We have studied the chicken spleen by light and transmission electron microscopy. Germinal centers are located at the beginning of the central artery which is surrounded by the periarterial lymphatic sheath (PALS). The central artery has no branch crossing the PALS, and there is no histologically identifiable marginal zone in the chicken spleen. The central artery continues as penicilliform capillaries.

The mid-portion of the penicilliform capillary is surrounded by the ellipsoid or Schweigger-Seidel sheath. The endothelial lining of this part of the pencilliform capillary contains small channels, formed between neighboring endothelial cells, which enter the ellipsoid. These channels allow circulating substances to accumulate in the ellipsoid.

The cells of the ellipsoid are reticular cells, round or ovoid in shape, exhibiting a limited number of cell junctions. At the surface of the ellipsoid are ellipsoid-associated cells (EAC) which have an affinity for toluidine blue. After perfusion fixation, the majority of the cells in the ellipsoid are lost; this suggests weak cellular connections between the ellipsoid cells. The ellipsoid-associated cells remain in loco. On the basis of their shape and cytological features, two stages of EACs can be distinguished. The round or ovoid EACs elaborate active Golgi zones surrounded by numerous small vesicles containing an electron-dense substance. Occasionally, mitotic figures can be seen among them which may suggest that they are immature forms. In the second or more mature stage, the EACs assume an elongated spindle shape. The spindle-shaped EACs reveal inactive Golgi cis-ternae and fewer but larger granules than the round or immature EACs. Unmyelinated nerve fibers and nerve ending were observed in the ellipsoid.

Perfusion fixation reveals that the distal portions of the penicilliform capillaries extend into the red pulp and become the sinuses. Therefore, the circulation of the chicken spleen is anatomically closed except for the channels in the mid-portion of the penicilliform capillary.

The periellipsoid white pulp (PWP) possesses a few small lymphocytes, macrophages, and plasma cells with the majority of cells being young blastlike cells which may replenish the EACs and ellipsoidal cells.

Carbon, injected intravenously, appears on the EAC membrane by 30 minutes, while Brucella abortus is completely phagocytized at this time. Binding the carbon triggers the EAC to detach from the ellipsoid and migrate to the periellipsoidal white pulp (PWP). The EACs phagocytize carbon by 2 hours, and by 5 and 8 days are present in the red pulp and surround the germinal centers at the border of the periarterial lymphatic sheath, respectively. Secretory cells which have been identified in germinal centers might originate from EACs.