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Development of the external ear in the dachs rabbit.

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Development of the External Ear in the Dachs Rabbit '
DORCAS D. CRARY
The Jackson Laboratory, Bar Harbor, Maine
ABSTRACT
Development of that part of the external ear which protects the external auditory meatus appears to be similar in all animals since the hillocks of His
have been described for reptiles, amphibia, and birds as well as mammals. Disagreement exists as to the origin and fate of the various hillocks, especially tragus and
antitragus. Most observations of the developing ear have begun with the formation
of the hillocks at which time the tragus is situated on the mandibular arch. Because
of the rapid early growth, transitional steps from the open branchial groove to this
stage have not been described previously. The retarded growth of the dachs rabbit
makes it possible to see intermediate steps. Here it is shown that the entire auricle
arises from an intact and continuous primordium, with the tragus arising from the
hyoid arch and migrating to the mandibular bar in the normal course of cell proliferation and differentiation. Variations in normal and heterozygous animals can be
found in the relative rates of development of the surrounding areas.
The dachs rabbit has typical features of
Ichondrodystrophy. The gene responsible
( D a ) produces various effects which contribute to the understanding of basic
developmental processes. These are primarily skeletal, but the external ear and
its musculature are also modified (Lamb
and Sawin, '63). The ear carriage is abnormal and a distinctive papilla projects
free and separate from the lateral margins
at its base (Sawin and Crary, '57). In the
heterozygote this papilla is only partially
free, being attached to the anterior margin
and projecting from it in varying degrees.
This structure, not manifest in the normal,
has been shown by Lamb and Sawin ('63)
to be the anterior cornu of the tragus.
Its presence or absence has been used
routinely to identify the three genotypes
a t birth. In an attempt to find the earliest
possible age at which the three genotypes
could be identified for further histologic
investigations, observations on the development of the external ear were made,
aiding materially the understanding of
such development.
They were re-examined at 9 X , 24 hours
later to verify the genotyping and to make
drawings of the external appearance of the
intact ear.
Table 1 shows the distribution of the
345 specimens examined. Of these 81
were dachs ( D a D a ) , 178 heterozygous
( D a d a ) , and 86 normal ( d a d a ) . A test of
the reliability of the typing was made for
each age since identification of the genotypes at early ages was difficult. This
showed no significant departure from the
expected single gene segregation based
either on any one age or on the pooled
values .
Comparative terminology used is as
follows :
Human (International
Anatomical Nomenclature
Commission, ' 5 6 )
T r a"m s
Antitragus
Crus helicis
Anthelix
Rabbit
(Lamb
Sawin,
'63)and
Anterior
Posterior
Anterior
Posterior
cornu of t r a-m s
cornu of tragus
helix
helix
Since many mammalian ears have a
simpler, more straightforward form than
that of the human, a simpler terminology
is more readily applicable, hence anterior
MATERIALS AND METHODS
and posterior helix rather than crus helicis
Embryos and fetuses were removed by and anthelix. In the rabbit, tragus and
C!aesarean section from DA race females antitragus can be shown to be processes of
at 11-23 and at 27 days postcopulation. a larger projection attached to the posteThey were placed in aceto-formol-alcohol rior helix (Lamb and Sawin, '63). The
for a few minutes to distinguish the ear
1 This investigation was supported in part by PHS
and its tubercles, tentatively identified for research grant C281 from the National Cancer Instltute, Public Health Service, and in part by Grant E-40
genotype, and then fixed in Bouin's fluid. from the American Cancer Society.
AVAT.REC.,150: 441-448.
441
442
DORCAS D. CRARY
TABLE 1
Distribution of genotypes
Age
postcopulation
DaDa
Dada
11
12
13
14
15
Subtotal
16
17
18
19
20
21 =
22 2
23
27
Totcl
1
3
2
3
1
10
5
5
5
4
3
14
20
9
6
81
1
6
5
6
2
20
1
2
12
6
10
9
13
56
37
10
5
178
dada
Total
1
3
12
9
13
6
43
19
14
20
18
24
99
72
3
2
4
3
13
2
3
5
5
8
29
15
3
3
86
22
14
345
___
0.3333
0.0
0.1112
0.2@00
2.0000
0.6195
2.0527
0.8572
0.0
0.1112
2.2500
1.7171
4.0906
3.4555
2.4285
2.1103
No chi square value deviated .from the expected 1 : 2 : 1 ratio at the 0.05 level of significance.
This sample contains some animals from normal x heterozygote matings.
process of migration, comes to lie in the
open end of the groove where it remains
free of the crus helicis. Its anterior margin projects downward and in later stages
becomes parallel to the posterior margin
OBSERVATIONS
(figs. 5-12). Thus it becomes the distincAs in all mammals the external ear of tive papilla of the dachs and as such is
the rabbit is a modification of the first identifiable as (the anterior cornu of) the
branchial groove with additions from the tragus (Lamb and Sawin, '63). In the
mandibular and hyoid arches (Arey, '54). heterozygote where the cleft is somewhat
At 11 days postcopulati6n (fig. 1 ) the narrower, usually only the posterior rnargroove is open ventrally in both dachs and gin of the tragus projects into the groove
heterozygous individuals and the diagnos- although variation may be almost contintic projections are just becoming visible. uous between dachs and normal. The skin
Arising from the hyoid arch is a rather fold covering the crus helicis bulges out
large appendage in the process of migra- to cover it and creates a ridge which protion around the open end of the groove. jects laterally where it covers the anterior
The heterozygote is slightly more ad- margin. In normal animals the tragus lies
vanced than the dachs and by comparing in proximity to the crus helicis and is
DaDa figures 1 and 2 , Dada figure 2 , and eventually almost completely obscured by
dada figure 1, in that order, the progress of the skin which overlies it. The ridge prothis migration can be readily seen. In the duced in the skin fold projects upwards.
normal the migration is nearly complete, This distinguishing feature separates the
and by 13 days (fig. 3 ) it is complete in three genotypes at all ages once migration
all three genotypes. The groove is closed of the tragus is completed (figs. 3-12),
and appears as an elongated slit with but it is somewhat less prominent after
small tubercles on each side. The width birth when the ear reopens and begins to
of this slit varies with genotype, being assume its ultimate adult conformation.
widest in the dachs, narrowest in normals, This ridge is particularly useful in typing
and intermediate in heterozygotes. Varia- early developmental stages where the
tion in development makes some speci- tragus is still prominent in normal animens identifiable only by the width of the mals. As the auricle grows, the ventral
groove. Because of the relatively broad portions of the ear are suppressed and
cleft in the dachs, the appendage, in the certain of the features seen in early develterms anterior and posterior cornu of the
tragus thus seem more apt. In using this
terminology, Lamb and Sawin followed
that of Meinertz ( ' 3 5 ) .
DEVELOPMENT O F RABBIT EXTERNAL EAR
opment become less noticeable. Thus, in
the normal of 16 days (fig. 6 ) the tragus
is quite visible, but at birth it is difficult
to discern and in the adult it is almost
never seen.
The auricular cartilage is first detectable
around 12 days (fig. 2) as a slight projection of the posterior dorsal edge of the
lbranchial groove. It projects out over the
groove (figs. 2-5) and by 16 days (fig. 6 )
covers it completely. As it grows, the
helices elongate and the ear opening narI*OWS until it closes completely. It does not
open again until after birth. At around
21-22 days (fig. 11) the ear folds back
revealing the now closed external auditory
meatus and assuming a position along the
back of the head where it remains until
after birth when it is erected in normal
and heterozygous animals. Because of the
abnormalities of skull (Sawin, Ranlett,
and Crary, '59) and musculature (Lamb
and Sawin, '63) the dachs ear never attains the erect position but projects dorsolaterally (Crary and Sawin, '52).
DISCUSSION AND CONCLUSIONS
That the external ear develops from hillocks (His, 1885, cited by Streeter, '22) or
ridges (Hammar, '02, cited by Streeter,
'22) arising from the hyoid and mandibular arches early in embryonic development
is a generally accepted fact. It is interesting to note that these hillocks have been
described for mammals with very different
types of ears (Gradenigo, 1888; Schwalbe,
1897, '16; Schmidt, '02, all cited by
Streeter, '22) and also for reptiles, amphibia, and birds, which never acquire a
distinct auricle (Schwalbe, 1891, cited by
Streeter, '22). This would seem to indicate
that the auricle, per se, is an independent
functional adaptation and that the development of that part of the external ear
which protects the entrance to the external
auditory meatus is probably very similar
in all animals. However, considerable disagreement exists as to the ultimate fate of
thle various hillocks or to the origin of the
definitive parts of the fully formed ear.
The greatest confusion appears to exist in
the origin of the tragus and antitragus.
His (1885, cited by Streeter, '22) in his
classic description, which has dominated
subsequent literature, attributed the origin
443
of the tragus to the mandibular arch and
the antitragus to the hyoid. Streeter ('22)
believed that too much emphasis had been
placed on the importance of the hillocks
which are transitory and incidental rather
than fundamental to the development of
the auricle. They are due to closely packed
masses of mesenchymal cells beneath actively proliferating ectoderm. The ectoderm of the hyoid arch shows more
marked changes than that of the mandibular bar and resembles that of the early
stages of limb buds. Streeter felt that the
auricle arises from an intact and continuous primordium which becomes elaborated
into its final form by the ordinary processes of differentiation. Although he makes
the statement that it is possible that the
auricle is entirely of hyoid origin, the mandibular elements being products of cells
which have migrated into this region from
the hyoid bar, he was unable to demonstrate it with his material and continued
to describe the tragus as of mandibular
origin. Wood-Jones and Wen ('34) concluded that the tragus was derived from
the mandibular arch, but that the remainder of the external ear was of hyoid origin.
Boas ('l2), in a comprehensive study of
the comparative anatomy of the adult
mammalian ear and its cartilage, perfected a method by which the cartilage
could be flattened out for ease of comparison. In so doing, he demonstrated that the
tragus is a part of the posterior border of
the ear cartilage which is rolled so that
the proximal portion lies over the anterior
edge. He pointed out the strong similarity
of all mammalian ears and discussed how
the individual parts vary from one form
to another, but he made no attempt to
trace their origins. His reclassification of
parts into anterons, posterons, and
notches, while perhaps facilitating his
comparisons, serves only to confuse the
picture. More recently, Lamb and Sawin
('63) prepared ear cartilages of dachs and
normal rabbits, again showing that the
auricular cartilage is nearly continuous
but with the margins unconnected anteriorly. The tragus is continuous for most
of its length with the posterior helix (anthelix) and lies just posterior to the anterior helix (crus helicis). In these preparations, the papilla-like structure of the
444
DORCAS D. CRARY
Stages in the development of the external ear in dachs ( D a D a ) , heterozygous ( D a d a ) ,
and normal ( d a d a ) rabbits.
Fig. 1 Eleven days postcopulation. Note projection (tragus) arising from the hyoid arch
which is in process of migration around the open end of the gill cleft in DaDa and Dada
animals.
Fig. 2 Twelve days. The tragus is still migrating around the gill cleft, Note beginning
of auricular cartilage.
Fig. 3 Thirteen days. Migration of tragus is complete in all three genotypes. Note difference in width of branchial groove.
Figs. 4, 5 Fourteen and fifteen days. Note projection of auricular cartilage over ear
opening.
Fig. 6 Sixteen days. Auricular cartilage (dotted lines) now covers ear opening completely.
445
DEVELOPMENT O F RABBIT EXTERNAL EAR
AC, Auricular cartilage
Ah, Anthelix
At, Antitragus
Abbreviations
BG, 1st branchial groove
CH, Crus helicis
HA, Hyoid arch
11, Incisura intertragica
MA, Mandibular arch
T, Tragus
Whole embryos and heads from Minot and Taylor ('05)
x 2.5.
Figs. 7-10 Seventeen-twenty days. Note angle of anterior margin of tragus in DaDa
animals and of the ridge produced by skin fold covering it in heterozygotes (Dada) and
normals (dada).
Figs. 11, 12 Twenty-two and twenty-three days. The auricular cartilage has folded back
along the head revealing the closed external auditory meatus.
446
DORCAS D. CRARY
dachs proves to be the superior anterior
cornu of the tragus (tragus) which in
normal rabbits develops in contact with
the anterior helix (crus helicis) where it
is not apparent beneath the skin. This
suggests that the tragus is, therefore, of
hyoid rather than of mandibular origin.
Most observations of the developing ear
have begun with the time of formation of
the auricular hillocks. Because of the rapidity of the early development of this
region, the steps in the transition from the
open branchial groove to this stage have
not been described previously and confusion concerning the origin of certain of
the parts of the ear has resulted. I n the
dachs rabbit growth in general is retarded,
making it possible to observe intermediate
steps in development not heretofore demonstrated. The projection seen i n the earliest embryos, later identifiable as the
tragus, becomes the distinctive diagnostic
papilla of the dachs because of the reduced
rate of growth and development which not
only slows the migration of the tragus but
keeps the groove relatively broad. Thus
the papilla, i n itself, is not a n abnormal
structure, but its abnormal exposure is
entirely the result of retarded growth. The
tragus normally remains free of the crus
helicis in some species of mammals, notably the camel (Camelus bactrianus) and
the wild pig of Sumatra (Sus) (Boas, '12).
Study of the development of the dachs
ear, especially when compared with that
of heterozygotes and normals, shows that
the entire auricle, which in cartilage preparations of adults appears as a single continuum, arises from a n intact and continuous primordium, as Streeter suggested,
with the tragus (anterior cornu) arising
from the hyoid arch and migrating to the
mandibular bar i n the normal course of
cell proliferation and differentiation. Variations in expression of this region in both
heterozygous and normal animals presumably can be found in the original width of
the branchial groove and the relative rates
of development of the surrounding areas.
LITERATURE CITED
Arey, L. B. 1954 Developmental Anatomy, 6th
ed. W. B. Saunders Company, Philadelphia.
Boas, J. E. V. 1912 Ohrknorpel und ausseres
Ohr der Saugetiere. Kopenhagen. 266 pp,
25 plates.
Crary, D. D., and P. B. Sawin 1952 A second
recessive achondroplasia in the domestic rabbit.
J. Heredity, 4 3 : 255-259.
Gradenigo, G. 1888 Die Formentwickelung der
Ohrmuschel mit Riicksicht auf die Morphologie
und Teratologie derselben. Centralbl. f . d. med.
Wissensch. 26:82-86, 113-117. Cited by G. L.
Strceter, Development of the auricle in the
human embryo. Carnegie Inst. Wash. Pub. 277.
Contrib. Embryol., 1 4 : 111-138, 1922.
Hammar, J. A. 1902 Studien iiber die Entwickklung des Vorderdarms und einiger angrenzenden Organe. I Abtheilung : Allgemeine
Morphologie der Schlundspalten beim Menschen. Entwicklung des Mittelohrraumes und des
ausseren Gehorganges. Arch. f. mikr. Anat.,
59: 471-628. Cited by G. L. Streeter, Development of the auricle in the human embryo.
Carnegie Inst. Wash. Pub. 277. Contrib. Embryol., 14: 111-138, 1922.
His, W. 1885 Die Formentwichelung des ausseren Ohres. Anatoinie menschlicher Embryonen. Part 111, 211-221. Cited by G. L. Streeter,
Development of the auricle i n the human embryo. Carnegie Inst. Wash. Pub. 277. Contrib.
Embryol., 14: 111-138, 1922.
International Anatomical Nomenclature Commission 1956 Nomina Anatoniica. The Willizms
& Wilkins Co., Baltimore.
Lamb, N. P.7 and P. B. Sawin 1963 Morphogenetic studies of the rabbit. XXXIII. Cartilages and muscles of the external ear as affected by the dachs gene ( D a ) . Am. J. Anat.,
1 1 3 : 365-388.
Meinertz, T. 1935 Die Hautmuskulatur der
Saugetiere. Yahrb. Morph. u. Mikrosk. Anat.
Abt. I. Gegenbauers Morph. Yahrb., 75: 15-51.
Minot, C. S., and E. Taylor 1905 Normal plates
of the development of the rabbit (Lepus cuniculus, L ) , In: Normentafeln zur Entwicklungsgeschichte der Wirbelthiere V. F. Keibel, Jena.
Sawin, P. B., and D. D. Crary 1957 Morphogenetic studies of the rabbit. XVII. Disproportionate adult size induced by the DA gene.
Genetics, 42: 72-91.
Sawin, P. B., Mary Ranlett and D. D. Crary 1959
Morphogenetic studies of the rabbit. XXV. The
spheno-occipital synchondrosis of the dachs
(chondrodystrophy) rabbit. Am. J. Anat., 105:
257-280.
Schmidt, J. 0. L.
1902 Vergleichend-anatoinische Untersuchuiigeii iiber die Ohrmuschel
verschiedener Saugetiere. Berlin. Cited by G .
L. Streeter, Dcvelopment of the auricle in the
human embryo. Carnegie Inst. Wash. Pub. 277.
Contrib. Embryol., 14: 111-138, 1922.
Schwalbe, G. 1891 iiber Auricularhocker bei
Reptilien; ein Beitrag zur Phylogenie des ausseren Ohres. Anat. Anz. 6: 43-53. Cited by
G . L. Streeter, Development of the auricle i n
the human embryo. Carnegie Inst. Wash. Pub.
277. Contrib. Embryol., 14: 111-138, 1922.
1897 Das aussere Ohr. In: Handbuch
der Anatomie des Menschen. K. H. von
Bardeleben. Vol. 5, part 2: 125-131. G. Fischer,
-
DEVELOPMENT O F RABBIT EXTERNAL EAR
Jena. Cited by G. L. Streeter, Development of
the auricle in the human embryo. Carnegie
Inst. Wash. Pub. 277. Contrib. Embryol., 14:
111-138, 1922.
1916 Beitrage ZUI Kenntnis des ausseren Ohres der Primaten. Zeitschr. f. Morphol. u. Anthropol. 19: 545-668. Cited by G.
L. Streeter, Development of the auricle i n the
447
human embryo. Carnegie Inst. Wash. Pub. 277.
Contrib. Embryol., 14: 111-138, 1922.
Streeter, G. L. 1922 Development of the auricle
in the human embryo. Carnegie Inst. Wash.
Pub. 277. Contrib. Embryol., 14: 111-138.
Wood-Jones, F., and I-C. Wen 1934 The development of the external ear. J. Anat., 68:
525-533.
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