Characterization of lymphatic and venous emigrants from the thymus.

Lymphocytes leaving the thymus via two different pathways were examined as to their morphology and phenotype. Cells leaving the thymus via lymphatics were obtained by a direct cannulation of thymic lymphatics and those leaving via the thymic vein were labelled within the thymus using an extracorporeal perfusion system and identified subsequently as fluorescent cells in the draining vein. In both cases the cervical thymus in lambs was used, since it is located in the neck region and ensures easy access to both blood and lymphatic vessels of the thymus without subjecting the animal to a major trauma or stress. Cells obtained from the thymic lymphatics or lymphatic emigrants were found to have distinct features different from peripheral T cells in terms of their surface morphology and expression of the MHC antigens. Venous emigrants were also slightly different from peripheral T cells in MHC expression. Estimation on the rate of thymocyte emigration into the periphery suggested that neither venous nor lymphatic emigrants represent a major fraction of de novo synthesized cells in the thymus of this animal species, as has been suggested in the mouse.