Puneet Khandelwal, Soman N. Abraham and Gerard Apodaca

layer of basal cells (Fig. 1). The intermediate cells are often
pear-shaped (pyriform), have a single nucleus, are �10–15
�m diameter, and are connected to one another and the overlying
umbrella cell layer by desmosomes (102) and, possibly,
by gap junctions (78). The number of intermediate cell strata is
species dependent: in rodents, the intermediate cells are one to
two layers thick (Fig. 1); in humans, up to five strata are
observed (102). The apparent thickness of the overall intermediate
cell layer varies with the state of bladder filling. The
strata of intermediate cells appear fewer in number in the
distended than in the voided bladder (85). How this phenomenon
is accomplished is unknown but could result from adjacent
strata of intermediate cells sliding past one another during
THE UROEPITHELIUM
AJP-Renal Physiol • VOL 297 • DECEMBER 2009 • www.ajprenal.org
Review
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Fig. 2. Ultrastructure of bladder uroepithelium.
A: scanning electron micrographs of
mucosal surface of the bladder. Left: overview
of the apical surface of rabbit umbrella cells.
Borders of an individual umbrella cell are
indicated by arrowheads. Right: ridge, consisting
of zippered apical membrane (ZM), at the
periphery of adjacent umbrella cells. B: transmission
electron micrograph of the apical pole
of a rat umbrella cell. Examples of discoidal/
fusiform-shaped vesicles (DFV) are marked
with arrows and hinges with arrowheads.
Plaques are located in the intervening membrane
between hinges. Apical cytoplasm of rat
umbrella cells has disc-shaped (�) and fusiform-shaped
vesicles. Multivesicular bodies
(MVB; i.e., late endosomes) are indicated by
arrows. C: asymmetric unit membrane (AUM)
at the apical surface of a rat umbrella cell.
Contrast was adjusted using Photoshop.
filling. Whether this involves reversible breakage of cell-cell
contacts is unknown. The layer of intermediate cells just
beneath the umbrella cells is partially differentiated, and these
cells can express UPs and have DFV (Fig. 1) (10, 230). As
described below, this population of cells rapidly differentiates
when the overlying umbrella cells are lost or removed. The
basal cell layer comprises a single-cell stratum that forms
intimate contacts with an underlying capillary bed (89) (Fig. 1).
The basal cells are mononucleate, �10 �m diameter, and
adhere to the intermediate cells by desmosomes and to the
basement membrane by hemidesmosomes (100, 102). The
latter attachment is mediated by �4-integrin (100, 102). Although
intermediate and basal cells express cytokeratin-13,
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