197mesoderm most closely associated with the tips, referred to as the cap mesenchyme
(CM). This feedback loop restricts the domain of GDNF-Ret signalling to the very
periphery of the expanding organ to form what is referred to as the nephrogenic
zone. Lineage tracing using the CM marker, SIX2, has established that all epithelial
cell types within the nephron arise from the CM (Kobayashi et al. 2008 ). The CM
has also been shown to represent a self-renewing progenitor population critical for
driving organ growth by supporting ongoing ureteric branching but also able to
commit to nephron formation. The balance between self-renewal as CM and com-
mitment to nephron formation via a mesenchyme to epithelial transition must be
tightly regulated, although it remains incompletely understood. What is known is
that signals from the adjacent ureteric tip, which represents the location of all pro-
genitors for ongoing collecting duct morphogenesis, support CM maintenance as
well as commitment (Kopan et al. 2014 ). Low levels of canonical Wnt signalling
(Karner et al. 2011 ) together with FGF signalling (Barak et al. 2012 ) support self-
renewal, while increased canonical Wnt signalling (Carroll et al. 2005 ) and subse-
quent non-canonical Wnt signalling (Tanigawa et al. 2011 ; Burn et al. 2011 ) are
required for nephron formation. The remaining mesenchyme represents a popula-
tion of stromal cells regarded as generating cells of the perivasculature (Kobayashi
et al. 2014 ). This mesenchyme, or an earlier intermediate mesodermal population,
is also likely to give rise to the vasculogenic endothelial progenitors that will con-
tribute to the formation of the capillaries within the kidney. There is also a thought
to be a contribution to the formation of kidney endothelium that arises via angio-
genic ingrowth.
Almost as soon as a nephron is formed, there is immediate patterning and seg-
mentation that occurs (Georgas et al. 2009 ). The cells closest to the ureteric tip as
the nephron forms will become the distal segments of the nephron, while those
further from the ureteric epithelium will form the proximal elements, including the
glomeruli and proximal tubules. There is a declining gradient of canonical Wnt
signalling from UB to glomerulus (Lindström et al. 2015 ), as well as distinctions in
the cell adhesion proteins and transcription factors marking each segment. While it
has been proposed that there are distinct progenitor cell types for different segments
of the nephron (Barker et al. 2012 ; Rinkevich et al. 2014 ), the focus with respect to
the recreation of the kidney has been the formation of the cap mesenchyme (neph-
ron progenitor) and ureteric tip (collecting duct progenitor). Both of these cell types
are known to arise from the intermediate mesoderm of the elongating trunk. Both
also appear to be lost around (Hartman et al. 2007 ; Rumballe et al. 2011 ) or prior
(Hinchliffe et al. 1991 ) to birth in mouse and humans, respectively, explaining why
the formation of new nephrons is not observed during postnatal life in mammals.
Indeed, what regulates this cessation of nephrogenesis is not understood although it
has been the subject of intense investigation. Recent studies suggest that these
populations are not transcriptionally uniform across morphogenesis, with both UB
and CM showing declining rates of cell cycle (Short et al. 2014 ). However, studies
into whether cessation is hard wired within CM cells suggest that placing a late CM
cell into an early CM will allow such a cell to continue to self-renew in vitro longer
than anticipated (Chen et al. 2015a). In addition, several studies have now developed
11 Recapitulating Development to Generate Kidney Organoid Cultures