Characteristics of the Erythron of the Head Kidney and Circulating Blood of the European Flounder (Platichthys flesus) throughout Its Annual Cycle

The composition of the erythron of the head kidney (pronephros) and circulating blood of the cold-loving European flounder (Platichthys flesus L., 1758) has been studied throughout its annual cycle. The erythron of the pronephros is represented mainly by erythroblasts (EBs) and basophilic normoblasts (BNs). The content of polychromatophilic normoblasts (PNs) is low (less than 2%). The maximum size of the erythroid germ of hematopoiesis in the pronephros has been recorded during the postspawning period (April–July). It reaches 17% of the cellular mass of the prints. BNs and PNs, which were not capable of proliferation, generally prevailed in the blood. The maximum content of these cell forms was also recorded during the postspawning period. Cells of earlier generations (EBs) were not detected at all in the blood. The increase in the production of erythroid cells by hematopoietic tissue coincided with an increase in the number of circulating erythrocytes in the blood of European flounder (R2 = 0.608 and 0.991), which indicated a shift in the erythrocyte balance in the red blood system towards production processes. Factors responsible for the generation of erythrocytes by the hematopoietic tissue in the fish in a spawning state are considered.