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Experiments were performed on CBA/CaLac mice with hyperplasia of hemopoiesis induced by 10-h immobilization. Erythropoietic and colony-stimulating activity increased under the influence of hemopoietic growth factors and other humoral substances, which plays a major role in the increase in bone marrow cellularity during immobilization stress. Erythropoietin stimulated proliferation of erythroid colony-forming units, while granulocyte colony-stimulating factor accelerated maturation of granulocyte precursors.

The content of hemopoietic precursors in the bone marrow, its morphological composition, production of hemopoietic growth factors by cells of the hemopoietic microenvironment, erythropoietic activity, and plasma content of endogenous erythropoietin were studied in CBA/CaLac mice with hypoplasia of hemopoiesis induced by adriamycin in a maximum permissible dose. Cytostatic treatment affected production of substances determining erythropoietic activity by adherent and nonadherent cells of the bone marrow. Erythropoietic activity of peripheral blood plasma considerably increased. Recovery of erythroid hemopoiesis was determined by the effects of endogenous erythropoietin and other substances secreted by cells of the hemopoietic microenvironment and present in the peripheral blood.

Cell elements of peripheral blood and bone marrow and splenic hemopoiesis were studied in athymic BALB/c mice (nude or thymectomized animals). In athymic animals lympho- and erythropoiesis were suppressed, while granulocyto- and monocytopoiesis were activated. These changes were more pronounced in nude mice.

In experiments in mice in the model of immobilizing stress the role of glucocorticoids in development of blood marrow stimulation phenomenon has been studied. The glucocorticoids have been found to play a leading role in adaptive reaction of blood system formation in response action of extreme factors, not exerting the myelosuppressing effect. This reaction is characterized by blood marrow hyperplasia development. The glucocorticoids influence the haemopoiesis through the T-lymphocyte system: the homing takes place on the 2-3 day after the beginning of effect of T-cells myelopoiesis regulator, expressing on the surface Lyt-1+, Lyt-2+ antigens.

The role of thymus in the regulation of erythropoiesis during stress has been studied in experiments on immobilized mice. The development of pronounced leukopenia due to stimulation of erythro- and granulocytopoiesis was demonstrated. Thymectomy, performed a month before immobilization, abolished the phenomenon of erythropoietic cellularity, indicating an important role of thymus in the regulation of myelopoiesis during stress.