TIE is a receptor tyrosine kinase expressed in both mature endothelial cells and their precursors, as well as in some hematopoietic cells. Mouse embryos homozygous for a disrupted Tie allele die at midgestation due to impaired endothelial cell integrity and resulting hemorrhage. Here we have performed chimeric analysis to study further the function of the murine TIE in the development of embryonic vasculature and in the hematopoietic system. Cells lacking a functional Tie gene (tie^lcztie^lczn– cells) contributed to the embryonic vasculature at E10.5 as efficiently as cells heterozygous for a targeted Tie allele (tie^lcz+ cells). Thus, TIE does not play a significant role in vasculogenesis or in early angiogenic processes, such as formation of the intersomitic arteries and limb bud vascularization. At E15.5 tie^lcztie^lczn– cells still readily contributed to major blood vessels and to endothelial cells of organs such as lung and heart, which have been suggested to be vascularized by angioblast differentiation. In contrast, the tie^lcztie^lczn– cells were selected against in the capillary plexuses of several angiogenically vascularized tissues, such as brain and kidney. Our results thus support,a role for TIE in late phases of angiogenesis but not vasculogenesis. Furthermore, the results suggest that different mechanisms regulate early and late angiogenesis and provide support for a model of differential organ vascularization by vasculogenic or angiogenic processes. Analysis of adult chimeras suggested that TIE is required to support the survival or proliferation of certain types of endothelial cells demonstrating heterogeneity in the growth/survival factor requirements in various endothelial cell populations.

Chimeric analysis of adult hematopoietic cell populations, including peripheral platelets and bone marrow progenitor cells, revealed that tie^lcztie^lczn– cells were able to contribute to these cell types in a way indistinguishable from tie^lcz+ or wild-type cells. Thus, the primary function of TIE appears to be restricted to the endothelial cell lineage.