Granulopoiesis in the pineal gland of chickens.

Foci of differentiating heterophilic granulocytes in the pineal gland were studied by light and electron microscopy in chickens, from hatching until 56 weeks of age. Foci of granulopoiesis could be seen in the first 24 hours after hatching. Thereafter, their number and cellular density increased, becoming highest at 2 weeks. From then on, numbers decreased progressively until foci disappeared at 18 weeks. Granulopoietic cells established local associations with fibroblast-like cells. Mature granulocytes reached the bloodstream by the 2 mechanisms described for hematopoietic cells in the bone marrow, either passing between lining cells or through the cytoplasm of lining cells.