BRIEF REVIEW www.jasn.org How Do Mesangial and Endothelial Cells Form the Glomerular Tuft? Michael R. Vaughan*† and Susan E. Quaggin*‡§ *Samuel Lunenfeld Research Institute, Mount Sinai Hospital, ‡Institute of Medical Science, and §Department of Medicine and Division of Nephrology, St. Michael’s Hospital, University of Toronto, Toronto, Ontario, Canada; and †Division of Nephrology, University of Washington, Seattle, Washington ABSTRACT The glomerular capillary tuft is a highly intricate and specialized microvascular bed itor cells reside together in a region that filters plasma water and solute to form urine. The mature glomerulus contains known as the aortogonadal mesone- four cell types: Parietal epithelial cells that form Bowman’s capsule, podocytes that phros, where early hematopoiesis takes cover the outermost layer of the glomerular filtration barrier, glycocalyx-coated place. In lower organisms, hematopoi- fenestrated endothelial cells that are in direct contact with blood, and mesangial esis continues to occur in the adult kid- cells that sit between the capillary loops. Filtration begins only after the influx and ney, as in the pronephros of zebrafish. organization of endothelial and mesangial cells in the developing glomerulus. The mammalian kidney develops in Tightly coordinated movement and cross-talk between these cell types is required three successive stages known as the for the formation of a functional glomerular filtration barrier, and disruption of pronephros, the mesonephros, and the these processes has devastating consequences for early life. Current concepts of metanephros; only the metanephros the role of mesangial and endothelial cells in formation of the capillary tuft are gives rise to the definitive adult kidney, reviewed here. and it no longer functions as a site of hematopoiesis. It is still debated J Am Soc Nephrol 19: 24–33, 2008. doi: 10.1681/ASN.2007040471 whether angioblasts migrate into the developing metanephros or arise in situ from a common progenitor. This is an OVERVIEW OF TUFT EMBRYOLOGY ops from the most proximal end of the interesting question given the close Glomerular morphogenesis proceeds renal vesicle that is farthest from the bud geographic and functional relationship through several well-defined stages in tip. Distinct cell types in the glomerulus that these progenitors exhibit during embryonic development, beginning first are first identified in the S-shaped stage, evolution. Experimental data regard- as a renal vesicle, followed by the com- where presumptive podocytes appear as ing the origin of these angioblasts ma-shaped body, S-shaped body, a cap- a layer of columnar-shaped epithelial within the metanephros is discussed in illary loop stage, and then the mature cells. A vascular cleft develops and sepa- a later section. glomerulus (Figure 1). The epithelial rates the presumptive podocyte layer The movement of endothelial pro- components of the glomerulus—the pa- from more distal cells that will form the genitors into the vascular cleft depends rietal epithelial cells and podocytes—de- proximal tubule (Figure 1). It is into this on the expression of angiogenic factors rive from the metanephric mesenchyme. cleft that vascular endothelial cells mi- such as VEGF-A by presumptive podo- 7 8,9 Once inside the developing glo- These mesenchymal cells adjacent and grate followed by mesangial cells. cytes. merulus, endothelial cells proliferate in inferior to the tips of the branching ure- Individual endothelial progenitors, situ and aggregate to form the first capil- teric bud begin to condense at 11.5 d post or “angioblasts,” are easily identified in coitum in the mouse or after 5 wk of ges- the developing kidney because they ex- tation in humans.1,2 This collection of press typical vascular markers such as Published online ahead of print. Publication date cells is known as the pretubular aggregate vascular endothelial growth factor re- available at www.jasn.org. (Figure 1). In response to inductive cues ceptor 2/Flk1 (VEGFR2). At the com- Correspondence: Dr. Susan E. Quaggin, Samuel from the ureteric bud and surrounding ma-shaped stage, these angioblasts hug Lunenfeld Research Institute, Mt. Sinai Hospital, Uni- the outside of the developing nephron versity of Toronto, 600 University Avenue, Toronto, stroma, the aggregates undergo a mesen- Ontario M5G 1X5, Canada. Phone: 416-586-4800; chymal-to-epithelial transition forming and then “stream” into the vascular Fax: 416-586-8588; E-mail: [email protected] cleft (Figure 1). Much earlier in devel- the renal vesicle and then the comma- Copyright © 2008 by the American Society of shaped body.3–6 The glomerulus devel- opment, angioblasts and renal progen- Nephrology 24 ISSN : 1046-6673/1901-24 J Am Soc Nephrol 19: 24–33, 2008 www.jasn.org BRIEF REVIEW lary loops. Rather than using a process men that develops only later through se- lial cells differentiate, becoming flat- called “sprouting angiogenesis,” lective apoptosis of endothelial cell sub- tened, and acquire a fenestrated mor- whereby a vessel with a lumen invades a sets.10 This process of lumenation is phology. The fenestrae are pores lined by tissue, glomerular precapillary cords dependent on TGF-␤ signaling.11 As glo- plasma membrane that pass through the form as a result of homotypic interac- meruli mature, the cords become lume- endothelial cell. Unlike other fenestrated tions between adjacent endothelial cells. nal and the initial capillary loop divides endothelial cells found in pituitary and Initially, these capillary cords lack a lu- into six to eight loops. Residual endothe- endocrine organs, the majority of glo- Figure 1. Migration of endothelial cells into the developing glomerular tuft. (Top) An embryonic day 12.5 mouse metanephros is outlined in black. Endothelial cells express a VEGFR2-GFP transgene and stain brown. The glomerulus develops from a pretubular aggregate (agg) that forms immediately adjacent and below the tips of the ureteric buds (ub). These aggregates derive from metanephric mesenchymal cells that have been induced to condense and epithelialize by signals produced in the ureteric buds. (Middle and Bottom) VEGFR2-positive cells are seen to “hug” the developing comma-shaped–stage nephron. The s-shaped stage is defined by the presence of a layer of podocyte precursors (presumptive podocytes) (po) and a vascular cleft. Endothelial cells seem to be streaming into this cleft from the Metanephric mesenchyme (MM). At the capillary loop stage, pockets of endothelial cells sit right next to the podocytes, and mesangial cells are soon found inside a single capillary loop. By the maturing stage, capillary lumens are beginning to form and a large population of mesangial cells is present. Schematic diagrams of each developmental stage are shown above the photomicrographs. Podocytes have been digitally colorized for identification (pink). Adapted from Saxen.1 pa, parietal epithelial cell; po, podocyte; me, mesangial cell; cap, capillary loop. J Am Soc Nephrol 19: 24–33, 2008 Formation of Glomerular Tuft 25 BRIEF REVIEW www.jasn.org of mesangial cells (known as mesangioly- sis) results in dilation of the glomerular capillary loops. Although it is clear that mesangial cells are required for this loop- ing, the specific mesangial factors guid- ing such events have not been identified. ORIGIN OF ENDOTHELIAL AND MESANGIAL CELLS Conflicting views exist regarding the or- igin of glomerular endothelial and mes- angial cells. Although the metanephric mesenchyme contains VEGFR-2–posi- tive angioblasts from early stages of de- velopment, cross-species transplanta- Figure 2. Mesangial cells in the early glomerular tuft. Mesangial cells (green) express tion studies suggest that extrarenal cells desmin, whereas podocytes (red) express WT1. Vascular ␣-SMA, another marker of also contribute to glomerular vascula- mesangial cells, is expressed after desmin. Figure courtesy of J. Miner, PhD, Washington ture. This is perhaps not surprising, University, St. Louis. given the ability of donor endothelial cells to “hook up” with host vasculature merular fenestrae in adult kidneys lack cells are identified by their expression of under very artificial circumstances (Hel- typical diaphragms associated with the various markers, including Thy1.1 (in mutt Augustin, DVM, PhD, Joint Re- PV1 protein.12 However, they are rats), desmin, ␣-smooth muscle actin search Division Vascular Biology of the bridged by an electron-dense complex (␣-SMA), and the PDGF beta receptor Medical Faculty Mannheim, University that is referred to by some but not all (PDGFR␤) (Figure 2).16,17 The subse- of Heidelberg and the German Cancer groups as a “diaphragm.”13,14 Exactly quent looping of glomerular capillaries Research Center, Germany, personal how this diaphragm forms during devel- will not proceed in the absence of mesan- communication, March 2007); for ex- opment is unclear, but it is thought to gial cells or in glomeruli with defects in ample, human endothelial cells trans- arise from proteoglycans and other pro- basement membrane that prevent adher- planted under the skin of the mouse can teins produced by differentiating glo- ence of mesangial cells.17,18 This demon- form functional vessels that connect to merular endothelial cells. Disruption of strates that mesangial cells play a key murine vessels. What is also intriguing this barrier through injection of enzymes morphogenetic role in forming the cap- about kidney transplantation experi- (hyaluronidase, heparinase, and chon- illary tuft. In the prevailing