173stromal cell population within the kidney is essential for growth of the UB and the
differentiation of nephron progenitors (Mendelsohn et al. 1999 ; Das et al. 2013 ).
Because the stromal cells could be a mixed population from several distinct origins,
further developmental examination should be undertaken to clarify early lineage
specification processes.
Future investigations should examine how best to validate induced human meta-
nephric nephron progenitors. Although current available protocols evaluate the
quality of nephron progenitors based on epithelial nephron structure reconstitution
capacity and marker genes expression, several problems with this approach exist.
First, the precise quantitative gene expression profile of human fetal nephron pro-
genitors is unknown, complicating the evaluation of induced nephron progenitors.
We have evaluated the quantitative gene expression profile of murine ESC-derived
nephron progenitors using a pure fetal population, and similar information from
human equivalents is essential. One problem complicating this issue is the difficulty
in discriminating between mesonephric and metanephric nephron progenitors.
Because both populations differentiate into nephric tubules and glomeruli, they can-
not be distinguished by morphological examination. Additionally, the widely used
method of posterior Hox gene examination may be insufficient, because Hox expres-
sion is dose dependent and not all-or-none. Second, there is no reliable functional
assay to quantitatively assess early nephron differentiation or late functional neph-
ron maturation. The currently used chemical Wnt agonist-driven differentiation
method varies between protocols, and it may be less effective in inducing differen-
tiation of nephron progenitors when compared with conventional Wnt-producing
cell-mediated stimulation. This also complicates attempts to compare induction
protocols. Furthermore, the late developmental process by which the kidney matures
into a functional organ is still poorly understood, and there is no method to recapitu-
late this process at present. Further evaluation of the capacity for induced nephron
progenitor cells to produce a functional kidney organoid will require a more thor-
ough understanding of the mechanisms underlying late-stage kidney
organogenesis.
9.10 Concluding Remarks
Many years of scientific effort toward understanding kidney developmental pro-
cesses have enabled the production of three-dimensional kidney tissue in a dish.
This milestone is a product of the careful analysis and understanding of multiple
biological mechanisms, including interdependent kidney organogenesis, self-
organizing nephron induction, and early lineage specification. Although the cur-
rently available tissue remains immature and cannot replace the adult kidney
functions, continued progress in the field of kidney regeneration may one day yield
an era in which hemodialysis is no longer needed.
9 Early Kidney Specification and Its Recapitulation by Pluripotent Stem Cells