Figure 1. Embryo haematopoietic sites and the emergence of lymphopoiesis in mid-gestation mouse embryos. Primitive myeloerythropoiesis is the only process occuring in E7–8 mouse embryos. Adult-type multilineage haematopoiesis simultaneously appears in Sp/AGM and YS of >E8 embryos. The transversal view depicting AGM region includes subaortic and intraaortic haematopoietic cell clusters. FL is likely colonized by both YS- and Sp/AGM-derived progenitors (1, 2). The embryo thymus is engrafted with Sp/AGM- and FL-derived pre-committed precursors (3, 4). Whether haematopoietic progenitors migrate between both YS and Sp/AGM is unclear (5, 6). In red, embryo haematopoietic sites (except the lymphopoietic thymus). Neural tube, blue; dorsal aorta, green; mesonephros, brown; gonads, orange.

Figure 2. Differentiation routes from the haemangioblast/haemogenic endothelium up to mature blood cells. Full arrows indicate the scheme based on the existence of a primordial CLP/CMP branching. Dotted arrows show alternative roads of lymphoid differentiation. The cellular stages and pathways predominating in embryo are labelled in blue. ETP, early T-cell progenitors; GMP, granulocyte-macrophage progenitors; LTR, long-term reconstituting; MEP, megakaryocyte-erythrocyte progenitors; MMP, multipotential progenitors; MZ, marginal zone; STR, short-term reconstituting; other abbreviations, in the main text.

Figure 3. Central unit of the organogenesis of peripheral lymphoid tissues.