Clones I I . the Appearance and Localization of Precursor Units in Lymphoid Tissues of Neonatal Mice

M a n y mouse strains become immunocompetent to erythrocyte antigens during the 1st wk of life (1, 2). However, in vivo experimental techniques have, to date, been unable to answer the question as to where components of precursor units capable of interact ing with erythrocyte antigens first arise. Using the modified Marbrook tissue culture technique (3, 4), we have under taken a s tudy of the kinetics of the appearance and matura t ion of precursor units in lymphoid tissues of neonatal mice. Tissues examined have been bone marrow, thymus, and spleen. These experiments have shown tha t cells capable of interact ing with sheep erythrocyte antigens (SRBC) to produce hemolysin are usually first seen in the bone marrow and spleen, and, less frequently, in the thymus of mice 1-2 days old. However, in the reported system, these cells usually do not go on to produce a hemolysin-producing cell clone (HPCC). The first complete precursor units capable of interact ing with erythrocyte antigen to produce a clone of hemolysin-producing cells are generally seen in the spleens of 3-day old mice. Much more rarely, such units are also found in the thymus of mice 2-3 days old. None of the cells isolated from the bone marrow, capable of interact ing with erythrocyte antigen to produce hemolysin, have replicated to produce HPCC.