Vascularization of organs generally occurs by remodelling of the preexisting vascular system during their differentiation and growth to enable them to perform their specific functions during development. The molecules required by early vascular systems, many of which are receptor tyrosine kinases and their ligands, have been defined by analysis of mutant mice. As most of these mice die during early gestation before many of their organs have developed, the molecules responsible for vascularization during organogenesis have not been identified. The cell-surface receptor CXCR4 is a seven-transmembrane-spanning, G-protein-coupled receptor for the CXC chemokine PBSF/SDF-1 (for pre-B-cell growth-stimulating factor/stromal-cell-derived factor), which is responsible for B-cell lymphopoiesis, bone-marrow myelopoiesis and cardiac ventricular septum formation. CXCR4 also functions as a co-receptor for T-cell-line tropic human immunodeficiency virus HIV-1. Here we report that CXCR4 is expressed in developing vascular endothelial cells, and that mice lacking CXCR4 or PBSF/SDF-1 have defective formation of the large vessels supplying the gastrointestinal tract. In addition, mice lacking CXCR4 die in utero and are defective in vascular development, haematopoiesis and cardiogenesis, like mice lacking PBSF/SDF-1, indicating that CXCR4 is a primary physiological receptor for PBSF/SDF-1. We conclude that PBSF/SDF-1 and CXCR4 define a new signalling system for organ vascularization.

Mice homozygous for the recessive mutation osteopetrosis (op) on chromosome 3 have a restricted capacity for bone remodelling, and are severely deficient in mature macrophages and osteoclasts. Both cell populations originate from a common haemopoietic progenitor. As op/op mice are not cured by transplants of normal bone marrow cells, the defects in op/op mice may be associated with an abnormal haematopoietic microenvironment rather than with an intrinsic defect in haematopoietic progenitors. To investigate the molecular and biochemical basis of the defects caused by the op mutation, we established primary fibroblast cell lines from op/op mice and tested the ability of these cell lines to support the proliferation of macrophage progenitors. We show that op/op fibroblasts are defective in production of functional macrophage colony-stimulating factor (M-CSF), although its messenger RNA (Csfm mRNA) is present at normal levels. This defect in M-CSF production and the recent mapping of the Csfm structural gene near op on chromosome 3 suggest that op is a mutation within the Csfm gene itself. We have sequenced Csfm complementary DNA prepared from op/op fibroblasts and found a single base pair insertion in the coding region of the Csfm gene that generates a stop codon 21 base pairs downstream. Thus, the op mutation is within the Csfm coding region and we conclude that the pathological changes in this mutant result from the absence of M-CSF.

Stem cells which have the capacity to self-renew and generate differentiated progeny are thought to be maintained in a specific environment known as a niche. The localization of the niche, however, remains largely obscure for most stem-cell systems. Melanocytes (pigment cells) in hair follicles proliferate and differentiate closely coupled to the hair regeneration cycle. Here we report that stem cells of the melanocyte lineage can be identified, using Dct-lacZ transgenic mice, in the lower permanent portion of mouse hair follicles throughout the hair cycle. It is only the population in this region that fulfils the criteria for stem cells, being immature, slow cycling, self-maintaining and fully competent in regenerating progeny on activation at early anagen (the growing phase of hair follicles). Induction of the re-pigmentation process in K14-steel factor transgenic mice demonstrates that a portion of amplifying stem-cell progeny can migrate out from the niche and retain sufficient self-renewing capability to function as stem cells after repopulation into vacant niches. Our data indicate that the niche has a dominant role in the fate determination of melanocyte stem-cell progeny.

A monoclonal antibody, A7R34, that recognizes the high-affinity interleukin 7 receptor (IL-7Ra) and blocks the binding between IL-7 and IL-7Ra has been produced. Cell surface staining with A7R34 demonstrated that IL-7Ra is expressed in both B-and T-cell lineages. In the bone marrow, immature B-lineage cells that do not express surface IgM were IL-7Ra+. In the thymus, IL-7Ra was detected in CD4-8-T cells and also in CD4 or CD8 single-positive cells but not in CD4+8+ double-positive cells. In the peripheral lymphoid tissues, both CD4 and CD8 single-positive cells were the manor cell types that express IL-7Ra. Addition of A7R34 to a long-term B-precursor-cell culture inhibited proliferation of the B-lineage cells, indicating that IL-7 is an absolute requirement for in vitro B-cell genesis. Consistent with this in vitro result, continuous nijection of A7R34 into an adult mouse resulted in a decrease of B-precursor cells and also of thymocytes, whereas a considerable fraction of mature B and T cells in the peripheral tissues persisted over 2 weeks of the experiment. When A7R34 iniection is started from day 14 of gestation, it is possible to produce mice that lack B cells. These results indicate that IL-7 is an essential molecule for generation of both B and T cells in murine bone marrow and thymus, respectively. Moreover, IL-7Ra would be the sole receptor system regulating these processes.B-cell genesis in the adult mouse is regulated by a meshwork of stromal-cell components in the bone marrow (1, 2). Establishment of the stromal-cell lines that can support B-cell genesis from multipotent hematopoietic stem cells facilitated identification of the molecules that are required for this process (3-6). Among a number of molecules that are expressed in the stromal-cell lines, interleukin 7 (IL-7) is the first molecule that has been shown to be able to induce proliferation of B-cell precursors (7,8). Initial use of recombinant IL-7 demonstrated that IL-7 can induce proliferation of pre-B cells (8)(9)(10). Subsequent studies further demonstrated that IL-7, when used in combination with the stromal cell lines or the ligand for c-kit, can act on earlier stages of B precursors (11)(12)(13)(14)(15). Besides the proliferative activity on B-precursor cells, it was reported that IL-7 induces proliferation of both mature and immature mouse T cells (16)(17)(18)(19)(20)(21)(22). In fact, studies of mice that received continuous IL-7 injections or bore the IL-7 transgene indicate that elevation of IL-7 results in enormous expansion of both T and B cells (23)(24)(25)(26). Despite all such positive evidence suggesting the role of IL-7 in lymphopoiesis, whether IL-7 is a functional requirement for in vivo lymphocyte production is yet to be determined. To address this question, we have produced an antagonistic monoclonal antibody (mAb) to the high-affinity IL-7 receptor (IL-7Ra) (27) and have investigated its expression and function. Our study demonstrates that IL-7Ra is The publication costs of this article were defrayed in part b...

We have investigated the lymphohematopoietic potentials of endothelial cells (EC) and hematopoietic cells (HPC) sorted from embryos. Expression of VE-cadherin, CD45, and Ter119 was used to distinguish EC (VE-cadherin+CD45-Ter119-) from HPC (VE-cadherin-CD45+). Thus defined, EC population takes up acetylated LDL and coexpresses CD31, Flk1, and CD34. In E9.5 embryos, EC from yolk sac (YS) and the embryo proper generate blood cells, including lymphocytes. Thus, lymphohematopoietic EC do exist in the embryo, and they are generated both in YS and the embryo proper. On the other hand, HPC with lymphopoietic potency appear first in the embryo proper. These findings implicate involvement of multiple environmental cues for acquiring lymphopoietic competency during differentiation of HPC.