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Address reprint requests and correspondence: Department of Otolaryngology Head & Neck Surgery, Oregon Health & Science University, PV-01, 3181 Sam Jackson Park Rd, Portland, OR 97232.
External ear anomalies are common and range from mild asymmetries to severe deformity or complete lack of external ear development. An in-depth knowledge of the developmental stages and embryology allows for an understanding of patterns of ear malformation. Here we review the embryology of the ear as well as the anatomy of a normally formed ear as a preface for discussing reconstruction in future chapters of this issue.
Ear malformations are common and range from minor abnormalities to complete anotia (lack of an ear). Understanding the key steps in the embryologic development of the external ear is, therefore, critical to the understanding of various ear deformities. Although we briefly discuss embryology of the ear canal, the focus of this discussion is on external ear development.
Embryology of the external ear
In the development of the human ear, the branchial arches play a crucial role. They are separated by pouches composed of endoderm on the internal aspect and by clefts composed of ectoderm on the external side. The central mesoderm contains the muscle, cartilage, vessels and nerves that will ultimately supply and establish the surrounding anatomic structures (Figure 1). Molecular signaling from the ectoderm results in invasion of mesenchyme and gradual obliteration of the clefts and pouches. The importance of this mesenchymal invasion is highlighted by the fact that failure of this critical step leads to remnants of the brachial arches and clefts, resulting in various auricular anomalies. Type I branchial cleft cysts represent a persistence of the first branchial cleft.
Additionally, some authors suggest that when there is an anomaly in the molecular signaling between the ectoderm and mesenchyme it can lead to failure of chondrogenesis. This breakdown of signaling can result in different degrees of anomalies, from absence of specific auricle components to microtia or even anotia if there is complete chondrogenesis failure.
Development of the human ear begins with appearance of the otic placode and vestibulocochlear ganglia at 3 weeks of gestation. The external auditory canal begins to develop from the first branchial cleft at 4 weeks. A pit is then formed by ectodermal proliferation. By 28 weeks an epithelial core has canalized from medial to lateral resulting in the fully patent external auditory canal. When the external auditory meatus fails to canalize, it can result in membranous or bony stenosis or atresia.
in: Johnson J.T. Rosen C.A. Bailey B.J. Bailey’s Head and Neck Surgery-Otolaryngology. Wolters Kluwer Health/Lippincott Williams & Wilkins,
Philadelphia (PA)2014
Development of the auricle begins at 5 weeks gestation with development of the auricular hillocks numbered from 1 through 6, derived from the first (mandibular) and second (hyoid) branchial arches.
in: Johnson J.T. Rosen C.A. Bailey B.J. Bailey’s Head and Neck Surgery-Otolaryngology. Wolters Kluwer Health/Lippincott Williams & Wilkins,
Philadelphia (PA)2014
Fusion of the hillocks results in formation of the auricle. It has been proposed that failure of these hillocks to fuse results in the formation of preauricular pits or sinuses and cleft ear (Figure 2).
Although it is mostly agreed upon that the hillocks can be followed through development to specific components of the auricle, this theory has been doubted by some who state the hillocks are transient, representing intense foci of mesenchymal proliferation, which do not directly give way to specific auricle components.
Specifically there is controversy regarding the developmental origin of the ascending helix and crus helicis. Over time the exact contributions of the each hillock to the auricle have been modified, starting with His in 1885. Park proposed that the pinna may develop from hillocks 1 and 6 based on his surveillance of ear clefting deformities.
An additional contribution to the auricular primordium is the free ear fold, which develops posterior to the second branchial arch and ultimately gives rise to part of the helix, scaphoid fossa, and to the superior crus of the antihelix.
Table 2 summarizes the proposed contributions of each hillock to the final auricle whereas Figure 3 illustrates these contributions throughout embryonic development.
Table 2Contributions of the arches and hillocks to the human auricle
Branchial arch
Hillock number
Ear component
Arch 1
1
Tragus
Arch 1
2
Crus helicis
Arch 1
3
Ascending helix
Arch 2
4
Horizontal helix, upper portion of scapha, and antihelix
During development, the external ear gradually moves laterally and dorsally and in the cranial direction relative to the eyes and mouth. Kagurasho et al
determined that movement of the ear during development is based on changes in size and shape of the embryo also known as differential growth, rather than on migration from one area to another.
Anatomy
The auricle plays a minor but important role in hearing. The auricle serves to help localize sound and the concha has a resonance of approximately 5 kHz.
in: Johnson J.T. Rosen C.A. Bailey B.J. Bailey’s Head and Neck Surgery-Otolaryngology. Wolters Kluwer Health/Lippincott Williams & Wilkins,
Philadelphia (PA)2014
As mentioned previously, during embryologic development the ear’s components are derived from the hillocks. The central mesenchyme is essential as it carries components leading to vascular, nerve, and muscular supply for the ear. The vascular supply of the ear is composed of a complex network of interconnected vessels stemming mainly from the superficial temporal artery and the posterior auricular artery
(Figure 4). There are variable branch patterns of upper, middle, and lower terminal branches of the superficial temporal artery that contribute to the anterior blood supply. Further anterior arterial contributions come from the perforating branches of the posterior auricular artery. Venous drainage is variable but most often originates from venae comitantes that accompany 1 of the 2 main arteries.
Figure 4Anterior and posterior arterial supply to the auricle. (Color version of figure is available online.)
Skin over the anterior and posterior surfaces differs mostly in its mobility with the posterior skin being quite mobile and the anterior skin tightly adherent to the underlying cartilage. The postauricular region contains a layer of connective tissue carrying blood vessels and nerves that separates the skin from the perichondrium. This becomes relevant when pursuing reconstructive options as one must consider the fascial layer in addition to the cutaneous layer.
Sensory innervation of the auricle is variable as was demonstrated by Peuker and Filler.
The lobule, antitragus, scapha, superior and inferior crurae, and posterior helix are almost uniformly innervated by the great auricular nerve. The great auricular nerve overlaps with the auriculotemporal nerve in most cases to provide innervation to the tragus. The auriculotemporal nerve is the predominant nerve supply for the anterior helix and the root of the helix. The auricular branch of the vagus nerve also provides innervation to the antihelix the majority of the time. However, the auricular branch of the vagus nerve has accessory innervation roles in the superior and inferior crurae and contributes to innervation of the middle and lower one-third of the cranial (posterior) surface. The final contribution comes from the lesser occipital nerve. The lesser occipital nerve contributes to the innervation of the upper third and middle third of the cranial surface. Common innervation is illustrated in Figure 5.
Figure 5Sensory innervation to the auricle is demonstrated earlier in the anterior (A) and posterior (B) views. The great auricular nerve (GAN) auricular temporal nerve (ATN) and auricular branch of the vagus nerve dominate anterior innervation. There is overlap between the ATN and GAN along the antihelix, tragus, triangular fossa, and superior and inferior crura as demonstrated by the dots of color. Additional dots of color demonstrate the variable contribution of the facial nerve to the concha, which is mostly innervated by the auricular branch of the vagus nerves (ABVN). The dominant posterior innervation is from the GAN and the lesser occipital nerve (LON). There are overlapping contributions from the ABVN. (Color version of figure is available online.)
The topographic landmarks are important for creating the distinctive look of the human ear. Their names and relationships are important for describing anomalies and defects and must be considered during reconstruction. The most external portion of the auricle is the helix, which can further be divided into 4 sections: the root, anterior helix, superior helix, and posterior helix, depicted in Figure 6. At the inferior aspect of the helix is the lobule, the only part of the auricle that does not contain any cartilage but rather is made up of skin and fibrofatty tissue. Working from external to internal the next section of the auricle is the antihelix. The antihelix can be divided into 3 parts: the body, superior crus, and inferior crus. Inferiorly the body blends with the antitragus, which opposes the tragus and the root of the helix on the anterior portion of the auricle (Figure 6).
Figure 6Anterior and posterior surface anatomy of the auricle. Anterior (A) and posterior (B) anatomic landmarks of the auricle. (Color version of figure is available online.)
The cartilaginous components of the anterior auricle consist of named depressions or fossae outlined by prominences. The scaphoid and triangular fossae are posterior and anterior to the superior crus of the antihelix, respectively. The middle fossa is the concha, which is divided into the concha cymba, superior to the root of the helix, and the concha cavum inferiorly. More inferiorly the intertragal notch is posterior to the tragus and anterior to the antitragus (Figure 6). On the posterior surface of the auricle are the scaphoid, triangular, and conchal eminences, whose names parallel the corresponding fossa on the anterior surface of the auricle. The superior and inferior crura on the anterior surface are complemented by the superior and inferior grooves on the posterior surface. The ponticulus, a vertical ridge which crosses the conchal eminence, can also be seen in approximately 66% of ears.
The muscles of the auricle are divided into intrinsic and extrinsic (Figure 7). The 3 extrinsic muscles include the superior, anterior, and posterior auricularis muscles. The 6 intrinsic muscles are considered vestigial but may play a role in modifying the shape of the auricle. The intrinsic muscles include the helicis major and minor, tragicus, antitragicus, traversus auriculae, and obliquus auriculae. Details of the origins, insertions and innervations of the auricular muscles can be found in Table 3.
Figure 7Anterior and posterior view of the extrinsic and intrinsic muscles of the auricle. Anterior (A) and posterior (B) views of the muscles of the auricle. The auricularis anterior, superior ans posterior muscles are the extrinsic muscles of the auricle. The intrinsic muscles include the helicis major and minor, tragicus, antitragicus, traverus auriculae, and obliquus auriculae. All muscles of the auricle are innervated by the facial nerve. (Color version of figure is available online.)
Overall, the average height of the adult auricle is approximately 58 mm for females and 62 mm for males. Projection of the auricle is defined as the distance from the posterior region of the middle helix to the mastoid skin and the normal range is 15-21 mm in both males and females.
The ear develops in a predictable manner, with various alterations in development resulting in predictable deformities when the process is interrupted. Knowledge of auricular development as well as normal external anatomy allows for precise evaluation of the ear and assists in selecting appropriate reconstructive techniques to optimize the complex 3-dimensional anatomical outcomes that will be discussed in the following articles.
Disclosures
The authors report no proprietary or commercial interest in any concept discussed in this article.
in: Johnson J.T. Rosen C.A. Bailey B.J. Bailey’s Head and Neck Surgery-Otolaryngology. Wolters Kluwer Health/Lippincott Williams & Wilkins,
Philadelphia (PA)2014
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