the utriculo - endolymphatic valve at the anteromedial wall of the utricle , was discovered by bast in 1928 in human fetuses .
the utricular duct connects the utricle with the saccular space , close to the entrance of the endolymphatic duct .
bast   suggested that the function of the valve was closing of the utricular end of the utricular duct .
bast himself proposed rotation of the valve lip at its base , where it is made - up of loose periotic tissue , as the functional mechanism . with rising intra - utricular fluid pressure
a second theory proposes the opposing utricular wall as the functional part of the valve : bending of the single cell layer of the highly compliant utricular wall opposite the lip is responsible for the opening or closure of the valve [ 35 ] . in the early decades of the twentieth century
bast   reported two cases of ruptured saccules in human ears , resulting in a closed valve and an expanded utricle .
bast and eyster   withdrew endolymph from the cochlear duct of the guinea pig , causing a collapsed saccule and cochlear duct , while the utricle was distended and the valve closed .
konishi   found open valves in the guinea pig at various stages of endolymphatic hydrops , after operatively obstructing the endolymphatic duct .
although there is no conclusive evidence up to now , bast s valve seems to be capable of protecting the utricle and the semicircular canal system when the evolutionary younger , cochlear part of the system ruptures . in the evolutionary chain of land living vertebrates , amphibians and reptiles have hearing organs that are small specialized regions of their vestibular systems . in birds , with a cochlear duct , a clear distinction can be made between an inferior and a superior part of the labyrinth .
if the function of bast s valve in mammals is a protective one , it is expected to find a similar structure in birds between the two parts of the labyrinth .
retzius   made highly detailed drawings of the labyrinth of the pigeon and depicted the connection between the utricle and the saccule as a simple hole in a membrane .
no utricular duct , nor a valve - like structure , is visible in fig . 
1drawing of the junction between the utricle and saccule seen from the utricular side by retzius ( 1881 ) ( arrow the hole between the utricle and the saccule ) drawing of the junction between the utricle and saccule seen from the utricular side by retzius ( 1881 ) ( arrow the hole between the utricle and the saccule ) the drawing of satoh   of a light microscopic slide shows a somewhat more detailed view of the same region in the pigeon . but also in this drawing little evidence for a valve - like structure between utricle and saccule can be found ( fig . 
2).fig .  2drawing of a light microscopical slide of the region between the utricle and saccule in pigeon by satoh ( 1917 ) ( s saccule , sp posterior part of utricle , tv tegmentum vasculosum , dsu utriculo - saccular duct ) drawing of a light microscopical slide of the region between the utricle and saccule in pigeon by satoh ( 1917 ) ( s saccule , sp posterior part of utricle , tv tegmentum vasculosum , dsu utriculo - saccular duct ) as part of a larger project to reconstruct the entire pigeon s inner ear in three dimensions , we investigated the transition region between utricle and saccule in detail using light microscopy .
our objective was to obtain detailed information on a structure which resembles bast s valve to some extent .
this structure consists of a duct between utricle and saccule , with an overhanging ridge and a slit - like opening on the utricular side of the duct .
detailed light microscopy results and 3d - reconstructions from the light microscopy slides of the structure and its location are presented .
three healthy pigeons ( columbo domestica ) , with a weight of 400500  g , were used . animal care and use were approved by the experimental animal committee of groningen university , protocol no .
2883 , in accordance with the principles of the declaration of helsinki general anesthesia was induced by intraperitoneal administration of pentobarbital ( 6% , overdose , hospital farmacist ) . after decapitation ,
all inner ears were fixated in an 8% neutral buffered formalin solution ( ph 7.34 ) for at least 24  h. then the specimens were rinsed in aqua - dest .
decalcification in ethylenediaminetetraacetic acid 10% solution ( edta ; sigma , ed5ss , ph 7.4 ) took place at a temperature of 50c in a microwave oven ( t / t mega microwave histoprocessor , milestone ) in eight sessions of 6  h. after decalcification , the specimens were again rinsed with aqua - dest and dehydrated in a graded seven - step ethanol series ( 30 , 50 , 70 , 90 , 96 , 100 , 100% ) .
the specimens were embedded in hpma ( hydroxypropyl methacrylate , polysciences inc . ) with addition of a catalyst ( n , n - dimethlaniline , peg 400 ; 15:1 , fluka chemie ag ) . as fiducial markers for 3d - reconstruction five holes ( diameter 0.5  mm ) were drilled in the hpma - block , perpendicular to the surface
. sections of 4  m were cut and each sixth section was stained with toluidine blue and contrast - stained with basic fuchsine to facilitate reconstruction .
the stored 2d images were processed with an imod ( http://bio3d.colorado.edu/imod ) software package for 3d - reconstruction .
the input of relevant contours in each 2d - image was manually performed with a writing tablet ( wacom cintiq 15x ) .
stacking of the lm slides was executed with midas software for manual alignment and adjustment ( midas is part of imod ) , using the drilled fiducial markers as references .
corrections to be made were at most a few percent of the 2d - image cross section .
if after 3d - reconstruction is larger irregularities were observed , the midas procedure was repeated for the relevant subsequent 2d - slides .
figure  3a  d shows subsequent ( 24  m distance ) light - microscopy images of a duct connecting the utricular and the saccular spaces .
the images show the shape of the entrance of the utricular duct at the uticular side .
3subsequent light - microscopy sections of the entrance of the utricular duct ( s saccule , us utricular space , u utricle ) .
4 subsequent light - microscopy sections of the entrance of the utricular duct ( s saccule , us utricular space , u utricle ) .
the structures inside the rectangular box are shown in more detail in fig .  4 the valve - like structure inside the box in fig . 
( dense ; non - compliant ) in the valve - lip surrounded by a single cell - layer and the opposing single cell - layer wall of cylindrical epithelia ( hypodense ; compliant ) , separating the perilymphatic space from the endolymphatic space in this region , can be distinguished .
furthermore , an accumulation of toluidine dyed endolymph at the entrance of the duct can be seen.fig .  4detail of fig . 
3a showing a valve - like structure consisting of a ridge overhanging the entrance of the duct ( single asterisk compliant cylindrical epithelia tegmentum vasculosum , double asterisks overhanging ridge consisting of rigid non - compliant connective tissue ) detail of fig . 
3a showing a valve - like structure consisting of a ridge overhanging the entrance of the duct ( single asterisk compliant cylindrical epithelia tegmentum vasculosum , double asterisks overhanging ridge consisting of rigid non - compliant connective tissue ) figures  5 and 6 show the slit - shaped opening of the utricular duct at the utricular side .
the duct is drawn open on both sides to give a good impression of the flat cylindrical shape of the ductfig . 
63d - reconstruction of the entrance ( slit - like ) of the utricular duct as seen from inside the utricule .
the dashed white line follows the edge of the valve - lip overhanging the slit entrance 3d - reconstruction the utricular duct .
the duct is drawn open on both sides to give a good impression of the flat cylindrical shape of the duct 3d - reconstruction of the entrance ( slit - like ) of the utricular duct as seen from inside the utricule .
the dashed white line follows the edge of the valve - lip overhanging the slit entrance figure  6 shows a raw 3d - reconstruction of the entrance of the utricular duct as seen from inside the utricle .
as far as we know this is the first description of the utricular duct and a utriculo - endolymphatic valve - like structure in the bird .
3b , the structure appears more like a hole than a duct and no clear valve - like structure is visible .
7 . because we had only 2d - slides with in - between distances of 24  m available the 3d - reconstructions in figs . 
5 and 6 are not very detailed , but they give a fairly good impression of the shape of the entrance of the utricular duct at the utricular side .
it is filled with rigid , non - compliant connective tissue , as can be seen in fig .  4 .
this rigid structure , in combination with the more compliant cell - layer opposite to it , could function as a valve , like it is postulated to do in mammals.fig . 
7schematic drawing by one of the authors ( hpw ) of the left whole labyrinth of the pigeon .
the arrow marked with an asterix points toward the utricular duct and the region of the valve .
bp basilar papilla , ca crista in anterior ampulla , cl crista in lateral ampulla , cp crista in posterior ampulla , es endolymphatic sac , ml macula of the lagena , mn macula neglecta , ms saccular macula , mu utricular macula schematic drawing by one of the authors ( hpw ) of the left whole labyrinth of the pigeon .
the arrow marked with an asterix points toward the utricular duct and the region of the valve .
bp basilar papilla , ca crista in anterior ampulla , cl crista in lateral ampulla , cp crista in posterior ampulla , es endolymphatic sac , ml macula of the lagena , mn macula neglecta , ms saccular macula , mu utricular macula the slit - shaped valve entrance at the urticular side ( fig . 
6 ) resembles this entrance in the guinea pig , as shown in fig . 
bast   also referred to this opening as slit - like in the guinea pig.fig . 
83d - reconstruction of the entrance ( utriculo - endolymphatic duct / bast s valve ) of the utricular duct in guinea pig as seen from inside the utricule ( hofman et al .
2008 ) 3d - reconstruction of the entrance ( utriculo - endolymphatic duct / bast s valve ) of the utricular duct in guinea pig as seen from inside the utricule ( hofman et al .
2008 ) shape and diameter of the utricular duct are different in the guinea pig and the pigeon . in the guinea pig
the shape of the duct is that of a flat funnel with a diameter in the order of 10  m at its narrowest passage . in this way
, the shape of the duct is that of a tube with an elliptical cross section with a largest width of about 200  m along its whole course .
a structure comparable to the utriculo - endolymphatic ( bast s ) valve in mammals can be found in the pigeon . like in mammals
its function is possibly the protection of superior part of the labyrinth against endolymph loss caused by a rupture of the membrane surrounding the inferior part .
 the first description of the presence of a utriculo - endolymphatic valve in human fetuses was given by bast in 1928 . since then this valve - like structure 
 is called bast s valve . 
 its exact function has not yet been established . 
 the general opinion is that it has a protective function by having the possibility to separate the superior endolymphatic compartments of the labyrinth from the inferior compartment . 
 phylogenetically seen birds are the first vertebrates with a cochlear duct and a distinct inferior and superior part of the labyrinth . 
 a structure in the pigeon inner ear , resembling bast s valve in mammals , is described . 
Introduction
Materials and methods
Results
Discussion
Conclusion