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Functional development of auditory sensitivity in the fetus and neonate
1Department of Physiology, Hebrew University-Hadassah Medical School, Jerusalem, Israel.
Insights
Human fetuses can hear in utero, while rats and cats develop hearing after birth. This study explores auditory development, focusing on the inner ear
Area of Science:
- Developmental biology
- Auditory neuroscience
- Otolaryngology
Background:
- Human fetuses exhibit auditory sensitivity in utero.
- Neonatal rats and cats serve as models for human fetal auditory development.
- Auditory development involves complex structural and functional maturation of the ear.
Purpose of the Study:
- To investigate the developmental stages of auditory sensitivity in mammals.
- To explore the role of the inner ear and middle ear development in auditory maturation.
- To understand the influence of oxygenation levels on auditory development and function.
Main Methods:
- Comparative analysis of auditory development in human fetuses, neonatal rats, and cats.
- Examination of inner ear responses to bone conduction stimuli.
- Investigation of middle ear developmental processes, including mesenchyme resorption and canal opening.
Main Results:
- The inner ear responds to bone conduction before the middle ear is functional.
- Key developmental stages include endocochlear potential (EP) augmentation and cochlear amplifier maturation.
- Thyroid hormone likely regulates ear maturation, including Na+,K(+)-ATPase activity for EP generation.
- Hypoxia in utero contributes to sensorineural hearing loss, which improves with increased oxygenation post-birth.
Conclusions:
- Auditory sensitivity develops progressively, influenced by middle ear maturation and inner ear function.
- The endocochlear potential, crucial for hearing, is regulated by oxygen supply and thyroid hormone.
- Transition to pulmonary oxygenation enhances auditory sensitivity by increasing EP magnitude.
Abstract:
The human fetus responds to sound stimuli while still in utero. The rat and cat begin to hear only after birth. Therefore neonatal rat and cat are used as models of the development of auditory sensitivity in the human fetus. The inner ear of rat responds to stimuli delivered directly to it (bone conduction) before the middle ear can conduct sounds to the inner ear. During this period, middle ear development involves mesenchyme resorption, ossicular hardening and opening of the external canal. The latter stages of inner ear development involve increased magnitude of the endocochlear potential which augments cochlear transduction and the active cochlear amplifier. These developmental stages are probably controlled by thyroid hormone which activates several genes leading to the synthesis of proteins and enzymes required for the structural and functional maturation of the ear. This likely includes the Na+,K(+)-ATPase of the stria vascularis which generates the endocochlear potential. The magnitude of the endocochlear potential is dependent on oxygen supply so that the human fetus in utero whose blood carries less oxygen than the newborn has a hypoxia-induced sensorineural hearing loss. Upon birth and transition from placental to pulmonary oxygenation, the oxygen content of blood is increased, the magnitude of the endocochlear potential is elevated and auditory sensitivity is enhanced.