The sites of intra-embryonic haemopoiesis prior to the hepatic haemopoiesis in the chick.

Abstract

The sites of intra-embryonic haemopoiesis of the chick embryo (stages 15-26) preceding the initiation of hepatic haemopoiesis was investigated by means of serial sections stained with azan liquid. Hitherto undescribed blood islands of large size and clear outline were found in the neck and lateral body fold of the embryo. They consisted of dense aggregations of erythroblasts and thin endothelial cells surrounding them, and could be clearly identified by azan staining. In the neck, the blood islands were located near the branchyal arteries at stages 15-18, while in the lateral fold they were seen mainly at stages 17-19. Examination of serial sections indicated that the blood islands extended in a cephalocaudal direction, assuming a rod shape. The largest blood island was found in the lateral fold, measuring 50 microns in diameter and 900 microns in length. Besides these islands, occasional sites of haemopoiesis were found in the mesonephros, mesentery, dorsal regions of the trunk, head and allantois after stage 18. As the blood islands found in this study alternated their localization sites according to developmental stage, they were considered to be temporary haemopoietic tissues rather than persistent ones.