Thymocytes are genuine lymphocytes. The largest lymphocytes of lymph nodes do not occur in the thymus; the smallest ones with pacychromatic nuclei and scanty cytoplasm are more numerous in the thymus. Some lymphoblasts similar to myeloblasts occur in the rabbit thymus.
All three types of granulocytes develop from lymphocytes and to some extent from the mesenchymatous reticulum in the rabbit. In the rabbit the heterophil and eosinophil granules are acidophilic when first formed, while in the human thymus many have a basophilic quota as in the marrow. The reticulum is largely of epithelial origin and retains its epithelial characteristics in cytoplasm and nucleus. In the rabbit some mesenchymatous tissue is blended with the reticulum and may give origin to lymphocytes and granulocytes. The epithelium may form macrophages but does not differentiate to lymphocytes and other types of cells. In imprints from the rabbit the epithelial cells usually can be distinguished from those of the mesenchymatous reticulum. During regeneration following irradiation the latter tissue can be seen to form some lymphocytes locally; other lymphocytes enter the organ through the lymph vessels.
Imprints stained with May-Grünwald and Giemsa are of great value for detailed cytology of the thymus and for determining cell relationships. Sections of thymus fixed in Helly’s fluid, stained with methyl green and pyronin and dehydrated in dioxan are excellent for lymphocytes and the transitional stages resulting from their development from fixed mesenchymatous tissue during regeneration of the organ following exposure to x-rays.