The Features of Erythropoiesis in Hibernating Ground Squirrels Urocitellus undulatus

Morphofunctional features of the bone marrow obtained from the femoral shaft and humeral diaphysis of long-tailed ground squirrels were studied in summer and fall, as well as during torpor bouts and short-term winter arousals (winter euthermic intervals). Histological analysis showed an increase in the count and size of bone marrow adipocytes in torpid animals, with a partial myeloid-to-adipose tissue replacement. Although the femoral bone marrow has a larger volume compared to the humeral, it contains significantly less nucleated cells, but significantly more erythroid islets, particularly during hibernation. In torpid ground squirrels, disc-shaped erythrocytes in the blood are replaced by atypical forms (oval- and target-shaped, macrocytes) whose counts drop significantly during winter euthermia. The reticulocyte count increases twofold during hibernation compared to the summer period. The results obtained are discussed in the context of maintaining high blood oxygen levels during torpor and erythropoiesis adaptation to conditions of prolonged hypothermia.