Anatomy and histology of cochleas in mammal

Igarashi, M.

Yoshinobu, T.

English

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COMPARATIVE COCHLEAR RECONSTRUCTION IN MAMMAS* 
Makoto Igarashi and Toshiko Yoshinobu# 
Bureau of Medicine and Surgery 
Subtask 1 Report No. 114 
Project MR005.13-6001 
NASA Order No. R-93 
Approved by 
.A- s .  
Captain Ashton Graybiel, MC USN 
Director of Research 
Released by 
Captain H. C. Hunley, MC USN 
Commanding Officer 
11 June 1965 
*This study was supported in part by the Office of Life Science Programs, National 
Aeronautics and Space Administration . 
#Present address; Kyoto University School of Medicine, Kyoto, Japan. 
U. S. NAVAL SCHOOL OF AVIATION MEDICINE 
U . S. NAVAL AVIATION MEDICAL CENTER 
PENSACOIA, FLORIDA 
SUMMARY PAGE 
THE PROBLEM 
The size, length, and shape of the cochleas have been compared in different 
mammalian species, with the use of graphic cochlear reconstructions. 
FINDINGS 
The ratio between the size of the cochlea and that of the skull generally decreases 
when the skull size increases. In other words, the size of the cochlea in  small mammals 
i s  considerably larger . 
The number of cochlear turns increases when the cochlea protrudes into the well- 
pneumatized mastoid bulla instead of being embedded in the thick otic capsule. 
ACKNOWLEDGMENT 
We gratefully acknowledge the excellent technical assistance of Miss Glenda J. 
Sessions. 
.. 
IN TRO DUCT I ON 1 
Anatomical development of the cochlea progresses from the amphibian in which the 
init ial form of the cochlea, macula basilaris, appears. Next in the progression are the 
cochlear duct (from the lagena), the perilymphatic ducts, and the organ of Corti (from 
the basilar macula); these are found in birds and crocodiles. The final stage i s  seen in 
mammals and appears as the cochlear spiral (1,6). 
Cochlear spirals of mammalian ears are al l  of different sizes and shapes, and the 
basilar membrane of each varies in length. In addition, mammals have vastly differing 
skull sizes. The present study was designed to compare these anatomical aspects in 
various species . 
PROCEDURE 
Length of the basal membranes and the number of cochlear turns were measured in 
17 human ears, 16 squirrel monkey (Saimiri sciureus) ears, 6 of the cat and rat, 4 of the 
guinea pig, 2 of the dog, and one ea= t h e i t ,  kangaroo rat, and bat. A l l  of 
these were adult inner ears. Only in the human, squirrel monkey, cat, guinea pig, and 
rat were the skull size and the diameter of the cochlea basal turn measured. 
The temporal bones were fixed, decalcified, dehydrated, and embedded in celloidin. 
Those of the bat and rat were serially sectioned in horizontal plane at 16 microns, those 
of the guinea pig and kangaroo rat at 18 microns, and the remainder at 20 microns. One 
of each ten sections which were stained in hematoxylin-eosin made up a pilot set. 
Cochlea spirals were then graphically reconstructed according to the technique of Guild 
(2) and of Schuknecht (3). 
Size of the skull was determined by measuring from the anterior edge of the frontal 
bone to the occipital point, long axis; and the distance between the two parietal bones 
at the widest point, short axis. 
RESULTS 
Average lengths of the basal membranes and the number of cochlear turns for each 
mammalian species examined are listed in Table I .  Difference between the longest in 
length, that of the human, and the shortest (the bat and rat) was 24 mm. The basal 
membrane measurements of the other mammals fe l l  in between. 
turns of a l l  species except the guinea pig and kangaroo rat was between 2-1/2 and 3. 
The number of cochlear 
A summary of the findings in the species whose skulls were measured i s  contained in 
Table II. From the measurement of the diameter of the cochlea basal turn,it can be seen 
that the size of the cochlea in the cat and in the squirrel monkey was approximately the 
same,yet the cat's skull was twice the size. In contrast, the skull of the guinea pig and 
rat was about the same size, but the cochlea of the former was larger and longer. 
1 
Table I 
The Lengths of Basal Membranes and the Numbers of Cochlear Turns in 
Different Mammals 
The Average Lengths of 
Species Basal Membrane (mm) Turns 
The Number of 
Human 
Dog 
Squirrel Monkey 
Cat 
Guinea Pig 
Rabbit 
Kangaroo Rat 
Rat 
Bat 
32 
25 
22 
22 
21 
14 
12 
8 
8 
2 1/2 - 2 3/4 
2 1/2 - 2 3/4 
2 1/2 - 2 3/4 
4 
3 1/2 - 3 3/4 
2 3/4 - 3 
2 1/2 
2 1/2 
2 1/2 
Table I I  
Summary of Findings in Five Different Species 
Skull Size Diameter of Length,Basal Turns 
(mm, long axis Cochlea Membrane in 
Species x short axis) Basal Turn (mm) (mm) Cochlea 
Human 180x 135 6.2 32 2 1/2 - 2 3/4 
Squirrel Monkey 47 x 35 3.7 22 2 3/4 - 3 
Guinea Pig 35 x 22 2.3 21 3 3/4 - 4 
Cat 70 x 40 3.8 22 2 1/2 - 2 3/4 
Rat 3 0 x  17 1.2 8 2 1/4 - 2 1/2 
2 
Representative spiral reconstructions of the mammalian cochleas are shown in 
Figurq 1-9. The arrow line at the top of each figure indicates the 1 mm scale to which 
the drawings were made. Inasmuch as these figures were drawn in the same scale, the 
ratio between each species demonstrates a comparison of the actual size of the cochleas. 
I 
DISCUSS I ON 
The discussion of the present study wil l  be limited to the comparative cochlear re- 
constructions. Since the cochlea i s  merely a part of the auditory system, i t  i s  critical 
to compare the auditory activities (including frequency range, discrimination, etc .) 
among the different species by comparing the cochlear structures. Obviously, there 
are many other factors, such as external and middle ear structures and functions, neural 
functions including cortical and efferent system functions, et cetera, which comprise 
the auditory function. 
The lengths of the basal membrane in several mammalian species were measured by 
von Bbk6sy and Rosenblith (4) who stated that the lower l i m i t  of hearing fel l  as the size 
of the animal increased, but the upper l i m i t  of resolution could not be determined because 
the vibratory amplitudes for the high frequencies were too small to measure. Thus, they 
observed no correlation between the lengths of the basal membrane and the hearing 
activity, especially in the frequency range. 
With the method used in the present study (2,3) the cochleas were reconstructed 
from a three-dimensional structure to a two-dimensional plane by using horizontal serial 
sections, and the technical errors inherent in such a procedure should be considered. 
According to Schuknecht's method (3), the measuring error was less than 5 per cent on 
the average, and the error from the height of the cochlea (or the steepness of the basal 
membrane) was less than 3 per cent. Therefore, when spiral reconstructions are com- 
pared among the different species, the actual error in comparison i s  usually negligible. 
The exact tangential section of the top of the Corti tunnel (contact of the tops of 
the inner and outer pillar cells) is not frequently seen on slides. Typical tangential 
cuts from the cat and the squirrel monkey are demonstrated in Figures 10-12. 
In comparing the size of the skull with the size of the cochlea, i t  becomes obvious 
that the ratio i s  not so pronounced as i t  i s  between other parts of the body and the skull. 
For example, the difference in size between the body of a small animal as compared to 
a larger one i s  much greater than that between the cochleas of the same animals. The 
ratio between the size of a human skull and the squirrel monkey skull i s  more than 1O:l; 
when the size of cochleas in the two are compared, the ratio drops to less than 3:l . 
The cochlea spiral of the elephant, according to von Bbkgsy (4),is 60 mm in length, 
but i t s  skull i s  tremendous in size. 
3 
The length of the basal membrane could not be related to the diameter of the Basal 
turn of cochlea, or to the number of cochlear turns. 
between the length of the basal membrane of the squirrel monkey and that of the guinea 
pig. However, the diameter of the cochlear base was 1.4 mm larger in the former, and 
the cochlea of the latter had 3/4 to 1 more turn. 
There was only 1 mm difference 
The cochlea of the guinea pig protrudes into the bulla while that of the other 
mammals was usually embedded in a thick, bony otic capsule. The kangaroo rat,which 
has about four cochlear turns, also has well-pneumatized mastoid bulla (5). 
The length and shape of the cochlea hook area, which is a protruding vestibular 
cecum of the cochlear duct, vary to a large degree among different mammals. The 
human, cat, and guinea pig ears have a much longer formation of this structure than 
those of other mammals, and these three species have considerably long, well-developed 
cochlear spirals. 
I 
5 
SQUIRR 
Figure 2 
CAT 
22.7mm 
Figure 3 
6 
DOG 
24.5mm 
Figure 4 
Figure 5 
7 
GUINEA P16 
Figure 6 
Figure 7 
BAT 
8.2mm 
Figure 8 Figure 9 
8 
S M  -
R' \ -s v  
A typica 
\ 
r N 
B 
Figure 10 
tangential section of  apical turn of cochlea (Squirrel Monkey). 
A l l  important structures are labeled. 225X 
SV - Scala Vestibuli 
SM - Scala Media 
ST - Scala Tympam 
R - Reissner's Membrane 
T - Tectoria 
0 - Outer Hair Cell 
H - Hensen's Cell 
C - Claudius' Cell 
D - Deiter's Cell 
X - Outer Pillar Cell 
P - Outer Pillar 
N - Tunnel Fiber (efferent) 
B - Basal Membrane 
9 
Figure 11 
10 
i 
Another typical tangential section of apical turn of the Squirrel Monkey 
cochlea. 115X 
When the animal i s  perfused successfully (intra-vital cardiac perfusion), 
a l l  small vessels along the cochlea (large arrow), vessels in stria vascularis 
(small arrow), and vessels in spiral ligament become free from blood cells. This 
i s  an important sign of  good fixation. 
Figure 12 
A typical tangential section of apical turn of the cochlea (cat). 150X 
Note the artistic arrangement of end organ structures in  cochlea. No  
post mortem change i s  seen. 
1 1  
REFERENCES 
1. Brock, W. , Phylogenese und vergleichende Anatomie des Gehoerorgans. In Denker, 
A .  ~ n d  Kghier, (3. (herausgegeben), Handbuch der  !-!ais-. idasen-, L-ilntenheil- n;T -- 
kunde. Berlin: Julius Springer, 1926. Pp 1-69. 
2. Guild, 8. R., A graphic reconstruction method for the study of the organ of Corti. 
Anat. Rec. 22:141-157, 1921. --- 
3. Schvknecht, H. F ., Techniques for study of cochlear function and pathology in 
experimental animals. Arch. Otolaryng. ,58:377-397, - 1953. -
4. von BGke/sy, G., and Rosenblith, W. A, ,  The mechanical properties of the ear. 
In Stevens, S .  S, (Ed.), Handbook of Experimental Psychology. N. Y.: John 
Wley  & Sons, 1951. Pp. 1075 - 1115. 
5. Webster, D. B., The ear apparatus of the kangaroo rat. Am. J .  Anat.,z:123-128, 
6. Werner, D s  F e i  Daz Gehoerorgcln der Wirbeltiere und des Menschen. 
- - -  
1961. 
Veb Goerg T K m e ,  1960. 
ieipzig: - -- 
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Comparative Cochlear Reconstruction in Mammals

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