Restriction of regenerative capacity of the rat thymus after the application of cyclophosphamide.

Restriction of thymic regenerative capacity, in contrast to the data in the literature, was found 30 days after a single (300 mg per kg) or fractionated doses (5 x 60 mg per kg) of Cyclophosphamide. Detailed cellular analysis revealed myelometaplastic changes in the thymic parenchyma, especially in the outer cortex and cortico-medullary zone. Plasmoid elements also appeared in the outer cortex. The capsular and septal connective tissue was markedly thickened. The thymic mass was decreased. The changes were more marked after the fractionated than after the single dose of Cyclophosphamide.

Patterns of thymic regeneration in rats after single or divided doses of cyclophosphamide.

Fractional doses of cyclophosphamide (Cy; 5 X 60 mg per kg) given to rats caused more severe initial destruction of thymic cortex and medulla, with a corresponding decrease of thymic mass, than did a single dose of the same amount (300 mg per kg). After the fractional dose, regeneration was retarded and repopulation was suppressed, so that on day 15 the thymus was almost totally alymphoid, in contrast to the initial regeneration seen after the single dose. On day 20 after the fractional dose of Cy an initial degree of regeneration corresponding to that seen on day 15 after the single dose, was found. The delayed restriction of regenerative capacity was more marked after the divided dose than after the single dose of Cy.

Reactivity of thymic metallophilic cells during the regeneration after the application of cyclophosphamide.

Metallophilic cells, which had dendritic morphology in vivo, were localized almost exclusively in the cortico-medullary zone of the control rat thymus. The patterns of thymic regeneration after a single (300 mg/kg) or consecutive injections (5 X 60 mg/kg) of Cyclophosphamide were well correlated with the changes in the number and topographical distribution of the metallophilic cell population. The specific staining with aldehyde fuchsin of metallophilic cells diffusely distributed throughout the cortex during the regeneration clearly demonstrated that these cells were identical with aldehyde fuchsin-positive metallophilic cells in the cortico-medullary zone of the normal rat thymus. The results presented suggest that metallophilic cells play some role in the process of normal thymocytopoiesis and represent a hitherto undescribed component of the thymic microenvironment.

Fractionated irradiation and haematopoiesis. II. The effect of bone-marrow cell transplants in conditions of split dose of irradiation.

Haemoregeneration after the irradiation with 600 R was studied using two different fractions given before and after the transplantation of bone-marrow cells. The dose of 600 R was divided in two uneven fractions: 500 + 100 R, 400 + 200 R and 300 + 300 R. During the free interval between the two doses (5 min) transplantation of bone-marrow cells was performed. Recolonization of bone-marrow and spleen was analysed on the 10th day after treatment. For analysis, samples of blood, bone-marrow and spleen were used. Maximal effect was found in the experimental group of animals irradiated with 500 R before and with 100 R after marrow-cell transplantation. Minimal haematopoietic response was in the group irradiated with 300 R before and after transplantation. This points at the importance of the primary dose for acceptance of the transplants and their activation.

Fractionated irradiation and haematopoiesis. I. The effect of a differently fractionated radiation dose of 600 R.

The effect of the same dose of irradiation (600 R) divided into two different fractions (500 + 100 R; 400 + 200 R; 300 + 300 R) upon the haematopoietic system of young rats (25 days old) was investigated. Free interval between the first and the second dose was 5 minutes. Changes were observed in bone-marrow, spleen and blood in all the experimental groups of 10 days after the irradiation. Spontaneous regeneration of haematopoiesis after a single dose of 600 R is higher than after division of this dose into two fractions, excluding the group where the dose was divided into two equal fractions. The most intensive haematopoietic repopulation was found in the group of rats irradiated with 300 + 300 R, where the primary dose was the smallest. In fractional conditions, the first dose is of primary importance for the repopulating capacity of the haematopoietic system regardless of the short free interval.

Stimulation and inhibition of erythropoiesis in donors and hematopoietic effect in irradiated recipient.

Regeneration dynamics in bone marrow and spleen was studied in rats after irradiation of 800 R and transfusion of bone marrow cells from donors treated in different ways. Priority of the microenvironment of the recipient or of the information obtained in cell donors with respect to further hematopoietic cell differentiation was studied in irradiated recipients. Rats irradiated with 800 R were used as recipients in the experiments. The donors of marrow cells were the rats with stimulated or inhibited erythropoiesis. Stimulation of erythropoiesis was induced by bleeding and experimental polycythemia was provoked by packed erythrocytes. According to our results, it can be concluded that the processes of postirradiation hematopoiesis after transplantation of the bone marrow cells depend on the number and proliferative state of both donors and recipient stem cells, and microenvironment, not excluding the information introduced with the donor cell transplant.