Evolutionary intraembryonic origin of vertebrate hematopoietic stem cells in the elasmobranch spleen

The electric ray (Torpedo marmorata Risso) provides an animal model for the detection of early intraembryonic hemopoietic stem cells (HSCs) in sea vertebrates. The spleen of this bone-marrowless vertebrate appears to be the major site of HSCs differentiation during development and in adulthood. Splenic development in this species was investigated and hemopoietic stem cells were detected in this organ by immunocytochemistry utilizing CD34 and CD38 antibodies. At stage I (2-cm-long embryos with external gills), the spleen contains only mesenchymal cells. At stage II (3-4 cm-long embryos with a discoidal shape and internal gills), an initial red pulp was observed in the spleen, without immunostained cells. At stage III (10-11- cm-long embryos), the spleen contained well-developed white pulp, red pulp and ellipsoids. Image analysis at stage III showed four cell populations, i.e. CD34+/CD38-, CD34+/CD38+, CD34- /CD38+, and CD34-/CD38- cells. The present findings, obtained from an elasmobranch, indicate that the CD34 and CD38 phenotypes are conserved through vertebrate evolution.