The development of the mammalian outer and middle ear

Abstract

The mammalian ear is a complex structure divided into three main parts: the outer; middle; and inner ear. These parts are formed from all three germ layers and neural crest cells, which have to integrate successfully in order to form a fully functioning organ of hearing. Any defect in development of the outer and middle ear leads to conductive hearing loss, while defects in the inner ear can lead to sensorineural hearing loss. This review focuses on the development of the parts of the ear involved with sound transduction into the inner ear, and the parts largely ignored in the world of hearing research: the outer and middle ear. The published data on the embryonic origin, signalling, genetic control, development and timing of the mammalian middle and outer ear are reviewed here along with new data showing the Eustachian tube cartilage is of dual embryonic origin. The embryonic origin of some of these structures has only recently been uncovered (Science, 339, 2013, 1453; Development, 140, 2013, 4386), while the molecular mechanisms controlling the growth, structure and integration of many outer and middle ear components are hardly known. The genetic analysis of outer and middle ear development is rather limited, with a small number of genes often affecting either more than one part of the ear or having only very small effects on development. This review therefore highlights the necessity for further research into the development of outer and middle ear structures, which will be important for the understanding and treatment of conductive hearing loss.

Keywords: Eustachian tube cartilage; development; embryonic origin; external auditory meatus; middle ear; ossicles; outer ear; tympanic membrane.

© 2015 Anatomical Society.

Figures

Figure 1

Schematic illustrating the structure of the ear, which is divided into three parts: the outer; middle; and inner ear. The outer ear includes the pinna and ear canal. The middle ear is between the tympanic membrane (eardrum; ty) and stapes (s), which is inserted in the oval window (ow). The middle ear is connected to the back of the mouth via the Eustachian tube. The inner ear converts the sound waves to electrical signals. ALM, anterior ligament of the malleus; EAM, external auditory meatus; I, incus; M, malleus; mn, manubrium; oa, orbicular apophysis; PLI, posterior ligament of the malleus; rw, round window; s, stapes; SLM, superior ligament of the malleus; st, stapedius; Tt, tensor tympani.

Figure 2

Schematic illustrating the early set‐up of the ear. (A) The pharyngeal arches are filled by neural crest streams; the first pharyngeal arch (PA1) is filled with neural crest cells from the midbrain (M), rhombomere 1 (r1) and rhombomere 2 (r2), while the second pharyngeal arch (PA2) is filled with crest predominantly from rhombomere 4 (r4). The ear is derived from the tissues of PA1 and PA2. (B) The first and second pharyngeal arches are divided internally by the endodermal first pharyngeal pouch (1pp) and externally by the ectodermal first pharyngeal cleft (1pc). (c) The middle ear is a composite structure derived from mixed embryonic origin, as seen in this schematic of an embryonic mouse middle ear. EAM, external auditory meatus; g, gonial; I, incus; M, malleus; mm, manubrium of the malleus; o, orbicular apophysis; ow, oval window; s, stapes; Tr, tympanic ring.

Figure 3

The development of the external auditory meatus (EAM) in mice. The development of the EAM in the mouse seen in frontal sections at E13.5 (top), E15.5 (middle) and P11 (bottom). The EAM (black arrow) extends towards the tubotympanic recess (white arrow) as an open structure at E13.5. By E15.5 this has collapsed to form a meatal plug (black arrow, and enlarged box) close to the tubotympanic recess (white arrow). At P11, the ear canal is opening and keratin squames can be seen within it (black arrow head). mec, middle ear cavity.

Figure 4

The glandula ceruminosa of the mouse. Trichrome‐stained frontal sections of P6 mouse outer ear from caudal to rostral, showing: (A) a lower power image for orientation; (B) the ceruminosa gland duct (black arrow) where it joins to the EAM (white arrows), at its heel; (C) the large duct of the ceruminosa gland (black arrow); and (D) the gland itself (black outline). me, middle ear; oe, outer ear.

Figure 5

Eardrum development in the mouse. (a) Schematic of the eardrum with pars flaccida and pars tensa. (b) Schematic showing the pars flaccida (dorsal) and pars tensa (ventral) within the ear. oe/ec, outer ear/ear canal; mec, middle ear cavity.

Figure 6

Development of the malleus and incus from Meckel's cartilage in the mouse. (A, B) Alcian blue‐stained section of Meckel's cartilage at E13.5 (A) and E14.5 (B). (C) Expression of the joint marker, Gdf5 on a sagittal section at E15.5, showing expression in the joint region between the malleus and incus. (D, E) Alcian blue‐ and Alizarin red‐stained skeletal preparations of the caudal end of Meckel's cartilage, showing the connection to the malleus at P0 (D) and its separation at P3 (E). (F) Schematic representing the separation of the ossicles, between E13.5 and P3. At E13.5 (A, F), the malleus and incus are still fused to the caudal end of Meckel's cartilage. At E14.5 (B, C, F), the malleus and incus have separated (arrow), and the joint marker Gdf5 is expressed in the joint region between the malleus and incus at E15.5 (arrow). At P0 (D, F), Meckel's cartilage is still fused to the malleus (arrow). At P3 (E, F), the malleus has separated from Meckel's cartilage (arrow). I, incus; M, malleus; mk, Meckel's cartilage; S, stapes. (A–C taken from Amin & Tucker, 2006, © 2006 Wiley‐Liss, Inc; D–E taken from Anthwal et al. 2013, © 2012 The Authors. Journal of Anatomy © 2012 Anatomical Society).

Figure 7

The embryonic origin of the stapes. (A) Alcian blue‐stained stapes at P6, showing it is completely cartilage at this stage. (B) X‐Gal‐stained stapes from a P10 Wnt1cre/R26R (Danielian et al. 1998; Soriano, 1999) mouse showing part of the footplate as unstained (arrow). (C) X‐Gal‐stained stapes from a P10 Mesp1cre/R26R (Saga et al. 1999; Soriano, 1999) mouse showing part of the footplate as stained (arrow). (D) Schematic illustrating the stapes in the oval window and the orientation of sectioning of (E) and (F). (E) Frontal section through X‐Gal‐stained stapes from a Wnt1cre/R26R mouse showing the stained and unstained regions of the footplate and the unstained annular ligament (arrowheads). (F) Frontal section through X‐Gal‐stained stapes from a Mesp1cre/R26R mouse showing the stained and unstained regions of the footplate and the stained annular ligament (arrowheads). (G) Schematic illustrating the embryonic origin of the stapes in the oval window. Most of the stapes is of neural crest origin (green), while the outer part of the stapedial footplate, the annular ligament (arrowheads) and the otic capsule surrounding the oval window is of mesodermal origin. c, crus of stapes; fp, footplate of stapes; ie, inner ear; m, mesodermal footplate of stapes; me, middle ear; oc, otic capsule; sh, head of stapes. (A–F taken from Thompson et al. 2012).

Figure 8

A schematic illustrating the cavitation of the mouse middle ear. (A) Representing ~E15.5. The first pharyngeal pouch extends towards the developing middle ear structures. (B) Representing ~ E17.5 where the tubotympanic recess ruptures and mesenchyme enters the distal end of the cavity. (C) Representing ~P6, the structures of the middle ear cavity have grown and developed, the ruptured endoderm is situated towards the edge of the future cavity and the cavity is filled with mesenchyme. (D) Representing ~P11, the middle ear mesenchyme is regressing away from the future tympanic membrane. (E) Representing ~P13, the middle ear cavity is clear up to the level of the ossicles in the attic region of the cavity. (F) Representing ~P16, the middle ear cavity is clear of mesenchyme and the neural crest cells have undergone a mesenchyme to epithelial transformation to form a continuous epithelial lining of the cavity. 1pp, first pharyngeal pouch; EAM, external auditory meatus; I, incus; M, malleus; s, stapes.

Figure 9

The Eustachian tube cartilage is of dual origin. Frontal sections of a Wnt1cre/R26R (Danielian et al. 1998; Soriano, 1999) P16 mouse stained with X‐Gal. The Eustachian tube cartilage (red dotted line), surrounding the dorsal Eustachian tube (black dash lined), is stained blue (arrow) at its dorsal‐most tip, as it is neural crest‐derived. However, the majority of the cartilage is unstained and therefore of mesodermal origin. (A) shows the Eustachian tube (ET) opening into the middle ear cavity (mec) and (B) a more caudal section.