Origins and comparisons of blood cells

Even the most primitive invertebrates, the protozoans, possess cell surface receptors, are capable of distinguishing self from nonself and contain lyso-somes. They thus have the potential to give rise to immunocytes. With the appearance of primitive multicellular organisms or metazoans, well-organized cell layers with division of functions are formed. In sponges, for example, which lack a blood system, food is trapped by the choanocytes and carried around the body by phagocytic amoebocytes. Simple diffusion of nutrients, excretory products and respiratory gases through the body wall serves the needs of the primitive coelenterates and flatworms but, once the third body layer, the mesoderm, evolved, a circulatory system was required to transport trophic substances and respiratory gases around the body. This latter evolutionary step would have freed the blood cells from their nutritional role so that they migrated from the connective tissue into the circulatory system to take up their defensive role. This evolutionary step can be seen in some advanced platyhelminthes in which the phagocytes migrate from the parenchymatous connective tissue into the hemolymph. Once the blood cells took on a purely defensive role then differentiation into other cell types would have occurred because of various environmental and internal pressures detailed below.

The phagocytic cell type has been conserved throughout phylogeny. In invertebrates, it has a food-gathering role but in higher forms it also phagocytoses and encapsulates invading agents. In the vertebrates, the phagocytic cell evolved into polymorphs, monocytes and macrophages which retained their phagocytic role. The macrophage also took on the function of presenting antigen to the lymphocytes during specific acquired immunity. Vertebrate phagocytes also bear surface receptors for Fc and C3b which, with the exception of one echinoderm, appear to be absent on invertebrate phagocytic cells.

Much research has attempted to detect the origin of the vertebrate lymphocyte and indeed cells resembling lymphocytes in morphology are present in many invertebrates. In most cases, these are immature progenitor cells although some workers believe that invertebrates possess cells homologous to vertebrate lymphocytes. In annelids, the presence of allograft and xenograft rejection with specificity and short-term memory, adoptive transfer, graft infiltration by lymphocyte-like cells and blastogenic responses towards transplantation and T cell mitogens, indicate that the blood cells involved may be analogous to vertebrate lymphocytes and have arisen by convergent evolution. Similar properties are reported for lymphocyte-like cells in tunicates, but the strongest case for true homology is provided by the echinoderms. In the starfish, Astenas ruhens, lymphocyte-like cells secrete an antibody-like substance as well as a lymphokine-like factor. These axial organ cells can also be fractionated by a method used to separate mammalian B and T cells and respond like these cells towards mitogens. Another echinoderm is also reported to have C3b-like receptors on the surface of its blood cells. The case for true homology, however, awaits detailed molecular evidence of the sort recently produced in studies on the evolution of the vertebrate immunoglobulins.

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