Abstract: The evolutionary changes in size, number, structural and ultrastructural features and sites of origin of erythrocyte are reviewed by comparing the early appearance of coelomocytic hemoglobin-bearing cell of Invertebrates with the typical erythrocyte of submammalian Vertebrates and its phylogenetic variations from the poikilotherm Agnathans, Elasmobranch and Teleost Fishes, Amphibians and Reptiles up to the homeothermic Birds. The earliest hemoglobin-bearing cell becomes apparent in a limited number of marine Invertebrates, such as the Phylum Annelida (segmented marine worms), like a coelomocyte in which hemoglobin pigment is carried together with other constituents. The occurrence of erythrocytes during evolution allows an increasing concentration of hemoglobin that is carried into cells rather than free in hemolymph/coelomic solution. Moreover, cellular sequestration of hemoglobin simplifies its structure that otherwise should be polymeric to avoid its free diffusion out of hemolymph. The definitive structure of the submammalian vertebrate erythrocyte occurs in Fishes, whose erythrocyte feature, mainly ellipsoidal, biconvex and nucleated, is conserved through all submammalian Vertebrates. Among Fishes, the erythrocytes of Chondroichthyes are the largest cells: they are ellipsoidal, flattened and slightly biconvex, due to nuclear bulge. Their number is very low as compared to the Teleosts' counterpart, whose erythrocytes are smaller in size: this is explained by the inverse proportion between size and number of erythrocytes observed in all Vertebrates, including Mammals. The evolutionary advantage of small and numerous erythrocytes, as in Mammals, lies in being the best condition for gas exchange in active metabolic organisms. The endothermic Fishes display the highest values of erythroid parameters that are similar to those of Mammals. Conversely, Antarctic icefish erythrocytes contain the lowest level of hemoglobin, due to their low metabolic rate. The erythrocyte structure, phylogenetically established in Fishes, has been conserved in Amphibians except for its volume - the largest one among all vertebrate and invertebrate erythrocytes - due to the occurrence of a large amount of nuclear DNA, because of repeated DNA sequences typical of Amphibians. As observed in triploid fishes, both cellular and nuclear sizes increase in proportion with the increase in ploidy, while the nucleo-cytoplasmic ratio remains unchanged. Among Reptiles, with the exception of the archaic tuataras, sea turtles show the largest erythrocytes. Avian erythrocytes show a reduced size compared to Reptiles. Among Birds, the largest erythrocytes and the widest variations in the erythrocytic hemogram values between-species are generally described in the smallest birds (as well as in Reptiles). As compared to lower Vertebrates, the increase of avian erythroid parameters towards those of Mammals is a phylogenetic progression related to homeothermy.
The sites of origin of erythrocytes in embryonic Fishes are located in yolk sac and/or in intermediate cell mass, while in adulthood their localization varies from the presplenic tissue of Cyclostomes, to the splenic tissue, scattered in the submucosa of gut of Hagfish, to the neural body of adult lamprey - in which blast cells proliferate between adipocytes, as well as it occurs in bone marrow of Mammals - to the spleen in Elasmobranchs and to the kidneys in Teleosts. Nevertheless, erythropoiesis is generally located in separate sites from those of granulocytopoiesis in most Fishes, in all Urodela, which are both Vertebrates lacking bone marrow. The early bone marrow, during evolution, occurs in Plethodontidae, the Family of lungless salamanders, with the sole lympho-granulopoietic function. The phylogenetic onset of medullary erythropoiesis in the Anurans is a hematologic landmark due, very likely, to protective shielding from ionizing radiation for the radiosensitive hemopoietic stem cells, offered by the bone along with the evolutionary transition from life in water to life on land. In Reptiles, the bone marrow becomes the dominant erythropoietic site in most species, even though the spleen of some lizards can be the major erythropoietic (and thrombopoietic) organ. In Avians and Mammals, erythropoiesis is exclusively housed in bone marrow, with rare exceptions. In all submammals, erythropoiesis completes into the circulating blood through similar maturative stages. Mammalian erythrocyte denucleation is the final event in the phylogenetic progression of vertebrate red blood cells. In conclusion, the erythrocyte count, hemoglobin concentration, hematocrit and erythrocellular indices of all submammalian Vertebrates are explored through the analysis of their variations as expression of evolutionary progression. The significance of erythrocyte denucleation and loss of organelles, occurring in erythrocytes of all Mammals, in rare species of marine Fishes and Amphibians is discussed too.

Brief Biography of the Speaker:
Dr. Alessandra Pica is Confirmed Associate Professor in General and Comparative Hematology and Human Development, Growth and Anatomy, at Faculty of Mathematical, Physical and Natural Sciences of University of Naples, Federico II, Italy. She is teacher of Biology in Doctorate School in Bioethics of University Federico II and member of the Animal Experimentation Ethics Committee (CESA) of National Hospital "A. Cardarelli", Naples, Italy. She is consulting hematologist in the Sea Turtle Rescue and Rehabilitation Program at Zoological Station A. Dohrn and teacher of Hematology of Sea Turtles in Training Course on Sea Turtle Rescue and Rehabilitation RAC/SPA (Regional Activity Centre for Specially Protected Areas - United Nations Environment Program Mediterranean Action Plan (UNEP)) of Zoological Station, A. Dohrn (Naples, Italy) addressed to foreign researchers of Mediterranean Area. Moreover, she is Revisor as Hematology expert for Committee for Research Evaluation (CIVR-MIUR). She is member of the Italian Society of Anatomy, the Italian Society of Histochemistry, the Italian Zoological Society, the Italian Group of Neuromorphology, the Scientific Committee of Interdepartmental Center for Ultrastructural Biological Research (C.I.R.U.B. ), University of Naples Federico II.
She worked at Institute of General Pathology in Naples, Experimental Center of Endocrinology and Oncology, in years 1982-83. Since 1983 she has worked as Researcher at University Federico II, Department of Biological Sciences, Division of Evolutionary and Comparative Biology, and she has been Associate Professor since 2002. In the past years, she was teacher of Human Anatomy, Embryology and Experimental Morphology, Histochemistry and Cytochemistry in the degree course in Biological Sciences at University Federico II and teacher of Hemopathology of Sea water Vertebrates in 1st Level Master in Biotechnology applied to reproduction and repopulation of marine species in the School of Mathematical, Physical and Natural Sciences of University Federico II.
In 2006, she published a monograph entitled "Erythrocytes of the Poikilotherms: a Phylogenetic Odyssey", in collaboration with prof. Chester A. Glomski, State University of New York at Buffalo, USA (Foxwell & Davies, UK 1-432; ISBN 88-8448-010-8).
Her main fields of interest are Comparative Hematology and Oncology. Most of her scientific work is about the characterization of the circulating blood cells and their progenitors in one species at least for each Class of non mammalian Vertebrates, focusing on the function of blood cells of non mammalian Vertebrates compared to their counterparts of Mammals. In the field of Oncology, her research work is mainly about the effects of a treatment with a new anticancer agent on human breast cancer cells.
At present, her main research subjects are the hematological characterization of the Mediterranean loggerhead Caretta caretta, the detection of biomarkers of cell damage following X-ray pollution in sea water and the effects of treatment with a new anticancer agent on leukemia cells of children.