Lymph Node Structure. An Ontogenetic Explanation for Divergence in Eutheria, Metatheria, and Prototheria

The development of the mesenteric lymph node was studied inthe polyprotodont marsupial, Marmosa mitis. A primary mesenchymalradiation from the mesenteric arterial investment into the surroundingmesenteric lymph space yielded reticulum having lymphoid affinity.A secondary radiation yielded the supporting connective tissues,i.e., the node's hilum and trabeculae. This secondary radiationis limited to eutherian and metatherian mammals; its absencein prototheria explains both the absence of complex lymph nodesin these mammals and the singular follicles found in their lymphvessels. The mesenchymal source indicates that complex lymphnodes, like splenic follicles, derive from mesenchyme associatedwith portions of the arterial tree. The ensuing evolution fromsource mesenchyme to mature nodal configuration is describedin a tentative histogenetic scheme.