Effect of nicotine on the structure of cochlea of guinea pigs

Abstract

Smoking has been positively associated with hearing loss in human. However, its effect on the cochlea has not been previously evaluated. Aim of work is to investigate the effect of nicotine, which is the primary pharmacological component of tobacco, on the structure of the cochlea of adult male guinea pigs. Fifteen male guinea pigs were classified into two groups: group I (control) and group II (nicotine treated group). Group II was further subdivided into two subgroups; IIA and IIB according to the dose of nicotine (3 mg/kg and 6 mg/kg, respectively). The cochlea was harvested and processed for light microscopy, transmission electron microscopy and scanning electron microscopy. Nicotine administration induced damage of outer hair cells which were distorted in shape with vacuolated cytoplasm and heterochromatic nuclei. Topography revealed damage of the stereocilia which included disorganization, bent and limp or complete loss and expansion of the surrounding supporting cells. These changes were more pronounced in the basal turn of the cochlea and mainly involved the outer hair cells. High dose induced more damage and resulted in protrusion of the apical poles of hair cells (blebing), particularly the outer two rows. Nicotine is proved to be harmful to the cells of the cochlea, particularly the outer hair cells of the basal turn. High doses induce blebing of hair cells.

Keywords: Cochlea; Guinea pigs; Nicotine; Scanning electron microscopy.

Fig. 1

Semithin section in the organ of Corti of group I, showing the normal organization of inner (I) and outer hair cells (O) and the surrounding supporting cells resting on the basilar membrane (BM). Note the tectorial membrane (TM), the inner (iP) and outer (oP) pillar cells enclosing the tunnel of Corti (TC), the Dieters' cells (D) and the laterally located Hensen's cells (HC) (toluidine blue staining, ×1,000).

Fig. 2

Semithin section in the organ of Corti of group IIA, showing preservation of a relatively normal organization of the cells. The inner (I) and outer (O) hair cells have densely stained nuclei and vacuolated cytoplasm. Note the encroachment of Deiters' cells (D) toward the reticular lamina and Hensen's cells (HC) toward the piller cells. BM, basilar membrane; IP, inner piller; OP, outer piller; TC, tunnel of Corti; TM, tectorial membrane (toluidine blue staining, ×1,000).

Fig. 3

Transmission electron microscopy of a portion of the organ of Corti of group I, showing the soma of an outer hair cells (O). Note the straight stereocilia (St) protruding from the cuticular plate (CP). Deiters' cells (D) have rounded nuclei (N) and microtubules running along their lateral plasma membrane (arrows) and surround their basal parts. Curved arrow, outer pillar (×2,900).

Fig. 4

Higher magnification of the previous figure showing the cuticular plate (CP), and the junctional complex (arrowheads) between the outer hair cell (O) and the Deiters' cell (D). Note the characteristic microfilament assembly appearing as electron dense structure running the depth of the junction and the subsurface cisternae (arrows) located under their lateral plasma membrane, stereocilia (St) and microvilli (mv) (×10,000).

Fig. 5

Transmission electron microscopy from organ of Corti of group IIA, showing outer hair cells (O1, O2, O3) with vacuolated cytoplasm and wrinkled irregular lateral plasma membrane and heterochromatic nuclei. CP, cuticular plate; D, Deiters' cell; St, Stereocilia (×5,800).

Fig. 6

High magnification of the previous figure showing the outer hair cells (O) heterochromatic nuclei (N), vacuolated cytoplasm (V) and dilated subsurface cisternae. A part of another hair cell shows electron dense nucleus with large heterochromatic clumps. CP, cuticular plate; D, Deiters' cell (×5,800).

Fig. 7

Scanning electron microscopy of the organ of Corti (the reticular lamina) of group I. A single row of inner hair cells (I) is separated from the three rows of outer hair cells (O1, O2, O3), and the three rows of Deiters' cells (D1, D2, D3). HC, Hensen's cells; iP, inner pillar cells; oP, outer piller cells (×2,000).

Fig. 8

Scanning electron microscopy of organ of Corti of group I. Hensen's cells are separated showing the cylindrical cell bodies of the third row of outer hair cells (asterisk) resting on the Deiters' cell bodies (D3). Note the phalangeal processes of Deiters' cells (↑) which extend obliquely to the reticular lamina. Outer hair cells (O1, O2, O3), inner hair cells (I) (×2,000).

Fig. 9

Scanning electron microscopy of the inner hair cells of group 1, showing linear stereociliary bundles arranged in three rows of graded heights. Note the tip links (arrow) and side links (arrowhead) between stereocilia and the surface granularity of stereocilia (curved arrow) (×15,000).

Fig. 10

Scanning electron microscopy of the outer hair cells of group I. The V-shaped stereociliary tufts are formed of three rows of stereocilia of graded height with the longest being in the outer side. Note the side links (arrowheads) between stereocilia (×15,000).

Fig. 11

Scanning electron microscopy of the reticular lamina of group IIA, showing disarray of the stereociliary bundles of the inner hair cells (I) and the outer hair cells particularly the third (O3) row (×2,000).

Fig. 12

Scanning electron microscopy of the reticular lamina of group IIA. The stereocilia protruding from the apices of inner hair cells (I) are bent and separated from each other with loss of their links (arrow). Note the relatively normal arrangement of stereocilia in the first row (O1) of outer hair cells (×7,500).

Fig. 13

Scanning electron microscopy of the reticular lamina of group IIA, showing the three rows of outer hair cells (O1, O2, O3). The stereocilia in O2 & O3 are bent (arrow), separated (arrowhead) or lost (curved arrow). Note the phalangeal scar extending from Deiters' cells in the site of lost outer hair cell (double arrow) (×2,000).

Fig. 14

Scanning electron microscopy of the reticular lamina of group IIB, showing frequent large blebs (arrowheads) arising from the cuticular plates of inner hair cells (×2,000).

Fig. 15

Scanning electron microscopy of the inner hair cell row of group IIB, showing blebs with granular surfaces of varying size (arrowheads). Note the disarray (arrow) and increased surface granularity (curved arrow) of the stereocilia (×15,000).

Fig. 16

Scanning electron microscopy of the reticular lamina of group IIB showing a phalangeal scar (double arrow) in the inner hair cell (I). Note the lost stereocilia (arrow) and the frequent blebs (arrowheads) (×2,000).

Fig. 17

Scanning electron microscopy of the reticular lamina of group IIB showing the three rows (O1, O2, O3) of outer hair cells with a phalangeal scar (double arrow) in the third row and the frequent blebs (arrowhead) (×3,500).

Fig. 18

Scanning electron microscopy at higher magnification showing a bleb and the stumps of the lost stereocilia (arrow) (×15,000).