The generally accepted concept regarding the differentiation of hemopoietic cells in vertebrates is that pluripotent stem cells that are initially differentiated in the yolk sac or its phylogenetic equivalent migrate via the circulation to colonize receptive organs, where they undergo subsequent differentiation into discrete lineages under the influence of specific microenvironments. Although this concept of hematogenous metastasis may be correct in general terms, the exact site of the origin of the stem cells as well as their state of commitment before and during migration in early embryos are still a matter of conjecture. More recent studies on the avian model have emphasized that definitive hemopoiesis is dependent not on the yolk sac blood islands, as had previously been believed, but on the intraembryonic stem cells localized in the mesenchyme associated with the dorsal aorta. A number of experiments on anuran amphibians, particularly Rana pipiens, seem to argue against the concept of the common hemopoietic stem cells that originate from the ventral blood islands of early embryos—the amphibian counterpart of yolk sac blood islands in mammals. This chapter briefly summarizes the knowledge of the early differentiation of lymphocytes mainly derived from the recent studies of the African clawed frog Xenopus laeuis, with particular attention to the site of origin of thymocytes in early embryos and its relevance to the differentiation capacity as stem cells. © 1986, Yamada Science Foundation and Academic Press Japan, Inc. All rights reserved.
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