Leucocytes in relation to the mechanism of thyroid-accelerated metamorphosis in the larval frog

In the larval frog the chief locus of blood-cell formation is the kidney. The intertubular regions of this organ are filled with many erythrocytes and leukocytes in various stages of development. The spleen and, to a less extent, the mesentery are also hemopoietically active, but are of minor importance. In the adult frog, however, conditions are different. The spleen is here the principal hemopoietic center, the kidney playing little part. At some time in the developmental history of the frog, therefore, the chief locus of blood-cell formation is shifted from the kidney to the spleen. A study has been made to determine the effect upon the hemopoietic centers of accelerated metamorphosis induced by feeding tadpoles with thyroid extract.

A few days after the first administration of thyroid extract definite changes may be seen in the kidney and spleen. The intertubular stroma of the kidney is largely drained of its blood-cells. Some stimulation of the hemopoietic tissue that is left occurs. Erythrocytes, lymphocytes, special granulocytes (pseudo-eosinophils or neutrophils) and eosinophils are drawn from the kidney in great numbers.

A marked shrinkage in size of this organ results which is evident microscopically. At the same time a myeloid metaplasia is effected in the spleen and mesentery. Erythrocytes are drained from the spleen and the splenic lymphocytes stimulated to differentiate into other erythrocytes. Lymphocytes from other parts of the body migrate to the spleen and are incorporated in it. The net result of this activity in animals in which metamorphosis is not too greatly accelerated is an increase in the proportion of lymphocytes to erythrocytes, and some increase in absolute size of the spleen. In these animals it may be said that the partial drainage of the intertubular regions of the kidney, and the stimulation of the spleen, apparently indicate the initiation of the shift of the main hemopoietic locus from the kidney to the spleen.