Acta Oto-Laryngologica

ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20

Extra Internal Hair Cells: A Scanning Electron Microscopic Study I. Kawabata & Y. Nomura To cite this article: I. Kawabata & Y. Nomura (1978) Extra Internal Hair Cells: A Scanning Electron Microscopic Study, Acta Oto-Laryngologica, 85:1-6, 342-348, DOI: 10.3109/00016487809121462 To link to this article: http://dx.doi.org/10.3109/00016487809121462

Published online: 08 Jul 2009.

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Acta Otolaryngol85: 342-348, 1978 EXTRA INTERNAL HAIR CELLS A Scanning Electron Microscopic Study I. Kawabata and Y. Nomura From the Department of Otolaryngology, University of Tokyo, Tokyo, Japan

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(Received June 21, 1977)

Abstract. The extra internal hair cell (EIHC) of the human cochlea was observed by means of a scanning electron microscope. The EIHC was found not infrequently in all turns of the human cochlea. It was located medial to the IHC row. The inner pillar cells showed an abnormal structure. The anatomical relationships between the displaced IHC and EIHC, and the inner pillar cell were classified into five types. The origin of these anomalies is discussed from an embryological viewpoint.

It has been said that the internal hair cells show less abnormality than the external hair cells in regard to arrangements of cells as well as morphology. However, extra internal hair cells (EIHC) are not infrequently observed when examining the surface of the organ of Corti under the scanning electron microscope (SEMI. As early as 1881, Retzius already reported on this and named this phenomenon “uberschussige innere Haarzellen.” Kolmer described a similar finding in a light microscopic study (Fig. 1). While surveying the surface of the human organ of Corti, we have found that abnormal shape and arrangement of the supporting cells such as the inner pillar cells and border cells are observed in the organ of Corti adjacent to EIHC. The purpose of this paper is to describe the morphology of EIHC and to classify cell arrangements of the extra internal hair cells and the supporting cells and to review the available literature. Aclu Otolaryngol85

MATERIAL AND METHOD Temporal bones from aged people of over 80 years old who died of pneumonia and heart diseases were used in this study. The bones were removed at autopsy about one hour post mortem and fixed in 10% formalin solution. The membranous labyrinth was removed from the temporal bone and, was then fixed in 2.5 % glutaraldehyde solution (0.2 M phosphate buffer) at pH 7.4 for 12-24 hours. Subsequently, the materials were fixed with 2 % tannic acid solution and 2 % osmic acid solution according to Murakami’s method. Specimens were dried using the critical point drying method after dehydration through graded alcohol and amylacetate. They were then coated with gold palladium in a vacuum evaporator (ion sputtering). The specimens were observed under a scanning electron microscope (JSM-S type). RESULTS The internal hair cells (IHC) form a single row in the organ of Corti. In the aged, the external hair cells showed a loss of stereocilia, whereas this loss was much less marked in the IHC. Instead, extra cells which lie outside of the usual single row were found in the IHC. The EIHC usually exists medial (toward the modiolus) to the normal position of the IHC row-never on the outside of it. Under low-power viewing,

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Extra internal hair cells 343

Fig. 2. A low magnification SEM picture of the surface of spiral organ. An extra internal hair cell (urrow) exists medial (toward the modiolus) to the normal position of the internal hair cell row (IHC). The internal hair cell row forms a convex curve laterally (toward the external hair cell (OHC)). x800.

Fig. I . ( a ) An extra internal hair cell. In 1881, Retzius drew a similar duplication of an internal hair cell and termed it “iiberschussige innere Haarzellen” (arrow). ( 6 )In 1927, Kolmer described a similar finding in a tangential section of the spiral organ with a light microscopy (arrow).

the IHC row formed a convex curve laterally at the point where the extra cell existed (Fig 2). There was no predominance of the localization of the EIHC in any part of the cochlea. They were found in the upper, middle and the basal turns of the human cochlea. Under a high-power view, no particular structure was found in the cuticular plate and stereocilia in these EIHCs. However, the supporting cells around the EIHC showed unusual cell arrangements.

Fig. 3 indicates the normal arrangement of the IHC and supporting cells. The cuticular plate of the IHC is elliptical and it protrudes a bit further than the free surface of the surrounding supporting cells. The long axis of the elliptical cuticular plate lies parallel with the row of IHC. Lateral to the IHC there is a head of the inner pillar cells. Its head shows a rectangular shape, the long axis of which is perpendicular to the row of IHC. It is tightly in contact with the cuticular plate of the IHC. Medial to the IHC there is a border cell which has microvilli on its surface. The border cell extends partly into the intercellular space between IHCs. Therefore, the boi-der cell and the head of the inner pillar cell contact each other. As the terminal web was poorly developed in the border cell, its cell margin was not clear. The shape and size of the border cell is, therefore, obscure. Medial to the border cells is the inner sulcus cell, the free surface of which has no microvilli. Acta Otoluryngol85

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Extra internal hair cells 345

Fig. 9 . Extra internal hair cells are located closely (a, arr o w ) or side by side (b, arrow). a : X800. b: X2400.

The anatomical relation of the EIHCs and supporting cells can be classified as follows. The type A (Figs. 4 and 10A): This is a displacement of IHC rather than EIHC. A displaced IHC is seemingly pushed medially from the normal IHC row by a head of the inner pillar cell.

Figs 3-8. High magnification SEM pictures of normal and extra internal hair cells. Fig. 3 shows the normal arrangement of an internal hair cell and the supporting cells. An internal hair cell (ZHC) located between the head of internal pillar cells (IPC) and border cells (BC). ~ 2 4 0 0 . Fig. 4 shows an extra internal hair cell corresponding to ( A )in Fig. 10. An extra internal cell (arrow) moves toward the modiolus with the head of the internal pillar cells (IPC). x2400. Fig. 5 shows an extra internal hair cell corresponding to ( B ) in Fig. 10. An extra internal hair cell (arrow) is surrounded by the border cells (BC) with numerous microvilli. The head of the inner pillar cell (ZPC) does not extend toward the modiolus. x 2 400. Fig. 6 shows an extra internal hair cell (arrow), located inside the row of internal hair cells. ~ 2 4 0 0 . Fig. 7 shows a similar finding to Fig. 6 . An extra internal cell (arrow) does not come into contact with ordinary internal hair cell and is surrounded by the border cells (BC). x 2 500. Fig. 8 shows an extra internal hair cell (arrow) corresponding to ( E ) in Fig. 10. This type appears to be a mixin Fig. 10. IPC: head of internal ture of types ( A ) and (0) pillar cell. x 2 400.

Type B (Figs. 5 and 10B): There is a gap in the IHC row. However, the head of the inner pillar cell is in normal position. This IHC is medial to the IHC row. The border cells with numerous microvilli surround the IHC. In other words, this type B has no extra IHC, but rather a dislocation of an IHC. The original area is replaced by the border cell. Type C (Figs. 6 and IOC): The IHCs lie side by side tightly. An EIHC is medial to and close to the IHC row. Type D (Figs. 7 and 1OD): Like Type C, there is no deficit of an IHC and in the IHC row. However, an EHIC is present medial to and away from the IHC row. The border cell with numerous microvilli is between the EIHC and the IHC row. Type E (Figs. 8 and 10E): This type shows a mixture of types A and D. An EIHC is located medial to and close to the IHC row. The head of the inner pillar cell, passing through the IHC row, is in contact with the EIHC’s cuticular plate. The head of the inner pillar cell is not rectangular but shows a curved shape. The EIHCs are observed at times, located side by side or closely (Figs. 9 a , b ) . A scheme of the above classification is shown in Fig. 10. Acta Otolaryngol85

346 I . Kawabata and Y. Nomura

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classification of the extra internal hair cells.

DISCUSSION Little has been mentioned about the extra internal hair cell (EIHC) since the first description of Retzius and later Kolmer. Recently Soudijn described briefly the EIHC of the guinea pig, using the SEM. The paucity of the literature is mainly attributable to a low incidence of EIHC (Kolmer, 1927; Engstrom et al., 1970). The EIHC exists in the organ of Corti of man (Retzius, 1881; Kolmer, 1927) rabbit (Retzius, 1881) and guinea pig (Soudijn, 1976). It is also found in rat, mouse and dog. Therefore, it is reasonable to assume that the EIHC is present in the organ of Corti of many species, and not specific to any one species. It is our impression that there are more EIHCs in the human organ of Corti than in other animals. As to the location of the EIHCs, Soudijn reported that they are found more in the upper turn of the guinea pig. Kolmer’s specimen was also from the upper turn of the cochlea. In the present investigation, the EIHCs were found Acta Otoluryngol85

more in the apical turn of the human organ of Corti, though they were also widely distributed in the middle and basal turns. The cytoarchitecture of the IHC and supporting cells was investigated in detail by transmission electron microscopy (Iurato, 1967; Kimura et al., 1965; Kimura, 1975). According to Kimura (1975), the supporting cells of the IHC are the inner pillar cell, the inner phalangeal cell, the border cell and the inner sulcus cell. The inner phalangeal cell is located between the inner pillar cell and the IHC. It is a small cell, irregularly shaped, and having many microvilli on the free surface. The border cell is located between the IHC and the inner sulcus cell. On observing the fine surface of the human organ of Corti by means of SEM, the inner pillar cells and the inner sulcus cells were easily distinguishable, whereas the inner phalangeal cells and the border cells were hardly distinguishable. The microvilli observed in the narrow space between the IHCs is possibly on the free surface of the inner phalangeal cell.

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Extra internal hair cells 347

As we found it difficult to distinguish the inner phalangeal cell from the border cell even when there was a relatively wide space between the IHCs (Fig. 5 ) , the border cell is defined as having numerous microvilli on its free surface. In this paper, we propose to classify the arrangement of EIHC and supporting cells into five types. The head of the inner pillar cell displayed an abnormal shape, whereas the border cell and the inner sulcus cell do not show any anomaly. Considering combinations of the presence or absence of abnormality of the IHC row and head of the inner pillar cell, the above-mentioned five types can be grouped as follows: Group Z: There is no gap in the IHC row. As the EIHCs lie medial (toward the modiolus) to the normal IHC row, the inner pillar cells show no anomaly. Group ZZ: There is a gap in the IHC row. No abnormality in the head of the inner pillar cell. Group ZZZ: There is a gap in the IHC row. The pillar head extends toward the modiolus. Retzius observed the EIHC in a newborn infant. Soudijn (1976) thought the EIHC to be a normal variation. Bredberg (1968) found the EIHC in a 6-month-old foetus. The cells in the organ of Corti are quite regularly arranged in the foetus. After birth, hair cells show an irregular arrangement. However, we are of the opinion that the EIHC observed in aged persons in the present study are formed during the development of the inner ear. It is still a matter of dispute as to which turn of the cochlea develops first (Pujol & Marty, 1970). However, the studies of Kolmer (1927), Kollmann (1898) and Minot (1894) have been generally accepted in regard to the development of the organ of Corti. According to Kolmer, differentiation of the organ of Corti begins as early as the 3rd foetal month in the epithelial mound of the cochlear duct. The organ of Corti develops rapidly from the 3rd to 4th foetal month. Thickened epithelium becomes the neuroepithelium. Three rows of the external hair cells are differentiated from a lateral part of

the neuroepithelium, whereas one row of IHC results from a medial part. In the 7th month, the inner sulcus is formed at the same time, and a space is built between the IHC and the external hair cells. This develops into the tunnel of Corti, which is formed by the inner and outer pillar cells. The hair cell is differentiated and completed from the neuroepithelium prior to the formation of the pillar cell. Considering the various arrangements of EIHC from the organogenesis of the organ of Corti, anomaly of cell arrangement occurs in the early stage of development in Group I. The EIHC with anomaly of head of the inner pillar cell observed in Group 111 seems to arise in a relatively late stage of development. The function of the EIHC is unknown. Since the IHC is innervated much more than the external hair cell in the human organ of Corti (Nomura, 1976), it is important to study whether or not the EIHC has a pattern of innervation similar to that of the IHC.

ZUSAMMENFASSUNG Die uberschussigen inneren Haarzellen der Schnecken von Menschen wurden mit dem Raster-Elektronenmikroskop betrachtet. Diese Zellen befinden sich auf der ganZen Windung der Schnecke, und zwar innerhalb der inneren Haarzellen-Reihe. Auch an den nebenliegenden inneren Pfeilerzellen wurden Anomalien beobachtet. Die anatomischen Beziehungen zwischen den verschobenen inneren Haarzellen, uberschussigen inneren Haarzellen und inneren Pfeilerzellen wurden in 5 Typen geteilt und die Bildung dieser Anomalien wurde aus den genetischen Ansichten besprochen.

REFERENCES Bredberg, G. 1968. Cellular pattern and nerve supply of the human organ of Corti. Acta Otolaryngol (Stockh) Suppl ,236. Engstrom, H., Ades, H . W. &Bredberg, G. 1970. Normal structure of the organ of Corti and the effect of noiseinduced cochlear damage. In A Ciba Foundation Symposium Sensorineural Hearing Loss (ed. G. E. W. Wolstenholme & J . Knight). J. & A. Churchill, London. Iurato, S . 1967. Submicroscopic Structure of the Inner Ear. Pergamon Press, London. Kimura, R. S., Schuknecht, H . F. & Sando, I. 1965. Acra Otolaryngol85

348 I. Kawabata and Y. Nomura

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Fine morphology of the sensory cells in the organ of Corti of man. Acta Otolaryngol (Stockh) 58, 390. Kimura, R. S. 1975. The ultrastructure of the organ of Corti. Int Rev Cytol. 42, 173. Kollmann, J. 1898. Lehrbuch der Entu~icklungsgeschlecht des Menschen. Verlag von Gustav Fischer, Jena. Kolmer, W. 1927. Gehororgan in Mollendorff's Handbuch der rnikroskopischen Anatornie des Menschen, Vol. 111. Springer-Verlag, Berlin. Minot, C. S. 1894. Lehrbuch der Entwicklungsgeschlecht des Menschen. Verlag von Veiut & Cony, Leipzig. Murakami, T. 1974. A revised tannin-osmium method for non-coated scanning electron microscopic specimens. Arch Histol Jap 36, 189. Nomura, Y . 1976. Innervation of the human organ of Corti. Acta Otolaryngol (Stockh)R2, 317.

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Pujol, R. & Marty, R. 1970. Postnatal maturation in the cochlear of the cat. J Cornp Neurol 139, 115. Retzius, G. 1881. Das Gehororgan der Wirbeltiere. Samson & Wallin, Stockholm. Soudijn, E. R. 1976. Scanning electron microscopic study of the organ of Corti in normal and sound-damaged guinea pigs. Ann Otol Rhino1 Laryngol Suppl. 29.

I . Kawabata, M.D. Dept. of Otolaryngology University of Tokyo Tokyo Japan

Extra internal hair cells. A scanning electron microscopic study.

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