157differentiation (Das et al. 2013 ; Mendelsohn et al. 1999 ). Therefore, the interactions
among the UB, metanephric nephron progenitors, and stromal progenitors are indis-
pensable for the maintenance of an immature cell population in the nephrogenic
zone during kidney organogenesis.
9.3.3 Wnt Triggers Self-Organizing Nephron Differentiation
A pioneering developmental biology study demonstrated that an inductive signal
from the UB promotes differentiation of the MM to an epithelialized nephron struc-
ture (Grobstein 1953 ). Later, the embryonic dorsal spinal cord was identified as an
alternative inducer that can trigger MM differentiation in an organ culture setting
(Grobstein 1955 ). Other reports have since investigated the efficacy of multiple
growth factors and cytokines in promoting the in vitro differentiation of MM,
including LIF (Barasch et al. 1999 ), HGF (Woolf et al. 1995 ), bFGF (Perantoni
et al. 1995 ), and TGFβ1 (Rogers et al. 1993 ).
An in vivo knockout study revealed that Wnt9b-driven Wnt4 upregulation within
the nephron progenitor is an essential regulator of nephron differentiation (Stark
et al. 1994 ; Carroll et al. 2005 ). The sufficiency of Wnt signaling for this process
was shown by an in vitro study in which isolated MM was cocultured with Wnt-
expressing feeder cells (Kispert et al. 1998 ; Carroll et al. 2005 ). Genetic stabiliza-
tion of β-catenin induced the differentiation program but blocked the completion of
MET (mesenchymal-to-epithelial transition), suggesting that canonical Wnt signal-
ing must be switched off at an appropriate time (Park et al. 2007 ). Indeed, an in vitro
study showed that transient application and subsequent removal of a chemical Wnt
activator could induce formation of an epithelialized structure in the cultured MM
(Kuure et al. 2007 ). Alternatively, others have provided evidence of a contribution
from the Wnt-calcium pathway to the MET process rather than the canonical path-
way (Tanigawa et al. 2011 ; Burn et al. 2011 ). Furthermore, ectopic activation of
Notch signaling can drive MET in the absence of Wnt9b and Wnt4 in vitro (Boyle
et al. 2011 ). Bmp7 signaling is also involved with this process by priming the
Cited1+/Six2+ naïve capping mesenchyme into a Cited1−/Six2+ state, thereby facili-
tating receptivity to Wnt (Brown et al. 2013 ).
Intriguingly, after the Wnt-driven MET step, the subsequent conformational
changes of nephron progenitors take place in a self-organizing manner. The epithe-
lialized nephron progenitors, or renal vesicles, polarize proximo-distally under the
influence of Notch-mediated patterning and elongate to form comma-shaped and
subsequently S-shaped tubular structures (Cheng et al. 2003 ; Kopan et al. 2007 )
(Fig. 9.2b). These S-shaped bodies further elongate in a corticomedullary direction
and form the loop of Henle presumably under the influence of Wnt-mediated signal-
ing from the medullary stromal cells (Yu et al. 2009 ). Recent in vivo and in vitro
studies have shown that the gradient of the Wnt signal may also play a role during
the proximo-distal patterning of the nephron (Lindstrom et al. 2014 ; Schneider
et al. 2015 ).
9 Early Kidney Specification and Its Recapitulation by Pluripotent Stem Cells