the eye is an optical device that transmits and focuses light onto the neural retina ( fig .
the retina , a 0.2-mm - thick central nervous tissue , is the first station of the visual system .
in addition to acting as a light receiver , the retina carries out considerable image processing through circuits that involve five main classes of cells ( i.e. , photoreceptors , bipolar cells , amacrine cells , horizontal cells , and ganglion cells ; fig .
these processes collectively amplify , extract , and compress signals to preserve relevant information before it gets transmitted to the midbrain and the thalamus through the optical nerves ( axons of the ganglion cells ; for review see baccus , 2007 ) .
the retinal information received by the midbrain is processed to control eye movement , pupil size , and circadian photoentrainment ( huberman et al . , 2008 ) . only the retinal input that terminates at the lateral geniculate nucleus of the thalamus is processed for visual perception and gets sent to the visual cortex . there , information about shade , color , relative motion , and depth are all combined to result in one s visual experience . the visual sense organ .
( a ) diagrams of the eye ; an enlarged diagram of the fovea is shown in the box .
retina forms the inner lining of the most of the posterior part of the eye .
the rpe is sandwiched between the retina and choroids , a vascularized and pigmented connective tissue .
r , rod ; c , cone ; b , bipolar cell ; h , horizontal cell ; a , amacrine cell ; g , ganglion cells ; m , mller cell .
the nuclei of the photoreceptors constitute the outer nuclear layer ( onl ) . the nuclei of the bipolar cells , amacrine cells , horizontal cells , and mller glial cells are found in the inner nuclear layer ( inl ) , and the nuclei of ganglion cells form the ganglion cell layer ( gcl ) . the outer plexiform layer ( opl )
contains the processes and synaptic terminals of photoreceptors , horizontal cells , and bipolar cells . the inner plexiform layer ( ipl )
contains the processes and terminals of bipolar cells , amacrine cells , and ganglion cells .
the processes of mller glial cells fill all space in the retina that is not occupied by neurons and blood vessels . reproduced from swaroop and zack ( 2002 ) , published by biomed central
the nuclei and processes of the retinal cells are segregated into alternate , anatomically distinctive layers ( see legends in fig .
photoreceptors lie in the outer part of the retina , the region farthest from incoming light .
light passes through transparent inner retinal layers before it can be captured by the photoreceptor .
though such an organization may seem counterintuitive , it allows the retinal pigment epithelial ( rpe ) cells that lie juxtaposed to the apical side of the photoreceptors to absorb scattered light or light unabsorbed by the photoreceptors .
visual perception begins when the captured photon isomerizes the chromophore conjugated with the visual pigment in the photoreceptor cell .
the photoexcited visual pigment then initiates a signal transduction cascade that amplifies the signal and leads to the closure of cation channels on the plasma membranes . as a result ,
the change in membrane potential is sensed by the synapses , which react by releasing fewer neurotransmitters ( see more in box 1 ; for review see yau and hardie , 2009 ) .
this information is relayed to the bipolar cells , and subsequently , the ganglion cells via a forward pathway .
this information is also modified by their lateral interactions with the interneurons - amacrine cells and horizontal cells .
light initiates the isomerization of the 11-cis retinal , a chromophore covalently conjugated to rhodopsin embedded within the disc membranes .
photoactivated rhodopsin activates the heterotrimeric g protein transducin by catalyzing the exchange of gdp for gtp .
the dissociated -subunit of transducin then activates cgmp - phosphodiesterase , which rapidly hydrolyzes cytoplasmic cgmp .
the decrease in cgmp concentration causes closure of cgmp - gated cation channels on the plasma membrane ( fig .
, the cells are hyperpolarized and release less glutamate transmitters to their connected bipolar cells . in the recovery phase , photoactivated rhodopsin is desensitized by phosphorylation and arrestin binding .
retinal guanylyl cyclase and guanylyl cyclase activator help to replenish cgmp levels in a ca - dependent manner . na / ca - k exchanger , which resides on the plasma membrane , regulates the ca homeostasis of the os ( molday , 1998 ) .
all these components are highly integrated in close proximity on the disc membranes and/or adjacent plasma membranes for fast processing ( fig .
photoisomerization converts 11-cis retinal to all trans - retinal , and finally to all - trans - retinol in the os .
all - trans - retinol leaves rods and  diffuses  to the rpe , where it undergoes a series of enzymatic reactions to be converted back to 11-cis - retinal .
11-cis - retinal returns back to the os where it regenerates rhodopsin and completes the visual cycle ( lamb and pugh , 2004 ) .
considerable progress has been made in characterizing all the major components of this visual cycle ; however , the cell biology of retinoid trafficking remains largely unknown .
humans and other higher primates ( but not other mammals ) have foveas that are 700-m - diameter foci near the central retina .
the fovea has the highest visual acuity in comparison with other parts of the retina due to several structural and compositional modifications of this region .
for example , the cell bodies of the proximal retinal neurons have been shifted to the side , enabling light paths to enter photoreceptors with minimal distortion ( fig .
the cone system has a higher resolution ; each fovea cone is connected to only one bipolar and one ganglion cell . in other areas of the retina ,
cones are named after their conically shaped light - sensing outer segments ( os ) .
the os is a modified cilium , a light - sensing organelle in which phototransduction occurs .
the cone os is commonly considered as a structure containing an orderly stack of membranous laminella that are continuous to the plasma membrane and form a highly convoluted surface membrane .
electron micrographs depicting cone oss containing closed discs have also been observed ( de robertis and lasansky , 1958 ; dowling , 1965 ; cohen , 1970 ) .
three cone pigments with maximal absorption for different wavelengths of light are packed into the os disc membranes of different types of cone cells .
unlike other retinal cells , there is only one kind of rod photoreceptor , and it is the predominant cell type in the retina .
humans have 130 million photoreceptors , 5 million bipolar cells , and 1 million ganglion cells .
rods outnumber cones by 20-fold , and are distributed throughout the retina with the exception of the fovea region .
the signal transduction , visual cycle , and electrophysiology of the rod have all been studied comprehensively ( box 1 ) .
the unusual structure of rod photoreceptor cells has also become a major source of interest in the field of cell biology as early as half a century ago , though limited accessibility in the past to these cells in vivo has made the study of these photoreceptor cells quite difficult .
genetic and mouse model studies from the past two decades have pinpointed the death of rod cells as the root cause of several inherited human retinal degeneration diseases , such as retinitis pigmentosa .
retinitis pigmentosa is a common form of progressive rod - predominant dystrophy that occurs in 1 in every 4,000 people .
patients with retinitis pigmentosa have impaired night vision and peripheral vision that precede the loss of central vision .
many causative genes for retinitis pigmentosa encode key players in phototransduction signaling , the visual cycle , and photoreceptor fate determination ( please refer to http://www.sph.uth.tmc.edu/retnet for a summary of the genes implicated in retinal degeneration ) .
interestingly , several of these genes encode molecules that play a significant role in morphogenesis and the vesicular trafficking of photoreceptors .
these findings have drawn considerable attention to the fascinating cell biology questions brought about by these neuronal cells . here ,
rods have evolved a unique structure to detect and process light with high sensitivity and efficiency ; human rods can detect single photons ( hecht et al . , 1942 ; baylor et al . , 1979 ) .
each rod contains four morphologically distinguishable compartments : the os , inner segment ( is ) , nucleus , and axon / synaptic terminal ( fig .
the length of the rod os ranges from 30 to 60 m in length ( and 1.410 m in diameter ) , depending on the species . basically , the rod os is a cylindrically shaped membrane sac filled with 1,000 flattened , lamellar - shaped membrane discs that are orderly arrayed perpendicular to the axis of the os ( sung and tai , 2000 ; molday , 2004 ) .
these discs appear to be floating freely , although filamentous structures bridging adjacent discs and disc rims to the nearby plasma membrane do exist ( fig .
2 , c and d ; roof and heuser , 1982 ; molday , 2004 ) .
the visual pigment of the rod , rhodopsin , comprises 95% of the total amount of disc protein ; it is densely packed within the disc lamellae ( i.e. , 25,000 molecules/m ) .
the high density of rhodopsin , together with its ordered alignment with respect to the light path , increases the probability of capturing an incident photon .
( a ) 3d cartoons depict the inter - relationship between rod and rpe ( left ) and is  os junction ( right ) ; rpe apical microvilli interdigitate the distal half of the os .
r , rpe ; v , microvilli ; o , os ; i , is ; n , nucleus , s , synaptic terminal .
( b ) a schematic drawing of a mammalian rod depicting its ciliary stalk and microtubule organizations ; the axonemal ( ax ) and cytoplasmic microtubules ( not depicted ) are anchored at the basal body in the distal is .
vlgr1whirlin complexes appear on both cc plasmalemma and the lateral plasmalemma of the is ridge complex .
( c ) electron micrographs reveal the hairpin loop structures of the disc rims and the fibrous links across the gap between the disc rims and plasma membranes ( arrowheads ) .
( d ) the os plasma membrane and disc membrane have distinctive protein compositions ; molecules are either expressed on the plasma membrane or the disc membranes , but not both .
the only exception is rhodopsin ; rhodopsin is present on disc membrane ( with a much higher concentration ) and plasma membrane ( not depicted ) .
the cgmp - gated channel : na / ca - k exchanger complex on the plasma membrane directly binds to the peripherin-2rom-1 oligomeric complex on the disc rim .
the cgmp - gated channel is composed of three a1 subunits and one b1 subunit .
abca4 , a protein involved in retinoid cycle , is also enriched on the disc rim .
retgc1 , retinal guanylyl cyclase ; cng channel , cgmp - gated channel . adapted from molday ( 2004 ) .
( e ) electron micrograph showing the longitudinal sectioning view of is  os junction of rat rod .
inset : a transverse section through the cc shows 9 + 0 arrangement ; an arrow points to the cross - linker that gaps the microtubule doublet and adjacent ciliary membrane .
( f ) electron micrographs of a low - power ( inset ) and high - power images of the rat retina , at the junction between the rod os and the rpe .
a white arrow points to a group of saccules from the tip of os curls and upwards .
white arrows in inset point to two distal os fragments that are engulfed by rpe .
the  spine  of the comb is the ciliary stalk , which contains axonemal microtubules that extend distally to about half the length of the os .
the axonemal microtubules are anchored at the basal body located at the distal end of the is , and traverse through the connecting cilium ( cc ) .
the cc is a narrow stalk ( 0.31.2 m in length ; 0.3 m in diameter ) that acts as the single physical bridge linking the is and the os .
the rod axoneme contains nine outer microtubule doublets and lacks the central pair of microtubules seen in motile cilia ( i.e. , 9 + 0 ; fig .
there are fibrous structures cross - linking each microtubule doublet to the overlying plasma membrane , and rows of intramembrane particles in the proximal cc ( i.e. , ciliary necklace ) ; these ciliary structures are speculated to serve as diffusion barriers ( for review see sung and tai , 2000 ) .
the distal os axoneme primarily contains microtubule singlets and no fibrous cross - linker . in terms of both morphology and molecular composition , the cc shares similar features with primary cilia .
primary cilia are hair - like cell protrusions present on most cells in vertebrate organisms , and are typically 36 m in length . these cilia sense extracellular signals by facing a fluid - filled luminal cavity ( rosenbaum and witman , 2002 ) .
the apical portion of the is forms a ridge - like structure that surrounds the circumference of the cc ( fig .
the apical surface of the is ridge is closely apposed to the basal os surface ( fig .
prominent microvilli extending from the apex of the is ridge surround the proximal os . resembling the calyx of a flower , these protrusions are named calycal processes ( fig . 2 , b and e ) .
the lateral surface of the is ridge ( or periciliary plasma membrane ) is closely apposed to the cc , forming a groove - like structure ( fig .
supramolecular complexes , which are made up of scaffolding and/or adhesion molecules ( e.g. , usherin , vlgr1 , whirlin , sans ) , are enriched in the specialized ( sub)plasmalemma of the lateral is ridge and/or cc ( fig .
it has been proposed that the long ectodomains of the adhesion molecules , usherin and vlgr1 , tether and form fibrous structures between the apposing membranes of these subdomains .
these fibrous links are analogous to the ankle links between adjacent stereocilia in developing hair cells .
consistent with this idea , mutations in each of these proteins lead to usher syndrome , a human deaf - blind disease ( kremer et al .
rod differentiation in rodents starts postnatally , and it takes 23 wk for the os to fully mature ( lavail , 1973 ) .
os morphogenesis among the rods is not completely synchronized throughout development until near the end .
it begins with the extension of a primitive cilium from the basal body anchored on the plasma membrane ( fig .
3 a ) . this rudimentary cilium has the morphological appearance of a typical primary cilium .
the apical end of a primitive cilium gradually becomes swollen and filled with a variety of membranous vesicles , tubules , and sacs . at postnatal days 810 , disc - like membranous
cisternae begin to fill the developing os ; however , these discs are often excessively long and highly disorganized ( fig .
3 b ; de robertis , 1956 , 1960 ; besharse et al . , 1985 ) .
many of them are aligned parallel to or at an oblique angle relative to the ciliary stalk .
the next phase of os differentiation involves a major remodeling that reorganizes the discs to align perpendicular to the ciliary stalk , making them stackable .
the terminal phase of rod differentiation is the elongation of the os ; the lengths of oss containing orderly disc stacks increase at an almost linear rate during this period until the oss reach their mature size ( lavail , 1973 ) .
( a ) a drawing depicting the transformation of developing rods during their os morphogenesis .
many discs are longer than the matured discs ; running in parallel to the ciliary stalk ( unpublished data ) .
( c ) a rod from a postnatal day 15 mouse lacking rpgrip1 ( rpgrip1 ) containing vertically oriented discs is shown .
somewhat unexpectedly , rhodopsin has an indispensable role in disc formation in addition to its well - known role in signal transduction .
rods of rhodopsin knockout mice form primitive cilia without disc - containing oss ( jansen et al . , 1987 ;
mice lacking one allele of rhodopsin develop oss that adopt a relatively normal appearance , but have lower levels of rhodopsin .
furthermore , transgenic mice that overexpress rhodopsin develop oss with increased diameters ( wen et al . , 2009 ) .
peripherin-2 ( also called peripherin / rds ) is a photoreceptor - specific tetraspan membrane protein confined to the rim region of the rod discs , which has a characteristic hairpin loop appearance ( fig .
2 , c and d ; molday et al . , 1987 , 2004 ) . like rhodopsin
null mice , peripherin-2 null ( or rds / rds ) mice form a bare cilium without os ( jansen et al . , 1987 ; connell et al . ,
microsome reconstitution studies suggested that peripherin-2 has an intrinsic ability to initiate the hairpin loop structure of the disc rim by simply forming disulphide - linked homodimers or oligomers ( wrigley et al .
peripherin-2 is likely to play an additional role in disc size control and/or stabilization of disc organization in adult os .
mice with one allele of peripherin-2 form oss characterized by overgrown disc membranes and a shape that resembles whorl profiles ( hawkins et al . , 1985 ) .
rom1 , which is also localized on the disc rim , is a homologue of peripherin-2 .
rom1 and peripherin-2 form a mixture of homo- and heterotetrameric core complexes ( loewen and molday , 2000 ) .
mice null for rom1 develop oss with globally normal appearances but larger basal discs ( clarke et al . , 2000 ) .
this indicates that rom1 has a modulating role in size control of newly synthesized discs .
the rom1 gene appears late during evolution and it has only been found in mammals . coincidentally , mammalian rods have a much smaller diameter ( 7-fold ) than lower vertebrate rods do .
monogenic rom1 disease is rare ; rom1 mutations in humans often have a digenic inheritance associated with either peripherin-2 or rhodopsin mutations ( kajiwara et al . , 1994 ) .
although disc genesis mediated by rhodopsin and peripherin-2 is a cellular event specific to the rod , initial ciliogenesis of the rod is most likely characteristic of other cilia as well .
it has been well established that intraflagellar transport ( ift ) is essential for the assembly of cilia in all organisms tested ( rosenbaum and witman , 2002 ) .
ift is a bidirectional movement of multi - protein ift particles along the microtubules of ciliary axonemes ; ift cargoes are either transported on the ciliary membrane or in a narrow subplasmalemmal space between the ciliary membrane and axonemal microtubule doublets .
anterograde and retrograde ift transport are powered by kinesin - ii and dynein 2 ( dhc1b/2 ) , respectively .
the zebrafish ovl mutant , which has ift88 ( a key ift component ) disrupted , does not form a primitive cilium in postmitotic rods ( tsujikawa and malicki , 2004 ) .
consistently , the oss of mouse rods with a hypomorphic ift88 mutation or a null kif3a ( a member of heterotrimeric kinesin - ii family ) allele are poorly differentiated and undergo degeneration ( marszalek et al . , 2000 ; pazour et al . ,
furthermore , kif17 ( a member of homodimeric kinesin - ii family ) is critical for rod os development in zebrafish ( insinna et al . , 2008 ) .
kif17 is homologous to caenorhabditis elegans osm-3 ; osm-3 is required for the elongation of singlet microtubules in the distal segment of the amphid channel sensory cilium ( evans et al . , 2006 ) .
two additional macromolecular complexes ( i.e. , nphp complex , bbsome ) associated with the ift complex are also important for early os development .
mutations of each component in either of these complexes are linked to syndromic retinitis pigmentosa diseases , now collectively referred to as ciliopathies .
retinal degeneration is a hallmark of many of the ciliopathies ( e.g. , senior locken syndrome , jourbet syndrome , bardet - biedl syndrome , alstrm syndrome ; katsanis , 2004 ; hildebrandt and zhou , 2007 ) .
affected patients also exhibit several other overlapping phenotypes such as cystic kidney , mental retardation , and polydactyly .
given the fact that a wide range of cell types are affected in patients with ciliopathies , the ift macromolecular complex clearly participates in a general function of cilia ( versus photoreceptor - specific function ) , such as the assembly and/or maintenance .
consistent with this idea , nphp1 knockout mice have delocalized ift88 and severely disrupted os development
mice mutant or null for the bbsome component , such as bbs1m390r knockin mice or bbs2 knockout mice , also exhibit poorly developed oss ( nishimura et al . , 2004 ; davis et al . , 2007 ) .
however , it was found that the oss in mice null for bbs4 developed normally , despite rhodopsin being mislocalized ( abd - el - barr et al . , 2007 ) .
this raises the interesting possibility that some of the bbsome subunits might have ift - independent roles . on the other hand , several additional animal models mimicking 
non - syndromic  retinitis pigmentosa ( e.g. , mice with null or mutant alleles of rpgrip1 , rp1 , prominin 1 , or protocadherin 21 ) have developed elongated oss , but their discs are vertically or randomly oriented ( rattner et al . , 2001 ; zhao et al . , 2003 ; yang et al . , 2008 ; zacchigna et al . , 2009 ) .
3 b ) , suggesting that these mutant rods are arrested at the developmental stage when their discs are still disorganized .
this further suggests that each of these molecules plays an important role in orienting the discs for stacking . among them , rpgrip1 is a structural component of the fibrous links between axonemal microtubules and the ciliary membrane ( zhao et al . , 2003 ) .
rp1 is a photoreceptor - specific microtubule - associated protein with particular enrichment in the os portion of the ciliary axoneme ( gao et al . , 2002 ;
rp1 can stabilize microtubules in vitro ; rp1 mutant rods have shorter axonemal microtubules with a kinked appearance .
thus , we predict that a timely distal elongation of the os axoneme , likely involving rpgrip1 and/or rp1 , is critical to
 slant  all the disorientated discs to one side of the os and/or anchor discs to the axonemal microtubules . finally , prominin 1 is a tetraspan membrane protein that binds to cholesterol , a specialized lipid that is concentrated on the apical - most is plasmalemma and calycal process ( fig
. 2 b , green shade ; andrews and cohen , 1983 ; weigmann et al . ,
prominin 1 binds to protocadherin 21 and influences protocadherin 21 s distribution on the proximal basolateral os plasmalemma ( yang et al . , 2008 ) .
these findings indicate that the specialized surface domains in the is  os junction participate in the sculpting and/or maintenance of the topographic properties of the basal os , a step that may be important for the discs synthesized late in development to lay and stack .
photoreceptors are highly susceptible to environmental insult because they constantly face a high influx of photons and free radicals .
os disc stacking makes a scheduled daily repair feasible ; new discs are assembled at the base of the os , while aged discs at the tip are eliminated by neighboring rpe cells .
complete renewal of the os takes 10 d in higher vertebrates and 69 wk for lower vertebrates . this cardinal feature of rod cells
was first demonstrated by young in the 1960s using pulse - labeling and autoradiography ( young , 1967 ) . in these experiments
, newly synthesized radioactive proteins appear as a band at the base of the os .
the band is progressively displaced toward the tip of the os , eventually disappears from the os , and appears in the rpe .
the distal disposition of the os is not constant , but follows a cyclic rhythm with a burst of activity peaking 2 h after light onset at dawn in all vertebrates studied ( lavail , 1976 ) .
whether the rhythmic rpe phagocytosis is regulated by light or circadian rhythm is controversial , and may differ among species ( lavail , 1976 , 1980 ; besharse et al . , 1977 ; besharse and hollyfield , 1979 ; sung and tai , 2000 ) .
it is now clear that rpe cells are professional phagocytes , among other duties ; each rodent rpe cell engulfs one tenth of the distal os discs of 30 rods every day ( fig .
these cells serve as scavengers that degrade the protein / lipid components of ingested os fragments
. considerable insight concerning the molecular machinery that underlies rpe phagocytosis has developed over the years .
of particular interest , mutations in mer tyrosine kinase , a critical component for the phagocytic activity of rpe , are linked to retinitis pigmentosa in humans ( gal et al . , 2000 ) .
this underscores the importance of timely os renewal for the health and survival of photoreceptors .
based on a reconstruction of a series of high resolution electron micrographs , young proposed that the endmost os discs are actively shed in packets preceding rpe engulfment ( young and droz , 1968 ) .
however , williams and fisher , who conducted studies on frogs , demonstrated that there are no shed discs in the subretinal space , and that shedding does not take place in the absence of rpe ( williams and fisher , 1987 ) .
thus , they concluded that disc shedding and rpe phagocytosis should be considered as concomitant and inseparable events , at least under normal physiological conditions .
to balance the apical loss , new discs and plasma membranes are assembled at the base of the os by incorporating proteins and lipids delivered from the is .
an estimated 77 cm of disc membrane must be synthesized each day per rat retina ; this surface area is equivalent to over 50 times the outer surface area of the eyeball ( mayhew and astle , 1997 ) .
how do rods achieve this extraordinary magnitude of membrane genesis , and how do they target the newly synthesized proteins to their respective sites with high fidelity ?
these have been challenging questions to study because the disc renewal process is difficult to replicate in vitro .
rods dissociated from the retina undergo drastic membrane remodeling , intersegmental fusion , and polarity loss ( townes - anderson , 1995 ) , making such investigations impractical . until recently ,
the dominant hypothesis for basal disc renewal has been a model proposed by steinberg et al .
( 1980 ) . based on an electron microscopic observation in which a few outfolded discs ( referred to as  open discs  ) were found at the basal os
, these authors proposed that discs are formed by evagination of the basal os plasmalemma , which is subsequently pinched off by a bilateral fusion .
although widely cited , no experimental data directly supports this model thus far . a conjectural piece of evidence in favor of the evagination model was an observation made by matsumoto and besharse ( 1985 ) . in studies of frogs , they found that a band of incorporated dye appears at the basal os of the rod shortly after a subretinal injection of lucifer yellow , and the resulting dye - containing discs migrate distally over time .
this result was interpreted to mean that the extracellular dye is incorporated into open discs during their sealing .
however , it is noted that no similar observation was found in any other mammals ( laties et al . , 1976 )
. rods of lower vertebrates ( e.g. , frog , zebrafish ) are a popular model system for photoreceptor studies because these cells are large and the procedure to generate transgenic animals is relatively easy .
however , comparative studies of protein localization , reporter targeting assay , and os renewal in amphibians and mammals have resulted in inconsistent conclusions ( sung and tai , 2000 ; ding and naash , 2006 ; chuang et al .
, 2007 ; baker et al . , 2008 ; kwok et al . , 2008 )
, indicating that the morphological differences between these dissimilar species might play an important role in their divergent strategies to adapt to their respective cellular physiologies and habitats .
a recently proposed vesicular targeting model suggests that basal discs are assembled by the repeated fusion of rhodopsin vesicles ( chuang et al . , 2007 ) .
in this model , rhodopsin vesicles at the basal os axonemes carry smad anchor for receptor activation ( sara ) and syntaxin 3 , and sara binds directly to both the c terminus of rhodopsin and syntaxin 3 ( fig .
4 c).using an in vivo retina transfection method , these authors showed that perturbation of the function or the expression of sara ( or syntaxin 3 ) in rods rendered targeting inefficient and led to aberrant accumulation of vesicles at the base of the os , the axonemal shaft , and the cc ( fig . 4 b ) .
these results suggest the interaction between the fyve domain of sara ( on axonemal vesicles ) and phosphatidylinositol 3-phosphate ( pi3p ; on basal nascent discs ) primes the axonemal vesicles to discs for snare - mediated fusion ( fig . 4 c ) .
these findings also provide convincing pieces of evidence that the precursors of the discs are membranous vesicles .
vesicular targeting model explains how new discs are assembled at the base of the os .
in contrast , sara ( green ) is specifically concentrated at the base of the os and especially enriched on the vesicles residing in the basal 3-m axonemal spaces .
( b ) electron micrograph reveals the aberrant vesicle accumulation at the basal os and at the cc shaft as a result of disruption of sara function ( arrows ) .
( c ) new discs at the base of the os are assembled and  grow  via sara- , pi3p- , and snare - mediated vesicular trafficking and membrane fusion .
the fyve domain of sara tethers axonemal vesicles to nascent discs through its high - affinity interaction with pi3p located on the immature discs .
the close proximity of these membranes then permits the snare - mediated fusion event to happen .
the vesicular targeting model suggests that the disc incorporation of rhodopsin and the formation of nascent discs occur concomitantly .
4 a ) , syntaxin3 , and pi3p all display a gradient of expression along the proximal 38-m os , with the strongest signal toward the base . spatial restriction of these proteins could function as a programmed timer that determines both the proper size and the number of new discs to be generated per day .
( 2007 ) also showed that the basal os plasma membrane enwraps the entire set of disc stacks , with no  open discs 
a similar conclusion can be drawn by using cryofixation techniques , namely , rapid freezing or high pressure freezing combined with freeze substitution ( obata and usukura , 1992 ) .
thus , the open disc  like structures are conceivable artifacts due to the histological damage of the basal os plasmalemma .
rhodopsin has an autonomous os targeting signal located within its cytoplasmic c - terminal tail .
this was first shown by the observation that the transgenically expressed retinitis pigmentosa rhodopsin mutant , which lacks the last five residues of the protein , loses its polarized os distribution ( sung et al . , 1994 ) .
the c - terminal region of rhodopsin is highly conserved among species , and is a hot spot for human retinitis pigmentosa mutations ( sung and tai , 2000 ) .
the sufficiency of rhodopsin s c terminus for os targeting was later demonstrated in transgenic frogs and zebrafish using a reporter assay ( tam et al . , 2000 ;
the manner in which rhodopsin s c terminus confers its vectorial transport and polarized os targeting has been delineated by identifying proteins that interact with this region of rhodopsin .
in addition to the aforementioned sara , which is critical for the final docking / fusion of rhodopsin onto discs , two additional molecules , arf4 ( adp ribosylation factor 4 ) and tctex-1 ( or dynlt1 ) , also interact with this region of rhodopsin ( tai et al . , 1999 ; mazelova et al . ,
the bar domain of asap1 facilitates membrane curving and , hence , budding from the trans - golgi network ( mazelova et al . , 2009 ) .
the post - golgi rhodopsin vesicle is then transported on microtubules based on cytoplasmic dynein activity ( fig .
tctex-1 , which is a cytoplasmic dynein light chain , serves as a cargo adaptor linking the rhodopsin vesicle with the dynein motor ( tai et al . , 1999 ) .
the motor - driven vesicular translocation is especially important because the distance between the trans - golgi network and the distal is is quite far ; rods with cytoplasmic dynein ( dhc1 ) suppressed had aberrant post - golgi vesicle accumulation in the is ( insinna et al . , 2010 ) .
several c - terminal retinitis pigmentosa mutant rhodopsins display a reduced affinity for tctex-1 , which may explain their mislocalization in rods ( tai et al . , 1999 ) .
. both sets of microtubules have their minus ends anchored at the basal bodies ( troutt and burnside , 1988 ) .
cytoplasmic dynein 1 transports post - golgi rhodopsin over long distances to reach the apical is region .
at least two current working models have been proposed to explain how rhodopsin is transported through the cc and how this process is coupled with rhodopsin s disc incorporation .
panel a depicts a model in which four or more distinctive cellular processes are involved ( sedmak and wolfrum , 2010 ) .
step 1 : rhodopsin is fused with the apical is plasmalemma , then crosses a putative structure barrier and reaches the ciliary membrane .
step 2 : rhodopsin is moved on the ciliary membrane using kinesin ii  powered ift .
panel b depicts a model in which rhodopsin vesicles , which are generated in the is , are delivered to the basal os axoneme through channeling via the ciliary axonemal shaft . these vesicles then undergo fusion to form nascent discs . in this scenario ,
post - golgi rhodopsin vesicles may traverse the endocytic compartments to recruit sara and other elements for subsequent translocation and/or fusion .
besides rhodopsin , photoreceptor retinol dehydrogenase also uses a ( v / i)xpx motif for its os targeting ( luo et al . , 2004 ) .
no other major os proteins are known to contain a similar motif , suggesting that a variety of os targeting signals and/or strategies must be operating in rods .
for example , the piggyback / hitch - hiking model is likely to be a common strategy for os trafficking by other molecules simply interacting with rhodopsin or rhodopsin - bearing vesicles , which are abundant in rods ( concepcion et al . , 2002 ) .
protein ( or protein  lipid ) interactions . as an example , photoactivated rhodopsin snatches arrestin from its is localization upon illumination ( strissel et al . , 2004 ) .
it was also shown that the os plasmalemma location of the cgmp - gated channel relies on its direct binding with a membrane skeletal protein ankryin - g ( kizhatil et al . , 2009 ) .
it is plausible that many of these mechanisms occur redundantly and/or successively to ensure the fidelity of the targeting of these molecules to their respective os membrane domains .
finally , a strategy completely independent of rhodopsin s targeting must be used for the disc rim targeting of peripherin-2 because no mislocalized peripherin-2 has been found in any mouse model in which rhodopsin is mislocalized ( hagstrom et al .
, 2001 ; zhao et al . , 2003 ; abd - el - barr et al . , 2007
it was suggested that a  checkpoint  system selectively permits peripherin-2 prepacked as homotetramers or even higher oligomers , but not dimers , into the os ( loewen et al . , 2003 ) .
the ability of peripherin-2 to further oligomerize with rom-1 and to interact with the cgmp - gated channel adjacent to the os may further promote and/or stabilize its disc rim distribution ( fig .
one of the most pressing questions pertinent to rhodopsin trafficking is how rhodopsin passes through the bottleneck - shaped cc .
this might be a consequence of several technical limitations , such as the high concentration of rhodopsin in the os or poor penetration of reagents into the cc , among other possibilities ( besharse and horst , 1990 ; liu et al .
the presence of rhodopsin on the ciliary membrane has , however , been detected when a heavy - etching technique and a mixture of anti - rhodopsin antibodies were used in combination ( wolfrum and schmitt , 2000 ) .
a study of shaker1 mice , in which the myosin viia gene is disrupted , found an increased rhodopsin signal on the cc membrane ( liu et al .
the importance and direct nature of myosin viia on the cc s transport of rhodopsin remain unclear because rhodopsin was not detectably mislocalized in the is and other parts of the rods .
furthermore , cc is largely devoid of microfilaments , although g - actin immunoreactivity is detected ( wolfrum and schmitt , 2000 ) . instead
, the actin meshwork of rods is prominent at both the proximal and distal ends of the cc ( sung and tai , 2000 ) .
many reviews have suggested that rhodopsin at the apical is plasmalemma is able to laterally diffuse and then traverse through the ciliary membrane by using kinesin - motored ift anterograde transport ( fig . 5 a ) .
this proposition is analogous to a working model for the transmembrane protein that travels on primary cilia ( rosenbaum and witman , 2002 ) .
rab8 gtpase has a proposed role in docking / fusing cargoes near the base of primary cilia to replenish membrane materials for cilia elongation ( nachury et al .
, 2007 ; yoshimura et al . , 2007 ) . in agreement with this notion ,
transgenic frog rods that overexpress dominant - negative rab8 have aberrant vesicular accumulation at the base of the cc ( moritz et al . , 2001 ) .
paradoxically , rhodopsin can not be efficiently targeted to the primary cilia in epithelial cultures ( chuang and sung , 1998 ) .
rhodopsin is actually largely absent from the base of the primary cilium , a specialized lipid zone that prevents free diffusion between the apical surface and cilium ( vieira et al . , 2005 ) .
to the best of our knowledge , rhodopsin has not been shown to be a cargo of ift .
in addition , there is currently no evidence supporting the idea that rhodopsin is transported through ift . the improper rhodopsin distribution seen in the abnormally differentiated and rapidly degenerated hypomorphic ift88 and null kif3a rods
should not be used to conclude ift s role in rhodopsin transport ( marszalek et al . , 2000 ;
this is because rhodopsin is not completely polarized in the os until pn12 , and rhodopsin fails to integrate efficiently once the os is shortened and/or degenerated .
a recent study using a different strain of rod - specific kif3a knockout that has a slower degeneration rate demonstrated that rhodopsin s os targeting is kif3a independent ( avasthi et al . , 2009 ) .
the same study also showed that kif3a is not required for os plasmalemma localization of the cgmp - gated channel .
thus , the nature of the membrane cargo(es ) transported by ift in photoreceptors remains to be identified . in this regard
, a biochemical study showed that retinal guanylyl cyclase binds ( indirectly ) to ift88 , making it a potential ift cargo ( bhowmick et al . , 2009 ) . assuming that rhodopsin is transported on the ciliary membrane
in addition to the evagination model discussed before , the pinching off of internalized ciliary membrane has also been proposed ( fig .
5 a ; obata and usukura , 1992 ; usukura and obata , 1995 ) .
evidence for the latter is limited to studying fluid phase marker uptake performed in dissociated immature rods .
an attractive alternative model is that rhodopsin vesicles are shipped directly from the is through the axonemal shaft ( fig .
the sarasyntaxin3 axonemal vesicles then undergo fusion at the base of the os to produce new discs , as suggested by the vesicular targeting model ( fig . 4 c ) . this model is appealing because rods may have evolved from other ciliated cells to develop a means to effectively move cargo in bulk and quickly generate disc membranes .
no ift retrograde transport is needed to return the os transmembrane proteins back to the cell body for degradation .
however , three major signaling components ( i.e. , arrestin , transducin , and recoverin ) , which are all peripheral membrane proteins , shuttle back and forth between the is and os in a light - dependent manner .
this is one remarkable mechanism that is unique to photoreceptors , and is used for light adaptation ( strissel et al . , 2004 ) .
a parallel light - dependent translocation is also conserved in fly photoreceptors , in which the light - sensing organelles are actin - based microvilli called rhabdomeres .
light - dependent protein transport in photoreceptors has been comprehensively reviewed elsewhere ( strissel et al . , 2004 ) .
compared with other types of polarized cells , the high spatial and temporal resolution of photoreceptors has much to offer for those who are interested in cell polarity , vesicular trafficking , and organelle biogenesis . many discoveries and new working models described in this review provide a starting point for advancement .
there is much more to be learned about protein transport and turnover in cone photoreceptors , which are also rich in membranes .
the cell biology study of other neurons in the visual system is still in its infancy .
for example , 20 and 30 types of ganglion cells and amacrine cells , respectively , have been identified based on their different shapes , molecular , and electrophysiological properties ( for review see masland , 2004 ) .
each of these individual cell types appears to be specialized in distinct functions that are not completely understood .
how are the processes of neurons that are involved in visual perception wired in such a complex yet remarkably precise manner ? how do retinal neurons undergo morphological adaptation in response to environmental cues ( e.g. , light , circadian ) , and what are their functional implications ( burnside , 1976 ; vollrath and spiwoks - becker , 1996 ; spiwoks - becker et al . , 2004 ) ?
the recent completion of the inventory of all retinal neuron types ( wssle , 2004 ) and our increasing capability to target , manipulate , and visualize specific cell types in the visual system provide great promise for our deeper understanding of the cell biology of our vision system .
 humans possess the remarkable ability to perceive color , shape , and motion , and to differentiate between light intensities varied by over nine orders of magnitude . 
 phototransduction  
 the process in which absorbed photons are converted into electrical responses  is the first stage of visual processing , and occurs in the outer segment , the light - sensing organelle of the photoreceptor cell . 
 studies of genes linked to human inherited blindness have been crucial to understanding the biogenesis of the outer segment and membrane - trafficking of photoreceptors . 
Looking at the big picture: the retina is a neural circuit composed of different cell types
The vertebrate rod: elegance and efficiency
Seeing is believing: OS morphogenesis in developing rods
Forever young: scheduled daily disposal of aged discs
Building from the bottom up: basal disc assembly
Getting to the top: targeting of rhodopsin and other proteins to the OS
Crossing the narrow bridge: ciliary transport of rhodopsin
Our vision: concluding remarks