The Histogenesis of the Thymus as shown by Tissue Cultures,Transplantation and Regeneration.

The histogenesis of the thymus is still unsettled. The majority of investigators, on the basis of the work of Hammar and Maximow, 1 consider the thymus to be a lymphoepithelial organ in which the original entodermal epithelium of the primordium transforms into a reticulum and is infiltrated with common lymphocytes. Some workers have recently resuscitated the old idea of Stöhr according to which the small thymus cells merely resemble the lymphocytes in appearance and actually represent peculiarly modified epithelial cells. 2 Besides, the epithelial nature of the reticulum seems to be doubtful, mainly because in vitally stained animals, as some authors claim, the reticular cells store vital dyes in the same way as histiocytes.

The method of tissue cultures was applied by us to the study of the thymus problem. Pieces of the thymus were taken from newborn, half-grown and full-grown rabbits and cultivated in plain and carminized plasma with the addition of bone-marrow or embryonic extract.

In the deeper parts of the explant necrosis occurs; the peripheral layers of the lobules and sometimes patches of the medulla remain alive. During the first 24 hours the cultures show a massive emigration of the small cells which behave exactly like lymphocytes. The interstitial connective tissue also serves as a source of migrating lymphocytes, histiocytes and fibroblasts. During the second and third days, as a result of autolysis of the dead lymphocytes and emigration of the living ones, the volume of the lobules decreases, and a broad, pale epithelial layer develops on the periphery through the condensation of the reticular cells. Islands of epithelium eventually arise also in the debris of the deeper layers through contraction and collection of epithelial cells.