Ear

Huff, Dale S.

doi:10.1007/978-3-030-11425-1_31

Dale S. Huff⁵

2656 Accesses

Abstract

The ear is often involved in the major categories of causes of intrauterine diseases and deaths, such as congenital anomalies with or without chromosomal or gene defects, infections, perfusion abnormalities of the placenta, and congenital tumors. Therefore, a working knowledge of the normal anatomy and histology of the ear is important in the study of fetal tissues. This chapter is a comprehensive review of the anatomy, embryology, and histology of the various compartments of the external, middle, and inner ear.

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Congenital Malformations of the Ear

External, Middle, and Inner Ear

Common Ear Diseases

Keywords

Introduction

The ear is often involved in intrauterine and neonatal diseases such as congenital anomalies, chromosomal and gene defects, amniotic fluid infection sequence, transplacental infections, maternal-placental and fetal-placental perfusion defects, other causes of fetal hypoxia, congenital tumors, and other diseases. Therefore, histologic examination of the ear should be an important part of the perinatal autopsy. However, because the anatomy of the ear is complex, because the removal and plane of sectioning of the petrous bone are often haphazard, and because the specimen is often fragmented or incomplete, interpretation of routine microscopic sections of the ear is often exasperating and unhelpful. As a consequence, histologic examination of the ear is often neglected in perinatal autopsies. Understanding the anatomy of the ear, standardizing the method of removal and sectioning of the petrous bone, and making examination of the ear a routine part of the perinatal autopsy will make histologic examination of the ear more satisfying and fruitful. The purpose of this chapter is not only to demonstrate the histology of the ear but also persuade the reader that histologic examination of the ear is a worthwhile part of perinatal autopsy.

Anatomy

In Situ Surface Anatomy of the Petrous Bone in the Neonate

The inner and middle ear and part of the external ear lie within the petrous bone . The surface anatomy of the neonatal petrous bone serves two functions for the perinatal pathologist. First, understanding the surface anatomy allows the pathologist to develop a mental picture of the position and relationships of the components of the inner, middle, and external ears within the petrous bone. Second, the surface landmarks provide guideposts for the placement of resection lines for the removal of the petrous bone during the perinatal autopsy. This section describes those surface landmarks .

The petrous bone is an oblong wedge obliquely oriented in the floor of the cranium. When viewed in situ from above in the opened cranium, it presents two surfaces. The superior horizontal surface is the roof of the petrous bone and forms much of the floor of the middle cranial fossa. The posterior vertical wall attaches to the posterior edge of the superior wall and drops vertically downward, forming a right angle with the superior wall. The posterior wall of the petrous bone forms the anterior wall of the posterior cranial fossa. The angle formed by the junction of the superior horizontal and posterior vertical walls is the ridge of the petrous bone. The petrous bone is oriented obliquely in an anteromedial to posterolateral direction. The anteromedial end, which is narrower, abuts against the lateral wall of the sella turcica. The posterolateral end, which is broader, abuts against the inside of the lateral wall of the cranium near the posterior end of the squamous portion of the temporal bone. The anteromedial end of the petrous ridge is sharp; the posterolateral end is more rounded. In the neonate, the boundaries of the cranial bones are sharply outlined by the wide-open sutures. The superior horizontal surface, the posterior vertical surface, the petrous ridge, and the open sutures are surface landmarks that aid in identification of the petrous bone at autopsy (Fig. 31.1) [1, 2].

The arcuate eminence and the subarcuate fossa are additional landmarks on the superior surface. The arcuate eminence is a curved ridge on the superior surface. It contains the superior semicircular canal and duct. The subarcuate fossa is a blind-ending depression that extends under the arcuate eminence. It is lined with dura, which transmits the subarcuate artery to the inner ear. The tegmen tympani, though not delineated by surface landmarks, is an important area to visualize. It is a very thin area of the superior horizontal wall that forms the roof of the middle ear. Because it is so thin, it can easily be removed to unroof the middle ear and expose the epitympanic recess and the other structures within those cavities (Fig. 31.1).

The internal acoustic meatus, jugular fossa, ostium of the cochlear aqueduct, and the orifice of the vestibular aqueduct are additional landmarks located from anteromedial to posterolateral along the posterior wall. The internal acoustic meatus is the most anterior landmark. It is the orifice of the internal acoustic canal through which the eighth cranial nerve passes from the posterior fossa to the inner ear. The base of the cochlea lies adjacent to the internal end of the canal. The jugular fossa is important because it marks the site of the ostium of the cochlear aqueduct, which is tucked under the inferior edge of the petrous bone at the apex of the jugular fossa and is therefore not visible on the surface of the petrous bone. It contains the periotic duct. The orifice of the vestibular aqueduct is hidden in a depression in the posterior wall posterior to the jugular fossa. It contains the endolymphatic sac. The position of the cochlea can be estimated from the location of the internal acoustic meatus. The position of the saccular dilatation of the vestibule can be estimated from the location of the ostium of the cochlear aqueduct. The position of the utricular dilatation of the vestibule can be estimated from the location of the ostium of the vestibular aqueduct. The arcuate eminence marks the exact position of the superior semicircular canal and duct (Figs. 31.1 and 31.2).

Arrangement of the Inner, Middle, and External Ears Within the Petrous Bone

The spatial relationships between the petrous bone, inner ear, and middle ear are intricate (Figs. 31.2 and 31.3). They are parallel to each other, with the middle ear lateral to the inner ear. They are obliquely oriented from anteromedial to posterolateral. They extend from the lateral wall of the sella turcica anteromedially to the posterior end of the squamous portion of the temporal bone posterolaterally. The inner ear consists of an outer osseous labyrinth, which is also called the otic capsule , and the membranous labyrinth, which lies within the otic capsule (Figs. 31.3, 31.4, 31.5, 31.6, 31.7, and 31.8). The auditory and vestibular sensory end organs lie within the membranous labyrinth (Figs. 31.8, 31.9, 31.10, 31.11, 31.12, and 31.13). In addition, the otic capsule, membranous labyrinth, and middle ears are divided into three segments from anterior to posterior. The three segments of the otic capsule (osseous labyrinth) from anterior to posterior are the cochlea, vestibule, and semicircular canals. The vestibule is further divided from anterior to posterior into the saccular dilatation and the utricular dilatation (see Figs. 31.4 and 31.6). The analogous segments of the membranous labyrinth from anterior to posterior are the cochlear duct within the cochlea, the saccule within the saccular dilatation of the vestibule, the utricle within the utricular dilatation of the vestibule, and the semicircular ducts within the semicircular canals (see Figs. 31.5 and 31.7). The analogous segments of the middle ear are the anterior part, which is anterior to the tympanic membrane, lateral to the cochlea, and gives rise to the Eustachian tube from its anterior end; the middle part or isthmus, which is lateral to the vestibule and whose entire lateral wall is the tympanic membrane; and the posterior part, which is posterior and superior to the tympanic membrane, lies lateral to the semicircular canals, and is further subdivided from anterior to posterior into the epitympanic recess, the mastoid antrum, and the mastoid air cells. The inner and middle ears are so intimately pressed together that the basal spiral of the cochlear duct bulges into the tympanic isthmus anteriorly, forming the promontory on its medial wall. The isthmus of the tympanic cavity and the vestibule share a common boney wall. The lateral semicircular canal bulges into the epitympanic recess posteriorly (see Figs. 31.2 and 31.3).

The surface anatomy of the petrous bone of the neonate is different from that of the adult. As the bones mature, the surface landmarks change. For example, the sutures close, making the borders of the petrous bone less clear. The petrous ridge becomes sharper throughout its entire length. The arcuate eminence and subarcuate fossa are smoothed over and become barely visible. The orifice of the vestibular aqueduct becomes hidden in a linear crevice that is higher on the posterior wall in the adult than in the neonate. The changing anatomy of the petrous bone from the embryo to the adult has been described previously [2]. Compare the petrous bone of the neonate in Fig. 31.1 with that of the adult in any atlas of adult anatomy [3]. Knowledge of the changing anatomy of the fetal and neonatal petrous bone will aid the perinatal pathologist in its identification and removal at autopsy.

External Auditory Canal

The wall of the external two thirds of the external auditory canal is composed of soft tissue and cartilage. The wall of the inner one third passes through the temporal bone and is therefore composed of bone (see Fig. 31.3) [3, 4]. The bony tympanic ring surrounds the inner end of the bony canal. The tympanic membrane is attached to a groove in the tympanic ring.

Tympanic Membrane

The tympanic ring and the attachment of the tympanic membrane to it are tilted so that the posterior and inferior attachments are much deeper than the anterior and superior attachments. The tilt is so marked that the tympanic membrane is nearly parallel to the posterior-inferior wall of the external auditory canal. The tympanic membrane is the medial wall of the external auditory canal and the lateral wall of the tympanic cavity. It therefore is a common wall between the external auditory canal and the tympanic cavity. Because it is attached to the inner end of the bony external auditory canal, attempts to remove the middle and inner ears by an incision through the petrous portion of the temporal bone flush with the inner surface of the temporal bone often result in destruction of the tympanic ring, tympanic membrane, and the middle ear.

Middle Ear

The tympanic cavity is a long, narrow, rectangular cavity. It lies parallel to the full length of the lateral surface of the inner ear (see Figs. 31.2 and 31.3) [5]. The anterior end is anterior to the tympanic membrane. Part extends below the lower border of the tympanic membrane, especially anteriorly, and is called the hypotympanum . The anterior end is adjacent to the cochlea of the inner ear. The opening of the Eustachian tube is in the anterior-inferior aspect of the medial wall adjacent to the cochlea. The middle part is adjacent to the vestibule of the inner ear. It is the main chamber of the middle ear and is sometimes referred to as the isthmus because it is narrower than the anterior and posterior ends. Its lateral wall is the tympanic membrane. The tympanic membrane is therefore a common wall between the external auditory canal and the tympanic cavity. The isthmus contains the handle of the malleus attached to the tympanic membrane, the long process of the incus, and the stapes. The medial bony wall of the isthmus is the lateral wall of the vestibule. The lateral wall of the vestibule is therefore a common wall between the tympanic cavity and the vestibule. There are two windows in the medial wall: the round window, located anterior-inferiorly, and the oval window, located posterior-superiorly. The round window lies immediately behind the orifice of the Eustachian tube and is closed by a delicate membrane called the secondary tympanic membrane, which separates the tympanic cavity from the scala tympani on the vestibular side of the window. The oval window is located posterior and superior to the round window. The foot plate of the stapes fills the oval window. The radial ligament attaches the edges of the foot plate to the edges of the oval window. The oval window separates the tympanic cavity from the scala vestibuli on the vestibular side. The base of the cochlea pushes the bony wall toward the tympanic cavity, producing a bulge in the wall between the round and oval windows, which is called the promontory . The posterior part is a recess in the posterior roof of the tympanic cavity, which forms an attic-like space superior and posterior to the upper rim of the tympanic membrane (see Figs. 31.2 and 31.3). This space is the epitympanic recess. It lies adjacent to the semicircular canals and contains the head of the malleus and the body and short process of the incus. The roof of the epitympanic recess is a thin region of the bony floor of the middle fossa called the tegmen tympani (see Figs. 31.1, 31.2, and 31.3). The tympanic cavity can easily be unroofed by removing the thin tegmen tympani. The wall of the lateral semicircular canal bulges into the epitympanic recess. An aperture in the posterior wall of the epitympanic recess, the mastoid aditus, leads posteriorly into the mastoid antrum and from there to the mastoid air cells, which are part of the middle ear . In most fetuses and neonates, the mastoid antrum is the most posterior part of the middle ear because the mastoid air cells, which are the most posterior part in adults, usually do not develop until late fetal or postnatal life (see Fig. 31.2).

The Eustachian tube exits from the anterior-inferior aspect of the medial wall of the tympanic cavity anterior to and below the round window, passes under the cochlea, and opens into the upper nasopharynx. In the fetus and infant, the tube is oriented horizontally (see Fig. 31.3). The tympanic cavity and mastoid air cells are in direct continuity with the outside environment through the Eustachian tube. Like the lung, before birth the middle ear contains amniotic fluid; after birth it contains air. The horizontal orientation of the Eustachian tube in the fetus and infant makes entry of amniotic fluid or nasopharyngeal fluid into the middle ear very easy. If those fluids are infected, intrauterine or neonatal otitis media and sepsis will likely follow. The spectrum of aspiration of amniotic fluid and the amniotic fluid infection sequence affects not only the lung but also the middle ear.

Inner Ear

Osseous Labyrinth

The inner ear is an irregular, oblong cavity in the petrous portion of the temporal bone. It is surrounded by an osseous capsule called the osseous labyrinth , bony labyrinth , or otic capsule (see Figs. 31.4 and 31.6) [6]. These three terms are used interchangeably. The osseous labyrinth has three interconnected parts (from anteromedial to posterolateral): the cochlea, which is a snail-shaped spiral tube joined to the anterior aspect of the vestibule; the vestibule, which is the central chamber; and the three semicircular canals, which are joined to the posterior aspect of the vestibule. The common cavity of the osseous labyrinth is the perilymphatic space, which is filled with fluid, the perilymph. The base of the snail-shaped cochlea faces posteriorly and slightly medially and lies at the inner opening of the internal auditory meatus. The apex or cupola of the “snail” points anteriorly and slightly laterally. The cochlea has a bony central core, the modiolus, which has been likened to a screw with the head of the screw at the base of the cochlea and its tip at the apex. The spiral lamina of the cochlea is analogous to the thread of the screw, and it projects like a shelf into the lumen of the cochlear canal along its equator. Its tip reaches the center of the canal. The spiral lamina forms part of the floor or base of the triangular cochlear duct. The vestibule presents two dilatations: the saccular dilatation anteriorly at the junction of the vestibule with the cochlea and the utricular dilatation posteriorly at the junction of the vestibule with the semicircular canals (see Figs. 31.4 and 31.6). The anatomy of the lateral wall of the vestibule, round window, promontory, and oval window is described above in the description of the middle ear. Three openings in the posterior wall of the vestibule lead to canals through the posterior wall of the petrous bone. The three canals are the internal auditory canal (also called the internal acoustic canal ), the cochlear aqueduct (also called the cochlear canaliculus ), and the vestibular aqueduct (also called the vestibular canaliculus ) (see Figs. 31.1, 31.2, and 31.4). The most anterior is the internal auditory canal. Its opening in the osseous labyrinth is at the base of the spiral cochlea. Cranial nerves VII (facial) and VIII (vestibuloacoustic) pass through the canal and exit through its foramen, the internal auditory meatus (also called the internal acoustic meatus ), in the posterior wall of the petrous bone. The opening of the cochlear aqueduct in the osseous labyrinth is in the wall of the scala tympani at the junction of the cochlea with the vestibule, and it exits the posterior inferior wall of the petrous bone into the subarachnoid space at the apex of the jugular foramen, creating a direct communication between the perilymphatic and subarachnoid spaces. It contains the periotic duct and a fluid that may be perilymph or subarachnoid fluid. It has no connection with the membranous labyrinth [7]. The most posterior is the vestibular aqueduct. Its opening in the osseous labyrinth is in the utricular dilatation of the vestibule, near its junction with the semicircular canals. It exits through a slit-shaped foramen in the posterior wall of the petrous portion of the temporal bone posterolateral to the jugular foramen (see Figs. 31.1 and 31.2). It contains the endolymphatic duct, which is a tube of membranous labyrinth that arises from the utriculosaccular duct and ends blindly in the dilated endolymphatic sac in the epidural space (see Figs. 31.1, 31.2, 31.5, and 31.7). It contains endolymph. The posterior segment of the osseous labyrinth is composed of the three semicircular canals: anterior (also called superior), posterior, and lateral. Both ends of each canal join the utricle. The three anterior ends are dilated; they are called the ampullae at their sites of union with the posterior-superior aspect of the vestibule. The posterior limbs of the anterior and posterior canals join to form the common crus. Therefore, there are only two openings into the posterior aspect of the utricle for the posterior limbs: one for the common crus and one for the lateral canal (see Figs. 31.4 and 31.6).

Membranous Labyrinth

The membranous labyrinth is a closed system of epithelial ducts and sacs derived from the ectodermal otic vesicle. It lies within the cavity of the osseous labyrinth (see Figs. 31.3, 31.5, and 31.7). It is composed of three major parts (from anteromedial to posterolateral): the cochlear duct within the bony cochlea; the saccule and utricle, which are two sacs within the saccular and utricular dilatations of the vestibule; and the three semicircular ducts within the bony semicircular canals. The cochlear duct begins as a blind end in the vestibule adjacent to the promontory and below the saccule. It follows the osseous cochlear spiral to its apex, the cupola, where it ends blindly. The cochlear duct therefore has two blind ends. One abuts the bony promontory in the vestibule below the saccule. The other lies at the apex of the spiral cochlea (see Figs. 31.3, 31.5, 31.7, and 31.8). The saccule is located anteriorly within the saccular dilatation of the vestibule, above the blind end of the cochlear duct and below and in front of the utricle. It is oval in shape and smaller than the utricle. The utricle is located posteriorly in the utricular dilatation of the vestibule, above and behind the saccule. A blind-ended recess of the utricle, the anterior recess, arises near the anterior opening of the lateral semicircular duct. This recess extends horizontally over the roof of the saccule (see Fig. 31.7) [8]. The three semicircular ducts empty into the posterior-superior aspect of the utricle. One end of each forms a dilated ampule at its site of insertion into the utricle. The three components of the membranous labyrinth are interconnected by small ducts. The ductus reuniens connects the cochlear duct to the saccule (see Figs. 31.3 and 31.7). The orifice of the ductus reuniens in the cochlear duct is some distance from its blind end. The segment of the cochlear duct between its blind end and the orifice of the ductus reuniens is called the cecum of the cochlea (see Figs. 31.3 and 31.7). The utriculosaccular duct connects the saccule with the utricle. The endolymphatic duct arises from the utriculosaccular duct (see Figs. 31.5 and 31.7). The posterior segment of the membranous labyrinth is composed of the three semicircular ducts inside the semicircular canals. Each has an ampulla, and each necessarily follows the course of its corresponding canal. Unlike the canals that join with the vestibule, the semicircular ducts connect to the utricle. The various parts of the entire membranous labyrinth are interconnected, from the cochlear duct anteromedially to the semicircular canals posterolaterally (see Figs. 31.3, 31.5, and 31.7).

Perilymphatic Space

The perilymphatic space completely surrounds the membranous labyrinth and separates the osseous labyrinth from the membranous labyrinth. Like the osseous labyrinth, membranous labyrinth, and middle ear, it can be divided into three parts, from anteromedial to posterolateral. The anterior part, in the cochlea, is composed of two parts: the scala vestibuli and the scala tympani. The membranous cochlear duct divides the spiral bony cochlear tube into three tubes: the triangular cochlear duct (scala media) in the middle; the scala vestibuli, above the cochlear duct adjacent to Reissner’s membrane; and the scala tympani, below the cochlear duct adjacent to the basal membrane (see Figs. 31.3 and 31.8). The lumens of the scala vestibuli and scala tympani become continuous with each other at the apex of the spiral, through an aperture called the helicotrema. The scala tympani ends blindly at the round window. The scala vestibuli joins the perilymphatic space of the vestibule at the oval window. The perilymphatic space of the vestibule is the cisterna vestibuli. It surrounds the saccule and utricle and is continuous with those of the semicircular canals (see Fig. 31.3).

Periotic Duct

The periotic duct is a direct communication between the scala tympani part of the perilymphatic space and the subarachnoid space of the posterior fossa. The duct lies within the cochlear aqueduct, a bony canaliculus in the otic capsule and posterior wall of the petrous bone (see Figs. 31.1, 31.2, and 31.4). (The otic capsule is described above and in the Embryology and Histology sections below).

Auditory and Vestibular Sensory Organs

Two systems of sensory end organs are present in the inner ear: the cochlear system, which detects sound, and the vestibular system, which detects motion and gravity. The cochlear auditory sensory end organ is the organ of Corti, which is a strip of neurosensory epithelium located in the basal membrane (the floor) of the entire length of the spiral cochlear duct (see Fig. 31.8).

The vestibular system is composed of two groups of neurosensory epithelial end organs . The first group is composed of two otolithic maculae, the saccular macula and the utricular macula, which detect linear motion. The two otolithic maculae are an interrelated pair. The saccular otolithic macula is a thickened ellipsoid plaque of neurosensory epithelium in the epithelial lining of the vertical wall of the saccule, which lies below the anterior recess of the utricle. Its flat surface is oriented vertically, and its long axis is oriented anteroposteriorly (Fig. 31.9) [5, 9]. It detects vertical linear motion and gravity. The utricular otolithic macula is a thickened ellipsoid plaque of neurosensory epithelium in the epithelium of the horizontal floor of the anterior recess of the utricle. Its flat surface lies horizontally, and its long axis is oriented anteroposteriorly (Figs. 31.9, 31.10, and 31.11). It lies immediately above and perpendicular to the vertically oriented saccular macula [5, 10]. It detects horizontal linear motion.

The second group is the three ampullary cristae or ridges of sensory epithelium , one in each of the three ampullae of the semicircular ducts. The three ampullary cristae , an interrelated trio, are linear ridges of thickened neurosensory epithelium oriented transversely to the long axis of the ducts. They extend only partially around the inner circumference of the ampullae (see Figs. 31.9, 31.12, and 31.13). Each is oriented in a plane different from the others. The cristae detect angular motion.

Embryology

Both the epithelial and connective tissues of the ear are derived from two closely related embryologic primordia, both of which arise within the ectodermal ring [11, 12]. The inner ear is derived from the otic placode, which is an ectodermal thickening within the rostral segment of the ectodermal ring. The external and middle ears are derived from the first and second pharyngeal complexes, which arise from the pharyngeal segment of the ectodermal ring (Table 31.1).

Table 31.1 The embryological origins of the external, middle, and inner ears

Full size table

Embryonic development of the ear begins early in the sixth postmenstrual week with the appearance first of the otic placode (the future membranous labyrinth of the inner ear), followed by the first and second pharyngeal arches (the future auricle), the first pharyngeal cleft (the future external auditory canal), and the first pharyngeal pouch, also referred to as the tubotympanic recess (the future tympanic cavity and Eustachian tube). All appear early in the sixth week. During the rest of the sixth week, the first pharyngeal cleft and first pharyngeal pouch grow toward each other, but their distal ends remain separated by a layer of mesenchyme from the arches. The distal ends of the cleft and pouch and the intervening mesenchyme are the first pharyngeal membrane, which will become the future tympanic membrane. The primordia of the ossicles condense within the mesenchyme of the first pharyngeal membrane and the mesenchyme adjacent to the first pouch: first the malleus, in the mesenchyme of the first pharyngeal arch, located in first pharyngeal membrane; followed by the incus, in the mesenchyme of the first arch, adjacent to the first pharyngeal pouch; then the stapes, in the mesenchyme of the second arch, adjacent to the first pouch. Meanwhile, the otic placode develops into the otic pit. A layer of mesenchyme condenses around the epithelial pit. The mouth of the pit becomes narrow, and the pit becomes the otic vesicle. The epithelium of the otic pit becomes the membranous labyrinth of the inner ear. The condensed mesenchyme surrounding the epithelial pit becomes the osseous labyrinth of the inner ear. These primordia of the auricle, external auditory canal, tympanic membrane, ossicles, tympanic cavity, Eustachian tube, and membranous and osseous labyrinths are established by the end of the sixth week.

From the beginning of the seventh week to the end of the tenth week (the end of the embryonic period), the final form of all of the organs develops from these primordia. The following events are occurring simultaneously during the last 4 weeks of the embryonic period.

The auricle is completely formed by the middle of the tenth week. The first pharyngeal cleft becomes the external auditory canal. In the tenth week, the lining epithelium of the canal proliferates and forms a plug, the meatal plug, which completely occludes the lumen of the distal third of the canal [5, 13].

The tympanic end of the tubotympanic recess enlarges to become the tympanic cavity, and its pharyngeal end becomes the Eustachian tube. The head of the malleus forms in the mesenchyme of the future tympanic membrane in association with the posterior end of the cartilage of the first arch, Meckel’s cartilage . The body and short process of the incus form adjacent to the wall of the tympanic cavity, also in association with the cartilage of the first arch, Meckel’s cartilage . The head of the malleus and the body and short process of the incus come to lie in the epitympanic recess. The handle of the malleus arises from the mesenchyme of the second arch in association with posterior end of the cartilage of the second arch, Reichert’s cartilage , and remains attached to the tympanic membrane. The long process of the incus and the head and limbs of the stapes also form in association with the cartilage of the second arch, Reichert’s cartilage, and come to lie within the main part of the tympanic cavity, the isthmus. The footplate of the stapes arises from the otic capsule and fills the oval window. The parts of the ossicles derived from the first arch are located in the epitympanic recess, whereas the parts derived from the second arch are located in the isthmus of the tympanic cavity. By the end of the tenth week, the mesenchymal ossicles have nearly attained their adult morphology and have begun to convert to cartilage. The diarthrodial synovial incudomalleolar and incudostapedial joints are formed, and the stapediovestibular joint is formed by the radial ligament. The enlarging tympanic cavity envelops the ossicles and the mesenchyme that surrounds them, so that they lie within the tympanic cavity, and the tympanic cavity is filled with the primitive mesenchyme.

The development of the inner ear is even more complex [11, 12]. The otic vesicle detaches from the surface ectoderm and comes to lie in its final position, medial to the developing tympanic cavity. The epithelium of the otic vesicle is thick, multilayered, and composed of small, oval, basophilic, undifferentiated cells with the appearance of pluripotent placodal cells. This histology remains unchanged until the epithelium begins to differentiate during the tenth week. During the seventh week, the round otic vesicle elongates in the ventral and dorsal directions. The ventral elongation will become the cochlear duct; the dorsal elongation, the endolymphatic duct. During the eighth week, the semicircular ducts appear as three hollow plates at the base of the endolymphatic duct. The epithelium in the center of the plates fuses, leaving the patent semicircular ducts at the periphery of the plates. By the end of the eighth week, the fused epithelium in the center of the plates disappears, and one end of each duct dilates at its point of insertion into the utricle, forming the ampullae of the semicircular ducts. The otic vesicle between the developing semicircular ducts and the cochlear duct forms two dilatations. One, the saccule, is located anteriorly, above the developing cochlear duct. The other, the utricle, is located above and behind the saccule. Both ends of each semicircular duct enter the utricle. The end of the endolymphatic duct enlarges into the endolymphatic sac. The elongating cochlear duct begins to spiral. The connection between the saccule and the cochlear duct becomes a narrow tube called the ductus reuniens . Its origin from the cochlear duct is some distance from the blind end of the cochlear duct, which is called the cecum of the cochlear duct. The connection between the utricle and saccule becomes a narrow tube called the utriculosaccular duct . The endolymphatic duct becomes a branch of the utriculosaccular duct. The cochlea elongates and spirals.

During the tenth week, the epithelium of the membranous labyrinth begins to differentiate [12, 14]. The primitive, multilayered epithelium becomes a single layer of flat epithelium, except in the sites of the future auditory and vestibular sensory end organs. In these sites, the ingrowing peripheral branches of the eighth cranial nerve induce the epithelium to differentiate into plaques of tall, ciliated neurosensory epithelium that are the rudiments of the neurosensory sensory end organs. The auditory organ is the organ of Corti, located in the basal membrane of the cochlear duct. It first appears in the vestibular end of the cochlear duct and then extends along the full length of the duct. The vestibular system of balance consists of the vertically oriented macula of the saccule, the horizontally oriented macula of the utricle, and the three radially oriented cristae of the ampullae of the semicircular ducts. Each of these six organs begins with the differentiation of tall, ciliated, neuroepithelial sensory cells.

The epithelium of the inner ear continues to induce the condensation of adjacent mesenchyme around itself to become an increasingly dense envelope of connective tissue, which abuts directly against the membranous labyrinth [12]. This is the precursor of the osseous labyrinth, also called the bony labyrinth or otic capsule . There is no space between the mesenchymal otic capsule and the membranous labyrinth. By the end of the tenth week, the elongation of the cochlear duct has reached nearly two and one-half spirals, the mesenchymal otic capsule has begun to convert into cartilage, and the auditory and vestibular sensory end organs are rapidly differentiating.

At the end of the embryonic period (tenth week), most of the major components of the ear are well-developed. However, a few important processes in the histologic maturation of the ear remain for the fetal period:

In the external auditory canal, the canalization of the meatal plug and the concomitant final maturation of the tympanic membrane
In the middle ear, the resorption of the embryonic mesenchyme, the establishment of the tympanic cavity, and the maturation of the secondary tympanic membrane covering the round window
In the inner ear, the final elongation of the cochlear duct to two and three-quarter turns, the formation of the perilymphatic space, the final maturation of the sensory end organs, and the ossification of the ossicles and otic capsule

Histology

External Auditory Canal

The entire external auditory canal is lined by keratinizing, stratified squamous epithelium, which is continuous with the skin of the pinna. The development of the embryonic and fetal histology of the skin of the external auditory canal is similar to that of the skin elsewhere in the body. The skin of the outer cartilaginous segment of the canal differs from that of the inner osseous segment. Recanalization of the meatal plug begins during the 13th week and is complete by the 18th week [13]. In the outer cartilaginous segment, early keratinization and primitive hair follicles appear in the third month. Sebaceous glands, ceruminous glands, and extensive keratinization appear in the fifth or sixth month in utero. By the time of birth, the lining epithelium of the outer cartilaginous portion of the canal includes numerous hair follicles, sebaceous glands, and ceruminous glands, but no sweat glands (Fig. 31.14). The ceruminous glands are simple, coiled, tubular glands that are thought to be modified apocrine glands similar to those in the eyelids, axilla, areola, groin, and elsewhere. Their coiled secretory portions form large lobules deep to the hair follicles and sebaceous glands. Their ducts empty into the hair follicles above the mouths of the sebaceous glands and onto the epidermal surface. The epithelium of the ceruminous glands is composed of a luminal layer of large secretory cells with granular, acidophilic cytoplasm and apical secretory granules and an outer layer of myoepithelial cells (Fig. 31.15). The secretory cells may contain yellow-brown pigment.

The ceruminous glands continue to mature into childhood and do not become fully functional until puberty or adolescence. Therefore, ceruminous waxy plugs in the external auditory canal are unusual before late childhood, although vernix caseosa may fill the canal in late fetuses and neonates.

The lining epithelium of the inner bony portion of the external auditory canal is different from that of the outer cartilaginous portion, in that it is thin and lacks dermal papillae and adnexa (Fig. 31.16) [15]. Adjacent to the tympanic membrane, its deep surface is undulating. Early in the fetal period, the stratified squamous epithelium is thin and not yet keratinized (Figs. 31.16 and 31.17). By mid-gestation it is keratinized and may be pigmented (Figs. 31.18, 31.19, 31.20, and 31.21).

Before birth, the external auditory canal is filled with amniotic fluid. After birth, it is filled with air. However, routine microscopic sections of the middle and inner ears often do not include the external auditory canal, so these features are usually not appreciated.

Tympanic Membrane

The mature histology of the tympanic membrane cannot be appreciated until the meatal plug is removed at 13–18 weeks gestational age. The mature tympanic membrane is composed of three layers, except in the pars flaccida [15]. The outer layer is extremely thin, stratified keratinizing squamous epithelium lacking dermal ridges and adnexa. It is continuous with the lining of the inner bony segment of the external auditory canal, which derives from the ectoderm of the first pharyngeal cleft. In the early fetal period, it is extremely thin and nonkeratinized (Fig. 31.22). By mid-gestation, it is thicker and well-keratinized (Fig. 31.23). By 6 months postnatal age, it remains heavily keratinized (Fig. 31.24). Through a process of “auditory epithelial migration,” the keratin and outer layers of the epidermis migrate from the middle to the periphery of the tympanic membrane and then onto the surface of the inner bony segment of the external auditory canal, from which they are ultimately expelled. This process ensures that the epidermal surface of the tympanic membrane remains thin, devoid of keratin, and pliable [15]. The middle layer of the tympanic membrane is a connective tissue from the first pharyngeal cleft and pouch; it is composed of two layers, an outer layer of radial fibers and an inner layer of circular fibers, which are well-developed by 15 weeks gestation (Fig. 31.22). The inner surface of the tympanic membrane is continuous with the lining of the tympanic cavity. It is derived from the endoderm from the first pharyngeal pouch and consists of squamous epithelium (Figs. 31.23 and 31.24).

Middle Ear

The histology of the middle ear changes significantly during the fetal period. Ossification of the ossicles begins at 16 weeks, first in the incus and followed sequentially by the malleus at 16–17 weeks and finally the stapes at 18 weeks. Ossification is complete by 24 weeks [2], and the ossicles are adult size by term. Beginning with the embryonic period and extending into the early fetal period, the tympanic cavity is almost completely filled with embryonic mesenchyme, which surrounds the ossicles and leaves very little lumen (Fig. 31.25). In the early fetal period, the epithelium lining the small cavity consists of simple or slightly stratified flat endoderm. It is thinnest over the inner surface of the tympanic membrane and the convex surfaces of the mounds of mesenchyme that fill the lumen. In these thin areas, it is a single layer of flat cells that lack cilia (Figs. 31.25 and 31.26). It is thickest in the corners and crevices of the cavity, where it becomes irregularly stratified and cuboidal to columnar, with patches of cilia (Figs. 31.25 and 31.27). With increasing gestational age, the subepithelial mesenchyme becomes thicker and more edematous. At mid-gestation, it continues to surround the ossicles and other structures that traverse the tympanic cavity, such as the tendon of the tensor tympani muscle (Figs. 31.28 and 31.29). The mesenchyme contains variable amounts of extramedullary hematopoietic tissue (Figs. 31.30 and 31.31). The epithelium remains thin and inconspicuous over the inner surface of the tympanic membrane, where it is a single layer of flat endothelium (see Fig. 31.23), and over the mounds of mesenchyme, where it is a single layer of flat or cuboidal endothelium (see Fig. 31.30). In the corners and crevices, it becomes irregularly pseudostratified, columnar, and cuboidal, with many patches of cilia (see Fig. 31.31). As pregnancy approaches term, the subepithelial embryonic mesenchyme is reabsorbed, and the lumen becomes larger. Less mesenchyme surrounds the ossicles and other structures (Figs. 31.32 and 31.33). The mesenchyme disappears from the tympanic side of the secondary tympanic membrane in the round window. The secondary tympanic membrane attains its adult appearance of a thin, trilayered membrane: a single layer of squamous epithelium derived from that of the tympanic cavity on the tympanic side, a middle layer composed of a thin sheet of connective tissue, and an inconspicuous single layer of periotic reticular squamous cells on the vestibular side (Figs. 31.32 and 31.34). Extramedullary hematopoiesis gradually diminishes from the embryonic mesenchyme. The epithelium continues to become thinner and less conspicuous (see Figs. 31.32 and 31.33). By the last week of pregnancy or the neonatal period, the mesenchyme completely disappears, including that surrounding the ossicles and the tendon of the tensor tympani muscle. The lining epithelium lies directly upon the underlying bone and the ossicles, and the mucosa becomes a mucoperiosteum (Figs. 31.35 and 31.36). The ossicles become fully mobile for the first time, allowing auditory signals, which previously could reach the inner ear only by bone conduction, to now reach the inner ear through the tympanic membrane and ossicles. The tall, pseudostratified, ciliated columnar endothelium in the corners and crevices may increase in thickness (Fig. 31.37). During the late fetal and neonatal periods, the mucosa around the opening of the Eustachian tube may form complex rugae. The normal epithelium in this area may contain a few mucus-producing cells (Fig. 31.38), but the presence of serous or mucous glands or large numbers of goblet cells anywhere in the mucosa of the tympanic cavity is abnormal and is probably a metaplastic response to excessive amniotic fluid or infection. Before birth, the tympanic cavity contains amniotic fluid and debris. After birth, it contains air.

Eustachian Tube

The short segment of the tympanic end of the Eustachian tube is supported by a bony wall. The remainder of the tube is supported by a wall of elastic cartilage. The entire length is lined by pseudostratified, ciliated columnar epithelium. Its pharyngeal cartilaginous end includes seromucous glands and lymphoid organs referred to as Gerlach’s tubal tonsils . Like all lymphoid tissues, these do not develop germinal centers or mature plasma cells until 2–6 weeks postnatal age.

Inner Ear

Otic Capsule

Ossification of the otic capsule begins only after the cartilaginous capsule reaches adult size at 16 weeks. Multiple sites of ossification appear during the 16th week. Ossification of the otic capsule is complete by the 24th week [2]. The otic capsule is seen in many sections of the cochlea, vestibule, and semicircular canals. It is a thin rim of very dense, often basophilic, bone, which is distinct from the petrous bone in which it is embedded.

Perilymphatic Space

The formation of the perilymphatic space of the inner ear occurs during fetal life [11, 12, 14]. As early as the tenth week, the mesenchyme of the inner surface of the cartilaginous osseous labyrinth becomes vacuolated and is transformed into a reticulum called the perilymphatic reticulum. The reticulum is gradually resorbed, which results in the formation of a space, the perilymphatic space, between the osseous labyrinth and the membranous labyrinth. This space contains the perilymphatic fluid. This process begins in the scala tympani, then extends into the scala vestibuli, and finally spreads throughout the vestibule and into the semicircular canals. The scala tympani and scala vestibuli are completely devoid of reticulum by the end of the 12th postfertilization week (Figs. 31.39 and 31.40). The process may be completed in the vestibule and then in the semicircular canals at any gestational age from 20 weeks to term, but the perilymphatic reticulum may never be completely resorbed from parts of the vestibule and semicircular canals in some individuals. The periotic reticulum forms the perichondrium of the cartilaginous otic capsule and presumably the periosteum of the osseous otic capsule. Several of the figures of the vestibule and semicircular canal demonstrate various degrees of resorption of the perilymphatic reticulum.

Cochlear Aqueduct and Periotic Duct

The development of the periotic duct and cochlear aqueduct is related to the development of the perilymphatic space and the otic capsule and is completed during the fetal period. The primitive periotic duct develops in conjunction with the inferior cochlear vein and its bony canal and Hyrtl’s fissure (the tympanomeningeal fissure). The periotic duct forms a direct communication between the scala tympani component of the perilymphatic space and the subarachnoid space. Hyrtl’s fissure forms a direct continuation between the tympanic cavity and the subarachnoid space and is separate from the cochlear aqueduct and periotic duct. In early fetuses, subarachnoid hemorrhage in the posterior fossa commonly spreads into the perilymphatic space of the inner ear through the periotic duct and into the tympanic cavity through Hyrtl’s fissure. By 24 weeks, Hyrtl’s fissure is closed, and the direct communication between the subarachnoid space and the tympanic cavity is lost. The direct communication between the subarachnoid space and the perilymphatic space through the periotic duct becomes permanent. Therefore, after 24 weeks, subarachnoid hemorrhage in the posterior fossa may spread through the periotic duct into the inner ear but not into the tympanic cavity [2].

Cochlear Duct

During the 11th week, the cochlea and cochlear duct attain their final length, and their spirals consist of two and three-quarter turns [11, 14]. Growth and maturation of the organ of Corti begins at the base of the cochlear duct as a thickening in the epithelium in one side of the round cochlear duct. This side of the wall becomes flat and forms the basal membrane or floor of the mature cochlear duct. The tall, columnar epithelium covering the floor of the developing organ of Corti continues to become thicker and develops two longitudinal ridges separated by a valley—a larger inner ridge nearer the center of the spiral and a smaller outer ridge nearer the lateral wall of the spiral. The tall epithelium of the larger, inner ridge secretes the gelatinous tectorial membrane that covers the top of the epithelium. The outer, smaller ridge will become the organ of Corti. By 14 weeks, the round cochlear duct has attained its mature triangular shape in cross section. The floor of the duct is the basal membrane containing the organ of Corti. The wall opposite the basal membrane is the vestibular or Reissner’s membrane, which is the roof of the cochlear duct. Reissner’s membrane and the basal membrane join to form an acute angle, the apex of the triangle, toward the center of the cochlear spiral. The lateral wall of the triangle is formed by the stria vascularis and the spiral ligament. The space above Reissner’s membrane is the scala vestibuli. The space below the floor is the scala tympani . By 14 weeks, the inner and outer hair cells and the pillar cells are present. The inner margin of the tectorial membrane is attached to the spiral limbus, and the free outer edge is embedded in the cilia of the inner and outer hair cells. By 16 weeks, the epithelial cells of the inner ridge degenerate and disappear, forming the inner spiral sulcus. The tectorial membrane stretches across the inner spiral sulcus. Cells between the inner and outer hair cells degenerate and disappear, forming the tunnel of Corti. The development of the organ of Corti approaches completion as early as 20 weeks gestation [14], when primitive fetal response to auditory stimuli received through bone conduction is first detectable. The development of the organ of Corti may not be complete until 25 weeks or later in some fetuses. This discrepancy may be explained in part by the fact that maturation begins at the base of the cochlear duct and progresses toward the apex of the spiral, so that the time of complete maturation will depend upon where along the spiral the section is obtained (see Figs. 31.4 and 31.40). The sophistication with which the fetus responds to auditory stimuli increases simultaneously with the increasing maturation of the organ of Corti as gestational age advances. The organ of Corti is often autolyzed in routine sections even in otherwise well-preserved specimens (Figs. 31.41 and 31.42). Reissner’s membrane is delicate and trilayered. Its inner cochlear surface is covered by simple squamous epithelium derived from the ectoderm of the otic vesicle. The inconspicuous middle layer is a wisp of connective tissue. The outer vestibular surface is covered by simple squamous epithelium derived from the periotic reticulum. The histology of all free walls of the membranous labyrinth in the saccule, utricle, and semicircular ducts is similar to that of Reissner’s membrane (see Figs. 31.40, 31.41, and 31.42). The spiral ligament is a thick, curved, pyramidal pad of stroma on the inner surface of the lateral bony wall of the cochlea. It forms the lateral wall of the cochlear duct. It is thickened periosteum and may be a specialized remnant of the periotic reticulum (see Figs. 31.40 and 31.41). The stria vascularis is a unique vascularized, stratified epithelial membrane on the surface of the spiral ligament. Its surface is a single layer of dark, eosinophilic, cuboidal cells referred to as dark cells or marginal cells. Deep to the dark cells is a thick layer of haphazardly arranged, more lightly stained polygonal cells referred to as light cells or basal cells . There are numerous capillaries in the layer of light cells. Epithelial membranes are characteristically not vascularized. The stria vascularis may be the only vascularized epithelial membrane in the human body and is thought to contribute to the maintenance of the volume and ionic makeup of the endolymph (Figs. 31.41 and 31.43). The nerves from the sensory hair cells pass through channels in the center of the spiral lamina (see Fig. 31.42) and join the spiral ganglion in the periphery of the modiolus. Nerves from the spiral ganglion pass through bony channels and join to form larger nerves in the modiolus (Fig. 31.44). Branches of the cochlear and vestibular nerve join to form the eighth cranial nerve in the internal auditory canal and exit the posterior wall of the petrous bone through the internal auditory meatus (Fig. 31.45).

Vestibular System

The delicate membranes of the saccule and utricle are usually destroyed in routine sections of even well-preserved specimens of the middle and inner ears (Fig. 31.46). Occasionally these structures are reasonably intact in routine sections (Figs. 31.47 and 31.48). The periotic reticulum of the saccule, utricle, and semicircular ducts may be incompletely resorbed in some fetuses of mid to late gestational ages (Figs. 31.47, 31.48, and 31.49).

The progression of maturation of the utricular and saccular maculae and the ampullary cristae is similar [11, 14]. They first appear at 10 weeks, and their maturation is complete by 14 weeks. They differ in that the maculae are ellipsoid plaques covered by gelatinous otolithic membranes (see Figs. 31.9, 31.10, 31.11, 31.50, 31.51, 31.52, 31.53, 31.54, and 31.55), whereas the cristae are narrow transverse ridges capped by gelatinous tongue-like membranes called the cupola , without otoliths (see Figs. 31.9, 31.12, 31.13, 31.53, and 31.56). At 10 weeks, the maculae and cristae are composed of a thick layer of two types of epithelial cells: ciliated neurosensory cells and supportive cells. The tall epithelial cells surrounding the sensory organs secrete a gelatinous membrane in which the cilia of neurosensory cells are embedded. In the maculae, this membrane is flat and covers the entire surface of the ovoid organ. In the cristae, it is a tongue-shaped membrane called the cupola , which arises from the apex of the full length of the crista and extends like a valve flap into the lumen. A thick layer of stromal cells and nerves beneath the epithelium adds to the thickness of the organs. By the 14th week, the free surfaces of the membranes of the otolithic saccular and utricular maculae contain basophilic crystals of calcium carbonate and protein called otoliths, so that the maculae are referred to as otolithic membranes (see Figs. 31.11, 31.52, 31.54, and 31.55). The microscopic appearance of the otoliths varies. In some sections, they are nearly invisible (Fig. 31.52). In others, they are small and round, or in some others, they may be irregular and oblong (Fig. 31.54). In still others, the otolithic membranes may be fragmented and lie in the lumen of the labyrinth, in which case they are easily mistaken for infectious organisms (Fig. 31.55). In contrast, the membranes of the cristae, the cupola, are devoid of otoliths and lengthen to stretch across almost the entire lumen of the ampullae (see Figs. 31.12, 31.13, and 31.56).

The semicircular canals may be seen in perfect cross sections (Figs. 31.57 and 31.58) or in oblique sections of various degrees (not illustrated). The bony wall of the canal, the otic capsule, is distinct from the surrounding petrous bone. The duct is attached to the inner surface of the bony canal by perilymphatic reticulum. The canal is lined by the simple squamous epithelium that is derived from the perilymphatic reticulum and serves as periosteum. The duct is lined by simple squamous epithelium derived from the ectodermal membranous labyrinth. Various amounts of perilymphatic reticulum may remain in the canal.

Endolymphatic Duct

The endolymphatic duct arises from the utriculosaccular duct and passes through the vestibular aqueduct of the vestibular otic capsule and into the dura. Its distal end is a large, dilated sac in the dura of the posterior wall of the petrous bone (see Figs. 31.1, 31.2, 31.4, 31.5, and 31.7). The narrow duct is lined by simple epithelium, which varies from squamous to cuboidal (Fig. 31.59). The large sac has a very irregular wall. The epithelial surface is thrown into numerous rugae and fjord-like inlets (Fig. 31.60). The epithelium ranges from simple low cuboidal to irregularly stratified cuboidal (Fig. 31.61).

Special Considerations

Examination of the Ear in the Perinatal Period

Why should perinatal pathologists include the examination of the ears in perinatal autopsies? This question is frequently asked. At a time when perinatal autopsy pathology is forced to compete for resources with the increasing demands of pediatric surgical pathology, the tendency is to spend fewer resources on perinatal pathology, not more. Several factors challenge that tendency. First, most of us humans die before we are born. This is a difficult concept, but many studies show that between 60% and 80% of pregnancies end in embryonic or fetal death. Second, the ear is often involved in the major categories of causes of intrauterine diseases and deaths, such as congenital anomalies with or without chromosomal or gene defects, infections (either amniotic fluid infection sequence or transplacental infections), perfusion abnormalities (either maternal placental or fetal placental), and congenital tumors [16]. The intrauterine abnormalities of the ear often cause neonatal hearing loss in those who survive. Third, the rapidly advancing fields of fetal medicine, imaging, and surgery impart an increasing clinical relevance to perinatal pathology. If perinatal pathologists are to keep abreast of or lead our clinical colleagues in the study of perinatal diseases , we need to perform more perinatal autopsies, not fewer and, in more detail, not less—including the examination of the ears.

Examples of abnormalities of the ear that accompany perinatal death or threaten the hearing of those who survive are numerous [16,17,18,19]. The histologic continuum of normal aspiration of amniotic fluid, excessive aspiration of amniotic fluid, and amniotic fluid infection sequence is one of the most common findings in sections of the ear. Because the tympanic cavity is in direct continuity with the amniotic cavity through the Eustachian tube, and because the course of the Eustachian tube in fetuses and neonates is nearly horizontal, a small amount of amniotic debris is often seen in sections of the middle ear of fetuses and neonates. Normally, this small amount of amniotic debris is removed, as it is from the lung, but fetal distress may result in an excessive amount of amniotic debris in the tympanic cavity, similar to that occurring in the lung. Therefore, an excessive amount of amniotic debris in the tympanic cavity should suggest significant intrauterine fetal distress. In extreme situations, the amniotic debris may persist and incite a foreign-body inflammatory reaction resulting in polypoid nodules attached to the ossicles or the wall of the tympanic cavity. Glandular and squamous metaplasia of the lining epithelium may accompany retention of amniotic debris. This is more likely in neonates than in fetuses. In cases of amniotic fluid infection sequence, acute inflammatory debris and infectious agents—including cocci, bacilli, fungi, and spirochetes—may be found in the tympanic cavity. Acute inflammation of the embryonic mesenchyme of the tympanic cavity, the mucosa, or the subjacent bone indicates invasive otitis media. A normal amount of extramedullary hematopoiesis should not be mistaken for otitis media.

Blood-borne transplacental infections may affect the inner and middle ears of fetuses and newborns. Cytomegalovirus commonly involves the stria vascularis of the cochlear duct (Figs. 31.62 and 31.63) and the sensory organs of the saccule and utricle. Hearing loss is one of the most frequent clinical findings in congenital cytomegalovirus infection. Rubella, Herpes simplex infection, toxoplasmosis, and Listeria also may infect the middle or inner ears. H. simplex and Listeria infections can be either ascending or transplacental.

Fetal distress may cause acute hemorrhage in the middle and inner ears, as it does in other organs. Acute abruptio placenta with subependymal, intraventricular, and subarachnoid hemorrhage is especially likely to be associated with severe hemorrhage of the middle and inner ears. One cause for the hemorrhage in the ears could be hypoxic-ischemic damage to the tissues of the middle and inner ears. Another reason could be the direct communication between the subarachnoid space and the inner ear through the periotic duct throughout fetal and neonatal life and the direct communication between the subarachnoid space and the tympanic cavity through Hyrtl’s fissure prior to 24 weeks, when the fissure closes. These two communications allow subarachnoid hemorrhage in the posterior fossa to flow from the subarachnoid space into the inner and middle ears [7].

Rarely, congenital neuroblastoma, leukemia, teratoma, or Langerhans cell histiocytosis may involve the middle or inner ears. Many congenital anomalies affect the ears, but special care in removing and sectioning the temporal bone may be required to demonstrate many of these malformations.

Removing the Temporal Bone

The reliable demonstration of normal and pathologic histology depends upon the use of effective techniques in removal and sectioning of the temporal bone . Haphazard removal is likely to yield unsatisfactory results due to fragmentation, crushing, incomplete removal, and poor orientation. Several methods have been advocated, which vary from simple methods to complex methods that are labor-intensive and expensive. Each institution can choose a technique that best fits its goals and resources.

Four incision lines are needed to remove the petrous bone (Fig. 31.64). The first passes through the temporalis muscle, separates the soft tissue from the outside of the temporal bone, and transects the external auditory canal (see Fig. 31.3). The second separates the anteromedial end of the petrous bone from the side of the sella turcica and is nearly parallel to the first incision. It should be placed as close to the wall of the sella turcica as possible. The third cuts through the roof of the petrous bone anterior to the area of the tegmen tympani, as close to the anterior vertical wall of the middle fossa as possible. It is extended into the underlying bone. It is perpendicular to the first two. The fourth is a cut along the inferior edge of the posterior wall of the petrous bone to include the orifice of the cochlear aqueduct at the superior edge of the jugular fossa. This is extended anteriorly toward the third cut. The four cuts are extended until the bone is free. This method provides a complete petrous bone, including the distal external auditory canal, tympanic membrane, middle and inner ears, ostia of the external auditory meatus and cochlear and vestibular aqueducts, and a portion of the Eustachian tube. After removal, the specimen should be thoroughly fixed. After fixation, the amount of decalcification varies. Ossification begins in the third month and continues throughout fetal life and into infancy. Therefore, the amount of decalcification should be carefully monitored and tailored to each specimen. Specimens from first-trimester and early second-trimester fetuses may need no decalcification.

After decalcification, the specimen is bisected by a horizontal section through the external and internal acoustic meatuses in a plane parallel to the plane of the superior surface of the petrous bone (Fig. 31.65). An alternative is a vertical section perpendicular to the superior surface of the petrous bone, parallel to the petrous ridge. Additional cuts parallel to either choice can be made, depending on the size of the specimen.

The instruments used in removal and sectioning of the specimens are important. Unossified or minimally ossified specimens should be removed with a scalpel blade. Because of unossified sutures and minor fontanelles, specimens may crumble even with the use of scalpels unless special care is taken in their removal and sectioning. Partially or extensively ossified specimens require a saw for removal. Adult Stryker saws are usually too large for use in fetuses and newborns. Pediatric Stryker saws are more appropriate and are easily available. Small, sharp bone scissors can be used, but they may cause crushing and fragmentation of the specimen. The use of chisels should be avoided. Sectioning of the appropriately decalcified specimen can be accomplished with a scalpel blade or one of the several available longer, razor-like blades.

More extensive and detailed examinations enter the realm of research and are beyond the scope of this chapter. For instance, if the entire length of the Eustachian tube is desired, a more invasive excision will be required. If the best preservation for histology and immunohistochemistry is desired, the unfixed specimen can be cut into thin serial slices by precision saws. These slices require minimal fixation; various fixatives can be used, and minimal decalcification is required. These slices can be serially microscopically sectioned and stained with a variety of stains and immunohistochemical techniques. The microscopic sections can be utilized for three-dimensional reconstruction to document the details of congenital malformations.

All these histologic procedures are complicated by multiple artifacts. The fixed whole specimen can be studied by CT microscopy [20] or MR microscopy [21]. These techniques avoid the artifacts of histologic procedures and provide excellent anatomic and pathologic detail of the middle and inner ears.

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Department of Pathology and Laboratory Medicine, The Children’s Hospital of Philadelphia and Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, USA
Dale S. Huff

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Department of Pathology and Laboratory Medicine, NorthShore University HealthSystem, Evanston, IL, USA
Linda M. Ernst
Department of Pathology and Laboratory Medicine, The Children’s Hospital of Philadelphia and Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, USA
Eduardo D. Ruchelli
Department of Pathology and Laboratory Medicine, The Children’s Hospital of Philadelphia and Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, USA
Chrystalle Katte Carreon
Department of Pathology and Laboratory Medicine, The Children’s Hospital of Philadelphia and Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA, USA
Dale S. Huff

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Huff, D.S. (2019). Ear. In: Ernst, L., Ruchelli, E., Carreon, C., Huff, D. (eds) Color Atlas of Human Fetal and Neonatal Histology. Springer, Cham. https://doi.org/10.1007/978-3-030-11425-1_31

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DOI: https://doi.org/10.1007/978-3-030-11425-1_31
Published: 01 September 2019
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Congenital Malformations of the Ear

External, Middle, and Inner Ear

Common Ear Diseases

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Introduction

Anatomy

In Situ Surface Anatomy of the Petrous Bone in the Neonate

Arrangement of the Inner, Middle, and External Ears Within the Petrous Bone

External Auditory Canal

Tympanic Membrane

Middle Ear

Inner Ear

Osseous Labyrinth

Membranous Labyrinth

Perilymphatic Space

Periotic Duct

Auditory and Vestibular Sensory Organs

Embryology

Histology

External Auditory Canal

Tympanic Membrane

Middle Ear

Eustachian Tube

Inner Ear

Otic Capsule

Perilymphatic Space

Cochlear Aqueduct and Periotic Duct

Cochlear Duct

Vestibular System

Endolymphatic Duct

Special Considerations

Examination of the Ear in the Perinatal Period

Removing the Temporal Bone

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