree shrews), the spleen has evolved into an ‘archetypal type ’ with open circulation (peculiar to mice, gerbils, bats and marsupials). It has subsequently diversified into different models: blood-storing spleens (ungulates and carnivores) and defensive spleens (Old World monkeys and the majority of rodents)