No abstract

No abstract

No abstract

The organ of Corti and macula lagena were studied by scanning and transmission electron microscopy in two species of monotreme, the platypus and echidna. In both species, the organ of Corti had a fundamentally mammalian conformation, with distinct outer and inner hair cells, separated by a tunnel of Corti. However, unlike eutherian mammals, the monotremes had three or four rows of pillar cells, and four to five rows of inner hair cells. The organ of Corti was much shorter than in eutherian mammals, at 4.4 mm (platypus), and 7.6 mm (echidna). While the total number of outer hair cells (3,350 platypus, 5,050 echidna) was many fewer than in most eutherian mammals, the total number of inner hair cells (1,600 platypus, 2,700 echidna) was comparable with that in eutherian mammals. The stereocilia on both inner and outer hair cells underwent a systematic change in orientation across the cochlear duct, with those nearest the tunnel of Corti having their axis of symmetry oriented transversely across the duct, and those on the outer edge of the organ having the axis oriented nearly longitudinally along the duct. The macula lagena had signs of a vestibular epithelium, with tall bundles of stereocilia, a division into areas with bundles of opposing orientation and type I and type II hair cells.

Messenger RNAs coding for growth factors and receptor tyrosine kinases were measured by quantitative competitive and by semi-quantitative reverse-transcription polymerase chain reaction in whole and dissected chick inner ears. The fibroblast growth factor (FGF) receptor 1 chick embryonic kinase (CEK) 1 was expressed in all structures examined (otocyst, hatchling whole cochlea, cochlear nerve ganglion, and cochlear and vestibular sensory epithelia),although slightly more heavily in the otocyst. The related fibroblast growthfactor receptors CEK 2 and 3 were preferentially expressed in the nerve ganglion and in the vestibular sensory epithelium, respectively. FGF1 mRNA was low in early development, increasing to mature levels at around embryonic age 11 days, while FGF2 mRNA was expressed at constant levels at all ages. In response to ototoxic damage, FGF1 mRNA levels were increased in the early damaged cochlear sensory epithelium. Immunohistochemistry for CEK1 showed that normal hair cells expressed the receptor heavily on the hair cellstereocilia, while with early damage, CEK1 came to be expressed heavily on the apical surfaces of the supporting cells. In normal chicks, the CEK4 andCEK8 eph-class receptor tyrosine kinases were expressed relatively heavily by the cochlear nerve ganglion, and CEK10 was expressed relatively heavily by the cochlear hair cell sensory epithelium. The results suggest that the FGF system may be involved in the response of the cochlear epithelium to ototoxic damage. The eph-class receptor tyrosine kinase CEK10 may be involved in cell interactions in the cochlear sensory epithelium, while CEK4 and CEK8 may play a role in the cochlear innervation.