Biomedical Engineering Reference
In-Depth Information
The cortical interstitium also consists of a stem/progenitor cell that gives
rise to renal interstitial cells including the pericytes, smooth muscle, and the
mesangial cells within and about the renal corpuscles (
Humphreys et al.,
2010
). Signals provided by both the cap mesenchyme and the interstitium
regulate the branching morphogenesis of the ureteric bud and potentially
the patterning of the nephron and collecting ducts (
Costantini & Kopan,
2010; Yang et al., 2002
).
The epithelial components of the kidney undergo extensive growth dur-
ing the embryonic period and continue postnatally. Morphogenesis establishes
the ultimate diameter, length, and shape of the kidney tubules, which are ab-
solutely essential for normal organ function. The cellular and molecular pro-
cesses regulating this morphogenesis are still poorly understood although
studies over the past several years have provided some insights.
Karner et al. (2009)
found that, during the embryonic period, orientation
of the mitotic spindle of most of the epithelial cells appears to be random
relative to the proximal-distal axis of the tubule. Without some sort of com-
pensatory mechanism, one would then predict that tubule diameter (or at
least the number of cells within the cross-sectional circumference of the tu-
bule) would increase during the embryonic period. It does not. In fact, it was
found that the number of cells within the circumferential wall decreased as
the tubule elongated (
Karner et al., 2009
). How can this be? One possibility
is that a significant numbers of epithelial cells are culled by apoptosis. How-
ever, several groups have reported that the rate of epithelial cell apoptosis is
negligible during the embryonic period. A second possibility is that some
sort of cellular rearrangement is occurring that causes the tubules to thin
and contributes to their elongation. Indeed, cellular rearrangements of this
type have been observed during gastrulation and neurulation in multiple
species (
Axelrod & McNeill, 2002; Concha & Adams, 1998; Darken
et al., 2002; Goto & Keller, 2002; Heisenberg et al., 2000; Honda,
Nagai, & Tanemura, 2008; Nikolaidou & Barrett, 2005; Torban, Kor, &
Gros, 2004; Wallingford, Fraser, & Harland, 2002; Wallingford et al.,
2000; Wang et al., 2006; Winklbauer, 2009
). Although the mechanism
regulating tissue elongation and thinning varies, the phenotypic outcome
is essentially the same and these processes are collectively referred to as CE.
Several distinct types of cell behavior have now been described that lead
to CE movements including mediolateral intercalation and multicellular
rosette formation and resolution (
Fig. 8.2
). Each of these processes has dis-
tinct, defining characteristics that can indicate whether they are occurring.
For example, cells undergoing intercalation frequently show mediolateral
