Yolk sac origin

Moore MA, Metcalf D. Ontogeny of the haemopoietic system yolk sac origin of in vivo and in vitro colony forming cells in the developing mouse embryo. Br J Haematol 1970 18(3) 279-296. 

At this stage, a normal yolk sac presents a robust vascular network (Fig. la). When the yolk sac is positioned to view the ventral side of the embryo, separate origins of the vitelline and umbilical vessels should be evident (Fig. lb). The score is reduced according to deviations from expected morphology. 

Lymphocytes generally comprise between 25-30% of the white cells in human blood. These immunologically active cells are comprised of several classes, each of which has specific properties and functions. Lymphocytes are derived from stem cells located in the yolk sac and fetal Liver. Later, some stem cells originate from the bone marrow These cells then differentiate into lymphocytes in the primary lymphoid organs, principally the thymus and lymph nodes. 

The rat ceruloplasmin cDNA clone described above was further used to identify extrahepatic sites of ceruloplasmin gene expression in the rat. Although the liver was originally postulated to be the sole tissue source of ceruloplasmin (189), the above study resulted in the detection of ceruloplasmin mRNA in the choroid plexus, yolk sac, placenta and testis of the rat. Since these tissues are situated at the interface between adjacent extracellular compartments, it has been suggested that ceruloplasmin may be involved in maintaining copper homeostasis in these compartments (189). 

BMPs were originally noted for their capacity to induce ectopic bone formation however, multiple relations to the HSC system have been described and they play a critical role in the formation and patterning of mesoderm in the embryo, the commitment of embryonic mesodermal cells to a hematopoietic fate, and in blood island formation in the yolk sac (Marshall et al. 2000 Snyder et al. 2004). Additionally, BMP4 is secreted by pulmonary microvascular endothelial cells in response to hypoxia (Frank et al. 2005), and in differentiating ESC cultures hypoxia upregulates the mesodermal markers brachyury, BMP4, and Flkl (Ramirez-Bergeron et al. 2004). 

Up to now efforts to obtain immunological reactions or lymphoid cell differentiation of yolk sac cells have been unsuccessful. Yet there is so little known concerning the origin of the cells of the embryonic liver that an extrinsic source of the immunocompetent cells demonstrable in liver should be considered. In analogy to the red blood cell system, the yolk sac seems a logical source of such cells (cf. Barker, 1967 Auerbach, 1967). Such an origin would readily account for the... 

If the yolk sac were to serve as an origin of lymphoid cells, it would be attractive to reconsider the old hypothesis of a single cell precursor of all blood cells of both erythroid and lymphocytic series. Modifications induced by tissue interactions as well as a series of hormonal agents could then account for the divergence of cell lines. 

Attention is drawn to the recent paper by Moore and Owen (1967) which reexamines the question of the origin of the thymic lymphoid cells. The newer studies employ yolk sac fusion performed on day 4, prior to the formation of a distinct thymic rudiment. With this procedure they find thymic lymphoid chimeiism with as many as 70 of the lymphoid cells ultimately derived from the coupled parabiont. 

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