View source for Anatomical and physiological studies on the growth of the inner ear of the albino rat 2 (1923)

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=Anatomical and Physiological Studies on the Growth of the Inner Ear of the Albino Rat=

==II. Correlation Between the Inception of Hearing and the Growth of the Cochlea==

The present study aims merely to compare in the rat the 
size of each element of the cochlea just before and just after 
the appearance of hearing and to ascertain the changes in the 
cochlea which take place during this phase. The rats which 
have sense of hearing show the so-called ' Ohrmuschelreflex ' of Preyer and other responses to auditory tests. Both the guinea- 
pig and rat react most evidently to sounds. The former animal 
responds usually five to six hours after birth. In the rat, however, 
the development of the function is, as already stated, relatively 
retarded and usually first appears at about ten days of age. 

To test the presence of hearing there are several methods, 
as for example, Preyer V Ohrmuschelreflex' and the reflex 
closure of the eyelid, and these I used in making my observations. 
As the source of the sound I selected the hand clap, a whistle 
(Triller-pfeife, about c 4 ) and a low sound made by drawing in 
the breadth with nearly closed lips (about c 1 ). 

Since it is my purpose merely to determine the first appear- 
ance of a response to sound, it is not nescessary to carry out such 
refined examinations as did Hunter ('14, '15, '18) Watson 
('07), and others on white rats, and Marx ('09) on the guinea- 
pig, nor was it necessary to use the tuning-fork which by the 
way is not so good for tests on animals as for those on man. 
Since sometimes we may have a defect of hearing in animals, 
as shown by many investigators, it was deemed necessary to 
use several sources of sound and to take care not to produce 
ah* currents striking the test animal or to touch it in any way. 
For this purpose a large sheet of glass was placed between the 
source of sound and the rats to be tested. While the rat was 
resting I suddenly produced the sound and noted whether the 
rat responded. When the animals already had their eyes open 
the test was made from behind to avoid visual reflexes. 

==Observations==

Rats at birth show no response to auditory stimuli. Most 
of them respond at twelve days of age very clearly, sometimes 
at ten to eleven days Under certain circumstances, the time 
of the reflex can be rather accurately noted. For example, 
while in the morning at ten days no reflex was noted it was 
present in the evening of the same day. Fortunately, I obtained 
five nine-day-old rats belonging to one litter and in nearly the 
same condition of nourishment and developnent. One of these responded to the test very evidently at noon on the ninth day, 
but the others did not. The sound which was effective was 
fairly intense, but to a faint and low-pitched sound this rat 
did not respond. In this case the external auditory canal was 
open. In the others there was in some a small open canal, 
but more or less closed by a cellular plug. In the latter cases 
I removed this obstruction without much difficulty or damage 
by washing, yet no reaction could be obtained to the stimuli. 
As it was to be expected, that also in the latter the reflexes would 
very shortly appear, all the cochleas of these young rats, both 
the not-hearing and hearing, were fixed by the method pre- 
viously described and later examined. 

In chapter 1, in which I followed the growth changes in the 
constituents of the membranous cochlea and in its ganglion 
cells from birth to maturity, including ' not hearing ' and ' hearing ' 
rats, very evident differences were observed in rats between 
nine and twelve days of age. In view of this, it will be of in- 
terest to compare the measurements obtained from the 'not 
hearing' and 'hearing' rats nine days old and members of the 
same litter. From the differences thus obtained we can conclude 
concerning the developmental changes in the cochlea requisite 
for the first appearance of hearing, provided there are no obstacles 
in the sound-conducting apparatus or deficiencies in the central 
organ. 

In table 113 are given the values for the size of the several 
constituents of the cochlea 'from a rat which did not hear and 
one which did, at nine days. The former data are the averages 
from four cochleas, while the latter are from two. The table 
shows in a striking way that where there is a significant differ- 
ence. The values obtained from the rat which could hear are 
usually larger than those of the rat which could not hear. 

Among these measurements we see sometimes very marked 
and sometimes only slight differences. Only the radial distance 
between the labium vestibulare and the inner edge of the head 
of the inner pillar cell in the first two turns and the diameter of 
the nuclei of the inner and outer hair cells are in the former smaller 
than in the latter. In both instances, however, these smaller 



TABLE 113 

Comparison of the dimensions of the constituents of the cochlea in a hearing rat (H.) 
with those in a rat not hearing (N.) at nine days of age measurements 

in micro 



BAT 


AGE 


BODY 
WEIGHT 


(N.) 
(H.) 


days 
9 
9 


grams 
10 
11 


(N.) 
(H.) 


1430 
1434 


Average 


distance between two spiral ligaments 





Breadth of membrana tectoria 





TURN I 


II 


III 


IV 


Average 




Th ids ness 


(N.) 
(H.) 


243 

242 


270 
268 


304 
308 


306 
314 


281 
283 




27 
27 



Breadth of membrana basilaris 





TURN I 


II 


III 


IV 


AVERAGE 


ZONA ZONA 
ARCUATA PECTINATA 


(N.) 
(H.) 


169 
171 


189 
186 


202 
214 


201 
204 


191 
196 


79 112 
93 103 


Distance between the habenula perforata and the outer corner of the inner 
hair cell 




TURN I 


II 


III 


IV 


AVERAGE 




(N.) 
(H.) 


38 
40 


38 
42 


44 

58 


49 
60 


42 
50 


Distance between habenula perforata and the outer corner ol the outer pillar 
cell at foot* 




TURN I 


II 


ill 


IV 


Average 




(N.) 
(H.) 


70 

79 


76 

88 


86 
103 


86 
102 


79 
93 


Height of greater epithelial ridge 




TURN I 


II 


III 


IV 


AVERAGE 




(N.) 
(H.) 


36 
42 


40 
43 


41 

48 


42 
50 


40 
46 


Distance between the labium vestibulare and the habenula perforata 




TURN I 


II 


III 


IV 


Average 




(x.) 

(H.) 


83 

85 


108 
104 


137 
140 


145 
150 


118 
120 





TABLE 113 Continued 

Distance between the labium vestibulare and the inner edge of the head of the 

inner pillar cell 





TURN I 


II 


III 


IV 


Average 




(N.) 
(H.) 


94 

78 


131 

108 


168 
170 


179 
210 


143 
142 


Height from basal plane to surface of pillar cells 




TURN I 


II 


III 


IV 


AVERAGE 




(N.) 
(H.) 


32 
40 


33 

42 


35 

45 


36 
45 


34 
43 


Height of tunnel of Corti 




TURN I 


II 


III 


IV 


AVERAGE 




(N.) 
(H.) 




20 




18 




14 




11 




16 


Height of papilla spiralis at the third series of outer hair cells 




TURN I 


II 


III 


IV 


AVERAGE 




(N.) 
(H.) 


28 
42 


28 
43 


27 
39 


28 
36 


28 

40 


Height of Hensen's cells 




TURN I 


II 


ill 


IV 


AVERAGE 




(N.) 
(H.) 


20 
42 


23 
45 


23 

38 


24 
30 


23 

39 


Angle of lamina basilaris with plane of membrana basilaris degrees 




TURN I 


II 


HI 


IV 


AVERAGE 




(N.) 
(H.) 




+7 




+4 




-8 



7 






Length of the inner and outer pillar cells 





INNER 

TURN I 


II 


III 


IV 


AVERAGE 






(N.) 
(H.) 


35 
36 


39 
39 


41 
42 


40 
45 


39 

41 




OUTER 
TURN I 


II 


III 


IV 


AVERAGE 


WEIGHTED 
AVERAGE 


(N.) 
(H.) 


26 
45 


26 

54 


29 
50 


29 
40 


28 
47 


30 
46 



Volume of inner and outer hair cells 





INNER: AVERAGE 


OUTER: AVERAGE 


WEIGHTED AVERAGE 


(N.) 
(H.) 


1798 
1815 


1277 
1279 


1407 
1428 



150 




TABLE 113 Concluded 
Diameter of nuclei of the inner and outer hair cells 





INNER: AVERAGE 


OUTER: AVERAGE 


WEIGHTED AVERAGE 


(N.) 
(H.) 


8.3 
8.2 


8.0 
7.4 


8.1 

7.6 




DEITERS' CELLS: 

VOLUME M ' 


DIAMETER OF 
NUCLEI 


LENGTH OF CELL 
BODY 


PHALANGEAL 
PROCESS 


(N.) 
(H.) 


518 
1193 


7.0 

7.2 


14 
32 


22 
22 



Ganglion spirale: diameters computed 





CELLS 


NUCLEI 


(N.) 
(H.) 


13.6 
13.7 


8.5 
8.6 



values are marks of maturity. These changes in size accord 
with the results given in chapter 1 at twelve days of age, though 
there are some differences between them in the absolute values. 

Figures 7 and 8 illustrate the outlines of the tympanic wall 
of the membranous cochlea in nine-day-old rats which could 
not and could hear, respectively. These figures have been 
drawn from the corresponding sections at the beginning of 
the middle turn of the cochlea and are comparable with figures 
3 to 6 and 9 to 12. On comparing figures 7 and 8, the more 
noticeable differences appear to be the following. 

The membrana tectoria is a bit longer in the hearing rat. 
The appearance and the position of it with reference to the 
surface of the papilla spiralis is also different. In the not-hearing 
cochlea it has an infantile look. 

The outer end of the main part does not yet reach the second 
row of the outer hair cells and connects with the Hensen's 
prominence by a thick thread. There are also many fine fibers 
to be seen between the basal surface of the membrane and the 
papilla. In the hearing rat the fine fibers are absent. The 
membrane reaches already the row of the outer hair cells and 
there is a strong connection between this part and the terminal 

ame (Schlussrahmen) of the lamina reticularis by a thick read as shown in figure 8 Thus the position of the membrane above the papilla depends a little bit upon the increase of 
the length of the membrane itself, but chiefly upon other factors 
such as the inward shifting of the papilla. The membrana 
basilaris as a whole shows a small increase o" breadth in the 
hearing rat. The zona arcuate, however, increases much in 
its breadth, while the zona pectinata rather decreases. This 
is due to the development of the pillar cells. The base of the 
inner and outer pillar cell spread much on the membrane. At 
the same time the length of the cells, especially of the outer, 
increases nearly twice as much as in the not-hearing rat. 

Thus the foot of the outer pillar cell moves out ward, as Bottcher 
('69, 72) stated, while the inner corner of the inner pillar cell 
does not move in any way, as the habenula perforata stands at 
a fixed point. This results in a change in the form of the arch 
of Corti. The hitherto outward inclined arch tends to bend 
inward and between both inner and outer cells arises a space, 
the tunnel of Corti. The appearance of the tunnel seems to have 
some relation to hearing. The tunnel is always present in the 
cochleas of hearing rats. Sometimes the tunnel is present in 
the lower turns, but not in the upper turns in the not-hearing rats. 
We can say, therefore, that it probably appears through all the 
turns before the special function of the cochlea begins. In this 
way the zona arcuata of the membrana basilaris increases its 
breadth. 

The next striking change is the rapid increase in the size of 
the Deiters' cells, Hensen's cells, and the resultant change of 
the form, with an inward shifting, of the papilla spiralis. 

The Deiters' cells increase their height very rapidly; the length 
of the cell body becomes over twice that in the not-hearing rat, 
but the processus phalangeus changes only slightly. Hen- 
sen's cells develop also, but not so much as Deiters' cells. The 
papilla spiralis thus increases in height. On the other hand, 
the greater epithelial ridge vanishes inwards from the inner 
supporting cells and appears as a furrow the sulcus spiralis 
internus. Through the pressure of these outward-lying cells 
the papilla spiralis swings inward as a whole, without really 
moving on the membrana basilaris. The lamina reticularis becomes inclined inward instead of outward and subtends a 
slight angle with the plane of the membrana basilaris. The 
distance from the labium vestibulare to the inner edge of the 
head of the inner pillar cell becomes smaller through the inward 
shifting of the papilla. 

In the hair cells and the cells of the ganglion spirale we see 
a smaller difference between the hearing and not-hearing rats. 
Only the diameter of the nuclei of the hair cells in the hearing 
rat diminishes a little, as it continues to do in the adult cochlea. 

All the changes just enumerated begin at the base of the 
cochlea and progress to the apex. Therefore we see the high 
and outward ascending papilla spiralis in turn I, while in the 
upper turns the papilla is not yet so developed, is smaller in all 
the constituents, and shows in general the characters of a younger 
and less mature cochlea conditions which disappear with age. 
This upper and immature part seems not to respond to the test 
for hearing. Indeed, my testing result is positive for sounds of 
high pitch, but not for low pitch. Therefore we conclude that 
the papilla spiralis develops functionally from base to apex and 
that when the papilla spiralis has developed in the basal turns, 
but not in the upper turns, it responds to sounds of high pitch 
alone.

==Discussion==

If we assume that our tests for hearing are trustworthy, then 
the differences between the size of the constituents of the cochlea 
in nine-day-old rats which could and could not hear will indicate 
what developmental changes in the cochlea of the albino rat are 
necessary for the appearance of the hearing reflex. Whether all 
the differences found by us are necessary is difficult to determine, 
and the problem is open for further study; but as the matter 
stands, our results give the closest correlation between structural 
changes and the appearance of function which has as yet been 
reported. 

Kreidl and Yanase ('07) studied the differences between the 
not-hearing and hearing rat and summarized their results on 
page 509: "Kurz vor Eintritt des Horreflexes ist das Cortische Organ im wesentlichen fertig ausgebildet. " They publish no 
measurements nor data. The condition of the development of 
the organ of Corti described as 'fertig ausgebildet' is not suffi- 
ciently precise. 

Further, on the same page they say, "Der auffalligste und, 
soweit die Untersuchungen bis jetzt eregben haben, einzige 
Unterchied, der zwischen dem anatomischen Bild des Labyrinthes 
eines neugeborenen Tieres, das den Reflex eben nochnichtaufweist, 
und dem eines solchen, dasdenselben zum ersten Male eben erkenn- 
an lasst, ist der, dass beim ersteren noch ein Zusammenhang zwis- 
chen Cortischem Organ und Cortischer Membran besteht, beim 
letzteren dagegen dieser Zusammenhang bereits gelost oder gelock- 
ert ist." Their observation is quite different from mine. In my case 
the papilla spiralis shows in the development of its constituent 
elements pretty large differences between the not-hearing and 
hearing rats. Therefore it seems probable that the changes in the 
growth and form of the papilla just before the first appearance of 
the special function, take place very quickly. Also we cannot 
agree to then* statement concerning the relation of the membrana 
tectoria to the papilla spiralis. In our preparations there is 
still a connection of the membrane with the terminal frame of the 
lamina reticularis through a thick thread in the cochlea of the 
rat which could hear (fig. 8) and also in that of the rat which 
could not hear (fig. 7). 

This is a point on which opinions differ. While one opinion, 
represented by Kishi ('07) and others, is to the effect that this 
connection remains through life, the other, represented by Kolliker('67) and others, asserts the membrane projects free in the 
endolymph. I have never seen this connection in the adult 
cochlea, nor have I found such a connection of the membrane 
with the hairs of the hair cells, as Shambaugh ('10) described in 
the pig. In the young rats, at fifteen days for example, we very 
often see upon the terminal frame the broken remainder of this 
connecting thread. Whether this break arises through natural 
development or is the result of artificial manipulation it is hard 
to say. At any rate, Held's assertion ('90), that in an animal 
capable of hearing the membrana tectoria is never connected with the papilla spiralis, is not supported by my observation. 
That the freeing of the outer zone of the membrane is not absolutely necessary for the mediation of auditory impulses is 
demonstrated in the cochlea of birds, as shown by Hasse ('66), 
Retzius ('84), Sato ('17), and others. In these forms the membrane remains through life attached to the epithelial ridge. 

My results agree with those of Hardesty ('15) on this point, 
though he obtained a tectorial membrane which floats free in 
the endolymph with its outer zone. Lane ('17) studied the correlation between the structure of the papilla spiralis and the 
appearance of hearing in the albino rat, but his description is 
brief and does not touch on this relation of the papilla to the 
tectorial membrane. 

Thus the inception of hearing does not coincide with the 
detachment of the tectorial membrane from the papilla spiralis, 
but with the development of each constituent of the papilla 
spiralis and the membrane tectoria, as has been described. As 
these changes occur first at the base and then pass to the apex, 
the animal can perceive at first only the sounds of high pitch. 
One or two days later development is complete in all the turns, 
and then the rat can hear the sounds of lower pitch also. Thus 
the process of the development of the cochlea does not support 
the ' telephone theory' of audition, but on the contrary agrees with 
the conclusion that the papilla in different locations in the turns 
of the cochlea responds to sounds of a definite pitch, as Wittmaack ('07), Yoshii ('09), Hoessli ('12), and others have shown 
by experimental studies on the mammals. 

Concerning the exact age of the first appearance of the function 
in the rat, there are several different statements. Lane ('17) 
found no response to sound before the twelfth day after birth, 
and on the sixteenth day he reports hearing well established. 
Kreidl and Yanase ('07) state that hearing begins in the rat at 
from twelve to fourteen days. My rats responded usually at 
ten to twelve days, but one at nine days. These differences 
depend in all probability on the vigor of the young during the 
first days of postnatal life, and it seems probable that exceptionally well-nourished young might develop precociously in this respec t. The intensity of the stimulus is important in determining 
the hearing reflex, as Small ('99) has stated. In my cases the 
young rats responded very evidently to intense sounds, while 
they reacted weakly or not at all to those which were faint. 
Thus only the intense sounds were perceived by the rat of nine 
days.

==Conclusions==

1. The hearing reflex was never obtained in rats less than 
nine days of age. 

2. At nine days a single rat, one of five in a litter, responded to 
a sharp sound like clapping the hands and to a whistle of high 
pitch; the other four did not respond. At the tenth day some 
of the four reacted, and at the twelfth day all could hear. 

3. The hearing reflex probably occurs early in young rats 
that are vigorous and well nourished. 

4. To obtain the first hearing reflexes it is necessary to have 
rather strong sounds of high pitch. 

5. A comparison of the histological structure of the cochlea 
in rats of nine days, one of which could hear and the other could 
not, shows clear differences in its development of the cochlea. 
These consist not in the detachment of the tectorial membrane 
from the papilla spiralis, but in the degree of differentiation of 
the constituents of the papilla. The tectorial membrane is 
connected in both cases at its outer end with the terminal frame 
of the lamina reticularis by a thick thread. The papilla is more 
differentiated in the hearing rat in several characters. The 
tectorial membrane has reached with its outer end the outermost 
row of the outer hair cells, but in the not-hearing rat it has not 
yet reached the second row of the cells. 

6. The form of the papilla and its relation to the surrounding 
structures, especially to the tectorial membrane, are in the hearing 
rat at nine days very similar to those in the rat at twelve days 
of age, though there are some differences between them in ab- 
solute size. 

7. The freeing of the tectorial membrane from the papilla 
spiralis is not necessary to the appearance of the hearing reflex, but the differentiation of the papilla, its shifting inward, its 
change in form and position under the membrana tectoria, appear 
to be important. 

8. Since the papilla develops from the base toward the apex, it 
first reaches in the lower turn a high degree of differentiation, 
and this part first begins to function. Therefore the rat can 
hear only sounds of high pitch when it first responds. 

9. This result accords with that well-known fact that the 
papilla in the lower turn responds to sounds of a high pitch, 
while in the upper turn it responds to sounds of low pitch.