Granulocyte differentiation in the lymphoid organs of chick embryos after antigenic and mitogenic stimulation.

Sections of spleen, bursa of Fabricius or thymus glands from 17-day-old chick embryos stimulated with sheep red blood cells, phytohaemagglutinin (PHA-P), concanavalin A or lipopolysaccharide were stained by an "indirect" peroxidase technique that differentiates eosinophils from heterophils and cell counts were carried out. The tissues were also examined with the electron microscope. Whatever stimulus had been used, the light microscopical examination revealed that the predominant granulocytes belonged to the peroxidase-negative heterophil series, outnumbering the peroxidase-positive eosinophils. There were no significant changes in the number of cells between treatments and the saline-injected controls, except in the case of PHA-P stimulation, where a slight reduction in the number of heterophils and a concomitant increase in eosinophils was observed. This study has demonstrated that the heterophil is the main granulocyte present in the lymphoid organs during late embryonic life in the fowl. It challenges previously reported work that the predominant cells are eosinophils and monocytes.