The Ikaros gene encodes a transcription factor that, in mice, has been show
n to be essential for the correct differentiation of B and T lymphocytes an
d is expressed in all cells of the lymphoid lineage, including pluripotent
hematopoietic stem cells. During development in zebrafish, Ikaros expressio
n begins in lateral mesoderm, and continues in the intermediate cell mass (
ICM), which is derived from lateral mesoderm and has been shown to generate
primitive hematopoietic precursors. Cells expressing Ikaros were then seen
on the ventral side of the dorsal aorta, known to be a location of definit
ive hematopoietic precursors in birds and mammals. Ikaros-expressing cells
were also found in the pharyngeal region, near the forming thymus. Later, s
uch cells were seen in the pronephros, the site of hematopoiesis in adult f
ish. The timing of appearance of Ikaros-expressing cells suggests that, sim
ilar to other vertebrates, lymphocytes in the thymus arise from hematopoiet
ic tissue located near the dorsal aorta or in the ICM. (C) 2001 Wiley-Liss,
Inc.