establishment and maintenance of cellular polarization are critical features for development and organ function . apical - basolateral ( a / b ) polarity of epithelial cells serves to perform vectorial functions , like transport of fluid or directed secretion of specific proteins . besides the ubiquitous epithelial a / b polarization , most epithelial tissues present a second polarity axis within the epithelial plane , referred to as pcp .
pcp was initially discovered in insects [ 24 ] and studied in drosophila , where all adult cuticular structures display pcp [ 59 ] .
many processes and organs requiring pcp have been subsequently discovered in vertebrates ranging from skin and body hair orientation , positioning of primary cilia , and sensory epithelium in the inner ear to directed movement of mesenchymal cell populations during gastrulation among many other processes ( for example , [ 1014 ] ) .
how individual cells that are hundreds of cell diameters apart acquire the same polarity within the plane of an epithelial organ is a fascinating cell biological problem .
although much progress has been made , the mechanistic aspects of pcp establishment are far from being understood .
initial molecular insights identifying pcp signaling components came from genetic screens in drosophila ( table 1 ) [ 5,1619 ] .
functional studies in several tissues placed a set of the pcp regulators into different categories .
these consist largely of two groups : ( a ) the fz - pcp core group and ( b ) the ft / ds group . whereas the latter revolves around the heterophilic adhesion of the two protocadherins ft and ds and the associated secreted golgi kinase four - jointed ( fj ) , the so - called fz - pcp core genes historically comprise six proteins that interact with each other inter- and intracellularly to separate into two complexes on opposing sides of each cell , which provides a cell with a planar orientation ( see table 1 and figure 1 for molecular details and published reviews for further insight on pathways and regulators associated with these core pcp systems ) .
the transmembrane interacting components of the fz core system are , besides fz itself , the four transmembrane protein van gogh ( vang , also known as stbm / strabismus , vangl in vertebrates ) and the atypical cadherin flamingo ( fmi , also known as stan / starry night , celsr in vertebrates ) [ 79,13 ] .
all drosophila genes have equivalent functional orthologues in vertebrates ( often more than one ) .
the homologous vertebrate genes for fw and vhaprr have not yet been functionally characterized in the planar cell polarity ( pcp ) context .
several other intracellular binding partners exist with yet - to - be - defined functions in pcp .
note that fz and vang act in both the interpretation of the long - range signal(s )  wg and dwnt4 - as well as the short - range / local cell - cell transmembrane interactions .
the cytoplasmic components dsh , dgo , and pk are essential for the negative feedback loops within individual cells , as they antagonize each other .
dgo , diego ; dsh , dishevelled ; fmi , flamingo ; fw , furrowed ; fz , frizzled ; pcp , planar cell polarity ; pk , prickle ; vang , van gogh .
the two systems are thought to mediate both long - range and cell - to - cell ( short range ) interactions in all tissues where pcp has been studied in drosophila .
the fz / pcp core group appears fully conserved in vertebrate pcp contexts , although additional components , like receptor tyrosine kinases of the ror family , are involved in vertebrate models .
the function of the ft / ds group in vertebrates is less well defined , although there is good evidence in some tissues / organs for their requirement in pcp establishment [ 2022 ] .
nevertheless , the study of both pcp systems in vertebrates is complicated by redundancy issues of multiple related proteins for each component . also , whereas the fz / pcp core group appears fully dedicated to pcp regulation , ft / ds signaling has also been linked to growth control and to hippo signaling as an upstream activator module . despite these complications , an absolutely key question in pcp regulation is the molecular relationship of the two pcp systems or how their function is integrated within individual cells or groups of cells .
several interaction models have been proposed , but these remain controversial and are an active area of research ( see below ) . despite the prevalence of studies on the fz / pcp core group and ft / ds signaling in pcp establishment , it is noteworthy that additional cellular mechanisms function in cellular planar polarization contexts that appear to be largely ( or fully ) independent of these two molecular cassettes .
alone in drosophila , the planar cellular polarization in the embryo that drives germband extension ( a process comparable to vertebrate  convergence and extension  ) and that is based on polarized myosin ii localization seems to occur independently of either system .
the six components of the fz core group have been known for quite some time , the last of these  diego ( dgo , related to inversin and diversin in vertebrates)being added in 2001 .
however , recent work in drosophila identified two additional factors that might be linked to pcp establishment in a similar manner and thus could also be considered members of the fz core group .
these include the vhaprr accessory subunit of the proton pump v - atpase [ 2830 ] and furrowed ( fw ) , a drosophila selectin family member . both affect pcp in a manner similar to the core components in all tissues analyzed ( wing , eye , and thorax ) and can be co - immunoprecipitated with fz and fmi ( vhaprr ) or specifically with fz ( fw ) . at least for fw , its function appears to be similar to that of fmi , as it is required to stabilize fz in plasma membrane complexes , and it appears also to promote homophilic cell adhesion .
moreover , fw can mediate an intercellular binding / interaction between fz and vang , again a function related to what fmi is doing , but unlike fmi , fw appears to affect only fz stability and associates only with fz ( figure 1 ) .
strikingly , fw and fmi could be partially redundant in mediating an fz - vang interaction or fz membrane association , as the tissues where fmi mutants have a weaker phenotype ( thorax ) are where fw loss of function ( lof ) is most severe ; for the eye , the opposite is true .
importantly , the pcp defects in fw , fmi double mutants are not more severe than those in fz null flies , suggesting that both require the presence of fz for their function .
the role of the vhaprr protein remains to be clarified , but functional studies suggest that it can affect the trafficking of fmi and possibly fz to or from the membrane ( or both ) .
whereas vhaprr can be co - immunoprecipitated with both fz and fmi , it appears to directly physically associate with fmi only .
while the function of fw is restricted largely to pcp ( with only a mild overgrowth of the eye disc and a potential role in cytoskeletal regulation ) , vhaprr appears generally required for trafficking , affecting also fz2 ( canonical wingless [ wg ] signaling ) , notch , and e - cad .
the biggest conceptual questions in pcp establishment are how long - range signals might regulate the process and how this is relayed to and reinforced through local cell - to - cell interactions . from early studies with lof mutants , it was clear that cellular orientation was non - random ( best visualized in the wing ) and that cellular orientation would indeed follow / adjust to the neighbor 's patterns , easily observed in fz , ds , ft , fmi , or vang mutants .
this behavior was further clarified with clonal analyses and the discovery of non - cell autonomous effects .
an example is the non - autonomous behavior of the fz mutant or overexpression clones affecting the polarity of wildtype cells surrounding the clones in the wing , eye , and abdomen .
these observations set the stage for analyses of local and global signals to work on pcp orientation : local signals with short - range abilities to communicate on a cell - to - cell basis and global signals that coordinate the process at the organ level over a long range .
first , there needs to be spatial restriction or directional signaling for a given  signal  .
second , a connection between the long - range signal and the cell - cell communication mechanism(s ) should exist .
two global clues had been associated with long - range pcp signaling : ( a ) the ds and fj expression gradients in the ft / ds system   and more recently ( b ) the long - range effects of localized wnt expression on the fz / pcp core factors .
how are the ft / ds or fz - pcp core pathways interpreting these signals , and how are these two systems integrating the information ?
the ft / ds pathway covers both the ( long - range ) spatial restriction and the ( short - range ) cell - cell communication features of pcp establishment .
related to long - range effects , two of its key components have a graded / localized expression pattern : in the wing , fj is expressed in an ascending proximo - distal gradient [ 4547 ] and ds is expressed at higher levels in the proximal part , opposite to the fj gradient , although its slope is rather steep as compared with other gradients .
the third main component , ft , has a rather uniform expression in the wing . within the current model , ds acts as a ligand for ft , inhibiting its function and creating a distal - to - proximal ft activity gradient   ( reflected in anisotropic subcellular distribution of ft , along with the fj expression gradient ) .
this model applies also for the eye , where fj is expressed in a graded manner with its peak at the equator , ds forms gradients with peaks at the poles and ft is uniformly expressed .
similarly , in the eye , flat ds expression rescued the ds mutant as long as fj was expressed in a gradient .
next , polarity needs to be  translated  from the long - range ( body ) axes to the local ( short - range ) cell - to - cell communication .
how is the long - range ft / ds pathway information transmitted to the short - range cell - to - cell interactions ? at the single - cell level , the atypical cadherins ft and ds are localized subapically in epithelial cells near but not directly at the adherens junctions , where they form heterodimers .
fj functions as a kinase at the golgi , affecting the localization of ft and ds and the strength of their interactions .
for example , ft mutant tissues induce surrounding cells to  point  toward the clone , whereas ds clones reorient cells away from the mutant patch , and fj mutant patches also reorient cells toward the clone   several unresolved mysteries exist in ft / ds  signaling  .
heterodimer formation , though seemingly essential for cell - to - cell communication , is itself not required for ft / ds pcp  signaling  to work , since ds binding to ft is not necessary for ft activity , as shown by the electron capture dissociation ( ecd)-ft ( ft lacking the extracellular domain ) .
the mirror experiment adds even more mystery , as the ds extracellular region is sufficient to generate pcp defects , whereas its intracellular region is thought to stabilize dachs , an atypical myosin that serves as a good marker for asymmetric ft / ds interactions .
the open questions remain ( at least in part ) because downstream effectors of the ft / ds pathway in pcp are functionally largely not defined , although several proteins are known to bind to ft , for example .
downstream of these complexes , exhibiting a polarized localization ( already at the third instar larval stage ) , with ds recruiting dachs to the membrane and ft antagonizing its membrane localization .
although dachs localization is a very useful marker for this pathway , its function in pcp establishment is still elusive , largely because its lof phenotype is very mild ( and this is further complicated by the observation that the intracellular domain of ds , where dachs interacts , seems dispensable for ds pcp signaling ) . another downstream player that could provide answers is par-1 , which might be the connection between ft / ds and microtubule plus - ends orientation .
again , both pcp systems need to be  translated  from global / body axis - related polarity to short - range cell - to - cell communication / relay and ultimately to single - cell responses . in the developing wing , each epithelial cell shows polarized core pcp protein complexes of fz and vang early in the prepupal stage or even late third instar , fz / dishevelled ( dsh)/dgo on one side and vang / pk on the opposite side , with both complexes also containing fmi . at early stages , the pcp axis is perpendicular to the wing margin , displaying a radial pattern . as mentioned above ,
the respective complexes are stabilized via feedback loops among themselves ( figure 1 ) in a manner that , disturbing the localization of one component ( via either lof or gain of function ) , affects the localization of all the others , even from the  opposite complex    ( figure 1 ) .
since fmi also interacts with both fz and vang , it had been suggested that homophilic fmi bridges across cell membranes facilitate intercellular fz - vang interactions that are essential to propagate pcp [ 33,6366 ] . at the short - range level , cell - cell communication
clonal cells lacking fz reorient adjacent cells toward the clone , whereas cells lacking vang reorient neighboring cells to point away ; double - mutant clones for fz and vang reorient hairs to point toward the clone .
these data suggest that fz is required for sending a polarity signal and vang for its  reception 
nonetheless , cells can and probably do communicate in both directions , possibly also via fmi - fmi bridges .
however , although fmi is needed on both sides , the ability of cells to communicate via fmi - fmi bridges depends largely on the presence of fz and vang .
nevertheless , it will be interesting to determine whether other pcp regulators are directly involved in modifying fmi - fmi interactions .
it is important to mention that dsh , pk , or dgo mutant clones affect pcp within mutant clones only , suggesting that dsh , pk , and dgo are involved mainly in the intracellular interactions and interpretation of pcp signals .
what about long - range regulation ? in the wing and eye , pcp axes are orientated toward the prospective margins , the source of wg / dwnt4 expression .
recently , gain - of - function experiments demonstrated that dwnt4 and wg reorient pcp , altering the fz - pcp core complex localization and polarity axis orientation , and thus indicated that these wnts serve an instructive function in defining pcp axes .
moreover , through complicated genetic assays , it was possible to show that wg and dwnt4 are indeed necessary for pcp establishment but that they act redundantly . like lawrence and colleagues ( who mentioned that  to understand the machinery , one needs to define its components and then work out what they do  ) , we were able to show that wg / wnt4 modulates the intercellular fz - vang interaction . in agreement with
the proposed graded fz activity ( instructive capacity to interact with other core pcp components ) that directs pcp orientation toward lower fz activity   ( figure 1 ) , co - overexpression of fz and wnt4 in vivo buffers each other 's effects , confirming the cell culture - based modulation of fz - vang interactions by dwnt4 and wg .
thus , the current model for long - range core fz - pcp axis establishment includes wg / dwnt4 as instructive regulators of fz - vang polarization in a graded manner in drosophila imaginal discs ( figure 1 ) ( this view applies at least for the wing and eye , but other tissues are more complex ) . along these lines , in vertebrates , particularly in zebrafish , wnt5a and wnt11 are involved in pcp establishment , but their mechanism for pcp establishment remains unclear [ 44,7275 ] ; one mechanism involving wnt5a - mediated vangl2 phosphorylation during digit formation in mouse limbs   also involves ror2 , a receptor tyrosine kinase that does not appear to have a pcp function in drosophila , suggesting potential additional players within the fz - pcp network .
nonetheless , the drosophila experiments provide strong evidence that wnts serve as an instructive input in regulating fz - pcp axis definition .
although in vertebrates the  mechanistic mix  is likely to be more complex as wnt5a and wnt11 might employ distinct functions , an instructive role for wnt11 has also been suggested .
it is intriguing to think about wg / wnts as the master regulators for pcp : ( a ) wg itself , through canonical wg signaling , regulates graded fj and ds expression in eyes and wings ( in the eye , wg downregulates fj and upregulates ds ; in the wing , wg upregulates fj and downregulates ds ) , and ( b ) wg / wnt4 regulates fz - pcp activity / polarity axis establishment through regulating the interaction between fz and vang .
strikingly , sagner and colleagues   show in drosophila wings that pcp can be affected by 
morphogenetic organizers  and oriented cell divisions . in the larval wing disc , notch and wg signaling sets up the dorso - ventral organizer and hedgehog ( hh ) and decapentaplegic signaling sets up the antero - posterior organizer .
as the morphogens control wing proliferation , size , and axes within the wing , their positional information could provide the earliest cues for pcp orientation .
this is a point of view supported by the fact that uniform expression of morphogens during wing development can generate pcp defects .
an hh - and - wg / wnt4 interplay has been also demonstrated for pcp establishment in the abdomen and thus might be a more general mechanism ( reviewed in ) . although it is clear that both ft / ds and fz - pcp pathways are linked to wg / wnt gradients , it remains open as to how these two pcp pathways interact or get integrated at the cellular level .
initial proposed models suggested that ft / ds acts upstream of the fz - pcp core factors , where the ft / ds system would provide  global cues  that orient the initial polarity of fz - pcp complexes ; this was based largely on the observation that , at the cellular level , in ft or ds mutants , fz - pcp core protein complexes remained with polarized distribution in individual cells but with abnormal orientation .
different views emerged when overexpression or lof clones of ft or ds displayed non - autonomy in null mutant backgrounds of fmi or fz .
this opened two possibilities , either that ft / ds signals to other  tissue - specific modules  but not to the core module ( called the bypass pathway ) or that the ft / ds and fz - pcp systems act in parallel .
the latter is supported by many genetic experiments in the fly abdomen   and also in the late embryonic / larval abdominal denticle belt orientations . in denticle belts ,
orientation of denticles is pcp - dependent , but the ft / ds and fz - pcp systems act in a redundant manner .
whereas fz or ft / ds mutants display normal denticle orientations , the double - mutant combinations affecting both systems resulted in marked denticle pcp defects , indicating a clear redundancy in this context .
consistent with two parallel pathways ( possibly affecting different cellular features / functions ) are the following observations .
it was suggested that ds expression was necessary for correct pcp throughout the wing , but rescue experiments of ds mutants with exogenous transgenic constructs suggested that it was not the case , concluding that the ft / ds system was redundant with fz - pcp signaling in much of the wing and pointed out a major phenotypic difference between the two systems : whereas flies carrying mutations in fz / pcp core components exhibit pcp defects throughout the wing , the ft / ds system affects mainly polarity in the proximal wing half ( figure 2 ) , the wing area where pcp strongly depends on cellular realignments and rotations during the transition from the radial to the proximo - distal pcp axis ( figure 2 ) .
in fact , the realignment from the radial to the proximo - distal pcp axis is dependent on ft / ds - induced mechanical forces , and ds in the wing hinge is essential for the wing to respond to the associated anisotropic mechanical stress .
thus , the different pcp systems appear to regulate distinct cellular features , both affecting the final pcp establishment and orientation . the mutations in fz / pcp core components affect the whole wing blade .
this is seen in both intercellularly acting factors  for example , fmi , the fmi - stan allele ( top left)or the cytoplasmic intracellular components  for example , pk ( bottom left ) .
. consistently , ds and ft also influence mitotic figure orientations near the hinge at the larval stage .
ds , dachsous ; fmi , flamingo ; ft , fat ; fz , frizzled ; pcp , planar cell polarity ; pk , prickle ; stan , starry night . a potential link between the two pathways might be provided via the polarization / orientation of microtubules .
the uemura lab has shown that the ft / ds system can regulate microtubule orientation   and that fz and fmi can be seen moving along planar microtubule arrays in pupal wings . moreover , most recently , it has been suggested that the different protein isoforms of the fz / core group gene pk , pk and sple , provide a different bias toward microtubule 
, it is noteworthy that recently the two distinct pk - sple isoforms were suggested to provide a link between the fz / pcp core group and the ft / ds system : as the sple isoform specifically binds to ds ( and dachs ) through its n - terminal extension , it can coordinate ( co)localization between the ds and vang complexes ( by binding to both ) , whereas the pk isoform ( lacking the n - terminal domain ) only physically interacts with vang .
as such , the tissues where sple is expressed and genetically more important ( eye and leg ) would align the vang and ds complexes on the same cellular side , whereas those tissues using mainly pk ( wing and thorax ) would not align these complexes ( allowing fz and ds to cohabit on the same side of cells ) .
although this requires a more physiological and functional dissection , it provides an intriguing potential mechanism for cross - talk and coordination of the two systems . in conclusion , we support the view that the two main molecular systems regulating pcp establishment act in parallel by using distinct effectors and affecting different cellular responses ; as such , they can act together in a non - redundant ( imaginal discs ) or redundant ( embryo ) manner .
importantly , such a setup provides a high level of fidelity and potential for corrective measures if needed . along these lines ,
the non - autonomous defects associated with mutant clones of fz core components are expanded in ft / ds mutant backgrounds as compared with a wildtype background in wings , arguing that the two pathways can indeed help each other in  correcting  aberrant cell - to - cell interactions in the respective other system .
such observations and hypotheses are all in favor of two parallel pathways , to provide corrective functions when needed and fidelity to the process .
as it appears that both systems act through different effectors , they should converge somewhere downstream .
cortical microtubules align perpendicular to the margin in proximal wing regions , supporting a general role for the orientation of cortical microtubules in pcp   and in the realignment of pcp later in development
. it will be for future functional dissections to establish the specific intersection and integration points between the two systems .
 correct patterning and polarization of epithelial and mesenchymal cells are essential for morphogenesis and function of all organs and organisms . 
 epithelial cells are generally polarized in two axes : ( a ) the ubiquitous apical - basal axis and ( b ) polarity within the plane of the epithelium . 
 the latter is generally referred to as planar cell polarity ( pcp ) and also is found in several contexts of mesenchymal cell patterning . in drosophila , all adult structures display pcp features , and two conserved molecular systems ( the fat [ ft]/dachsous [ ds ] system and the frizzled [ fz]/pcp pathway ) that regulate this process have been identified . 
 although significant progress has been made in dissecting aspects of pcp signaling within cells , much remains to be discovered about the mechanisms of long - range and local pcp cell - cell interactions . here , we discuss the current models based on drosophila studies and incorporate recent insights into this long - standing cell and developmental biology problem . 