Morphological Differences between Thymus- and Bone Marrow-Derived Lymphocytes

Simple morphological criteria are described, enabling one to distinguish two types of lymphocytes in normal unstimulated mice: 'Th', predominant in the thymus (90-92%) and 'Bm', predominant in the bone-marrow (85-93%). Their distribution in the peripheral lymphoid organs agreed significantly with the observed average distribution of T and B lymphocytes. Thy-mectomized and irradiated bone-marrow-restored CBA mice, as well as nude mice, showed a striking lack of Th cells. In vitro depletion of θ-bearing or Ig-bearing lymphocytes lead to a correlated depletion of Th or Bm lymphocytes, respectively: blood lymphocytes treated with anti-θ serum + complement showed a decrease in Th lymphocytes corresponding to the percentage of cells killed by the anti-θ serum. The selective retention of Ig-bearing spleen cells on Wigzell's anti-lg columns caused a depletion of Bm lymphocytes. It is concluded that the Th type is the morphological expression of the T lymphocytes, and the Bm type that of the B lymphocytes. The implications of this way of distinguishing T and B lymphocytes by simple morphology, and of the actual relationships between T and B cell differentiation, are discussed.