The Evolution of Blood-Forming Tissues

Abstract

The phylogeny of blood-forming tissues may be traced from invertebrates to vertebrates via annelids and cy-clostomes. A lymphocyte-like cell represents the primordial element from which differentiate erythrocytes, granulocytes, thrombocytes and monocytes. Among the vertebrates the spleen constitutes the fundamental blood-forming organ. In the hagfish the spleen consists of a diffusely scattered, perivenous myeloid tissue throughout the submucosa of the gastro-intestinal tract. In the lamprey the spleen is aggregated within the spiral valve. In the lungfish the spleen is still intra-enteral, but no longer diffuse; it has become segregated within the wall of the stomach. The submucosa of the intestine retains considerable granulocytopoietic activity. In ganoid fishes the spleen is a sharply segregated extra-enteral myeloid organ attached to the mesentery. In elasmobranchs and teleosts and all of the higher classes the spleen exists essentially as in ganoids. In Amphibia it is still the essential erythrocytopoietic organ. In certain fishes and in larval Amphibia the intertubular tissue of the mesonephros has accessory erythrocytopoietic function. With the development of long hollow bones, beginning with the Anura, the spleen becomes increasingly less important as an erythrocytopoietic organ. In Aves and Mammalia it is substantially a vestigial organ; blood-formation is here restricted to the bone marrow. The substitution in evolution of bone marrow for spleen as the locus of blood formation finds its explanation in the peculiar vascular mechanism of marrow, and in the relatively larger area and more elastic nature of this tissue. The essential identity between the vascular arrangement of the spleen and bone marrow inheres in the presence of long, straight, non-anastomosing arterial capillaries leading to an extensive system of venous sinusoids. The essential factors for hemoglobin elaboration in these sinusoids, are relatively static blood and the consequent high C02 tension.