A surface preparation study on the effect of methyl mercury on the sensory hair cell population in the cochlea of the harp seal (Pagophilus groenlandicus Erxleben, 1777)

Abstract

Surface preparations of the organ of Corti of four harp seals were used to study the effect of prolonged ingestion of methyl mercury on the sensory cell population.A low level of damage to the sensory hair cells occurred throughout the length of the cochlea. Damage was confined to the three outer rows of sensory hair cells especially the third outermost row. At each location along the length of the cochlea, sensory hair cell damage in the seals on a daily dose of 25.0 mg/kg of methyl mercury exceeded the damage to the cochlea of the seals fed on a daily dose of 0.25 mg/kg of methyl mercury. Greatest damage in all the mercury-treated seals occurred in the middle coil of the cochlea. Seals on the higher mercury diet showed a 20–24% sensory cell damage at the upper middle coil, about 19–26 mm from the base, whereas only 4–5% damage was found within same region in the cochlea of the seals on the lower mercury diet.This lack of specificity and low level of damage to the sensory hair cells seems characteristic of mercury and is a direct contrast to other known ototoxic agents.

Mercury (Hg2+) suppression of potassium currents of outer hair cells

“…Neurotoxicity of organic (MeHg) or inorganic (Hg 2 + ) mercury has been well documented in both humans [8,13,14,19,29,32,33,35,48] and experimental animals [1,11,28,29,31,40,47]. The developing premature and immature CNS is more sensitive to damage from MeHg than the adult CNS [8,13].…”

“…The auditory deficit extends almost over the entire frequency range of hearing, and in the most severe instances, Hg 2 + can lead to clinically significant auditory deficits [4,19,35]. The sensory cochlea and vestibular apparatus of the inner ear containing the hair cells and central auditory structures have been a target of Hg 2 + ototoxicity [4,9,14,16,17,24,26,35,40,43,47].…”

Toxicology of Marine Mammals: New Developments and Opportunities

“…Reijnders 1986;Ross et al 1996a, b) compared to the older ones (e.g. Tillander et al 1972;Ramprashad and Ronald 1977) and general outcomes [fatalities were recorded in Ramprashad and Ronald (1977)]. Nevertheless, effects on the immune (Ross et al 1996a, b;De Swart et al 1996), sensory (Ramprashad and Ronald 1977) and reproductive systems (Reijnders 1986) were found thereby providing evidence that pollutants could be associated with adverse effects.…”

“…In the past, experiments using animals held in captivity were performed in a limited number of occasions (Tillander et al 1972;Ramprashad and Ronald 1977;Van de Ven et al 1979;Reijnders 1986;Ross et al 1996a, b;De Swart et al 1996;Thomas et al 2005). These studies differ in a number of ways such as the administration type (e.g.…”

“…Tillander et al 1972;Ramprashad and Ronald 1977) and general outcomes [fatalities were recorded in Ramprashad and Ronald (1977)]. Nevertheless, effects on the immune (Ross et al 1996a, b;De Swart et al 1996), sensory (Ramprashad and Ronald 1977) and reproductive systems (Reijnders 1986) were found thereby providing evidence that pollutants could be associated with adverse effects. To our knowledge, similar experiments were never performed using marine mammal species other than pinnipeds and were not performed for any marine mammal species in the last decade.…”