Primary Formation of Germinal Centers in the Chick Spleen after Injection with Sheep Red Blood Cells

: The primary formation of germinal centers was induced in the chick spleen by intravenous injection of sheep red blood cells (SRBC) at 21 days of age. In the spleen, SRBC were trapped in the ellipsoids soon after injection. The number of cells of periellipsoidal lymphoid tissues (PELT) was gradually depleted. Large pyroninophilic cells appeared in the periphery of periarterial lymphatic sheaths (PALS) 4 hrs after injection. Then they increased in number and migrated centralward in the PALS. On the 4th day they were accumulated in the vicinity of the central arteries to make nodular structures. Morphologically mature germinal centers encapsulated by connective tissue were formed for the first time on the 6th day after injection. They were most numerous on the 8th day and then decreased gradually. It was presumed that the primary germinal centers in the chick spleen might have been formed by the cells of the PELT which were originally derived from the bursa of Fabricius. The process of proliferation of germinal centers in the secondary immune response was discussed.