213(Sharmin et al. 2016 ). Xinaris et al. ( 2012 ) also demonstrated the maturation and
vascularisation of organoids aggregated from dissociated foetal mouse kidneys and
transplanted into athymic host rats. Such results are in line with what had previously
been shown when an embryonic kidney is transplanted into the omentum of a recip-
ient animal (Rogers et al. 1998 ; Dekel et al. 2002 ). Here, the formation of glomeru-
lar capillaries via the ingrowth of host vasculature resulted in the formation of a
urinary filtrate. Supplying the organoid with a functional circulation, and indeed the
initiation of filtration of the blood, may well assist in the functional maturation of
many of the component epithelial cell types along the forming nephrons, including
the proximal tubule cells and the podocytes. Indeed, the transplantation of intestinal
organoids for periods of 2–3 months can significantly improve intestinal epithelial
maturation and allow the formation of an appropriate smooth musculature within
the surrounding mesenchymal layers (Watson et al. 2014 ). These types of studies
with kidney organoids will begin to clarify whether renal tubular maturation requires
unidirectional flow of a glomerular filtrate.
Another critical challenge for kidney organoid implantation, following from
their vascularisation, will be to ensure a viable exit path for the urinary filtrate that
will be produced. In mammalian kidney development, the failure to develop a col-
lecting system results in multicystic dysplasia and involution of the kidney (Hains
et al. 2009 ). Similarly, transplanted, vascularised kidney organoids with no drainage
will develop hydronephrosis (Yokote et al. 2015 ).
While the Takasato et al. ( 2015 ) organoid approach appears to contain a ureteric
epithelium, the way in which an organoid is aggregated, which involves the com-
plete dissociation of cultures to a single-cell suspension followed by reassociation,
will not result in the formation of a single common ureteric tree and certainly not
one existing ureter. This was identified early as a major distinction between a kidney
organoid and a kidney (Davies 2015 ). For the formation of a replacement kidney, it
is likely that the ureter itself must be patterned first or separately and then sur-
rounded by nephrogenic epithelium. This is counterintuitive based upon our funda-
mental understanding of kidney morphogenesis. The branching of the ureteric
epithelium to form a single unified tree through which the urinary filtrate can flow
requires the close apposition of the ureteric tip with the cap mesenchyme. Conversely,
the formation and attachment of nephrons occur in a spatially restricted region
around the ureteric epithelial tips. While we know that the fusion of the nascent
nephrons with the ureteric tips to form a single contiguous lumen occurs at the late
renal vesicle stage via a process of invasion on CM-derived cells into the ureteric
epithelium (Georgas et al. 2009 ; Kao et al. 2012 ), how this process is regulated and
whether it can be induced without an initial nephrogenic zone is also not clear.
Studies into the process of fusion between new nephrons and existing collecting
duct epithelium are being performed in the setting of the zebrafish mesonephros
(Diep et al. 2015 ), but whether this is applicable in the mammalian setting is yet to
be determined.
11 Recapitulating Development to Generate Kidney Organoid Cultures