Yolk-sac hematopoiesis James Palis, Mervin C Yoder  Experimental Hematology  Volume 29, Issue 8, Pages 927-936 (August 2001) DOI: 10.1016/S0301-472X(01)00669-5

Figure 1 Development of yolk sac blood islands in the mouse embryo between E7.5 and E9.5. Yolk sac blood islands form between the single cell layer of endoderm cells (E) and mesothelial cells (open arrow). Blood islands develop from undifferentiated mesoderm cells (E7.5, closed arrow) that give rise to inner blood cells surrounded by an outer endothelial lining (E9.5, closed arrow) Experimental Hematology 2001 29, 927-936DOI: (10.1016/S0301-472X(01)00669-5)

Figure 2 Cytospin preparation of circulating mouse blood cells at E10.5. The vast majority of cells are differentiating primitive erythrobasts, some still in mitosis (arrow). Rare macrophage cells (m) are also present Experimental Hematology 2001 29, 927-936DOI: (10.1016/S0301-472X(01)00669-5)

Figure 3 Development of primitive erythroid (EryP-CFC), definitive erythroid (BFU-E), and macrophage (Mac-CFC) progenitors in the mouse yolk sac between E6.5 and E11.5 of gestation. Data derived from Palis et al. [42] Experimental Hematology 2001 29, 927-936DOI: (10.1016/S0301-472X(01)00669-5)

Figure 4 Stages of human yolk sac formation. (Top) The primary yolk sac (1°YSC) is formed 7–8 days postconception. Intermediate cells (IC; depicted as white cells) migrate from the endodermal (EL) layer (dark-colored cells) of the primary yolk sac to fill in the space between the endoderm and trophoblastic basal lamina (TBL). (Middle) The secondary yolk sac (2°YSC) is formed by the remnants of the primary yolk sac following its collapse and fragmentation. (Bottom) By 12 days postconception, the secondary yolk sac has expanded to the edge of the embryonic disk, is lined by endoderm cells (dark cells), and protrudes into the exocelom (EXO). The mesoderm adjacent to the trophoblastic basal lamina (TBL) has differentiated into mesothelium (MESO). Hematopoietic activity will be present in yolk sac blood islands arising in the secondary yolk sac by week 3 of gestation. Reproduced with permission of F.F. Nogales (ed): The Human Yolk Sac and Yolk Sac Tumors, Berlin: Springer-Verlag, 1993, pp. 35–36 Experimental Hematology 2001 29, 927-936DOI: (10.1016/S0301-472X(01)00669-5)