The origin of mesoderm in phoronids

Integrin alphaIIb is a cell adhesion molecule expressed in association with beta3 by cells of the megakaryocytic lineage, from committed progenitors to platelets. While it is clear that lymphohemopoietic cells differentiating along other lineages do not express this molecule, it has been questioned whether mammalian hemopoietic stem cells (HSC) and various progenitor cells express it. In this study, we detected alphaIIb expression in midgestation embryo in sites of HSC generation, such as the yolk sac blood islands and the hemopoietic clusters lining the walls of the major arteries, and in sites of HSC migration, such as the fetal liver. Since c-Kit, which plays an essential role in the early stages of hemopoiesis, is expressed by HSC, we studied the expression of the alphaIIb antigen in the c-Kit-positive population from fetal liver and adult bone marrow differentiating in vitro and in vivo into erythromyeloid and lymphocyte lineages. Erythroid and myeloid progenitor activities were found in vitro in the c-Kit(+)alphaIIb(+) cell populations from both origins. On the other hand, a T cell developmental potential has never been considered for c-Kit(+)alphaIIb(+) progenitors, except in the avian model. Using organ cultures of embryonic thymus followed by grafting into athymic nude recipients, we demonstrate herein that populations from murine fetal liver and adult bone marrow contain T lymphocyte progenitors. Migration and maturation of T cells occurred, as shown by the development of both CD4(+)CD8- and CD4-CD8(+) peripheral T cells. Multilineage differentiation, including the B lymphoid lineage, of c-Kit(+)alphaIIb(+) progenitor cells was also shown in vivo in an assay using lethally irradiated congenic recipients. Taken together, these data demonstrate that murine c-Kit(+)alphaIIb(+) progenitor cells have several lineage potentialities since erythroid, myeloid, and lymphoid lineages can be generated.