Features of the structural development of the inferior colliculus in relation to the onset of hearing in a marsupial: The Northern Quoll,Dasyurus hallucatus

Abstract

The time course of synaptogenesis and the arrival and myelination of afferent connections were studied in the developing inferior colliculus (IC) of a marsupial, the Northern Quoll, and related to the onset of hearing and patency of peripheral auditory structures in that species. The quoll is born after 3 weeks of intrauterine growth and completes its development in a pouch for a further 80 days before weaning. Synaptic terminals in the IC at 9 days after arrival in the pouch were extremely rare and were associated with very low vesicle numbers. The number of synapses increased smoothly during pouch life, whereas the number of neurons with nucleoli fell over the same time period. The ratio of synapses to cells steadily increased from day 9 to day 63, then rapidly accelerated to day 73; a similar high ratio was observed in adults. Retrograde labeling from the IC of fibers projected from the medullary auditory nuclei, first observed on day 36, became progressively denser during pouch life. Myelination of lateral lemniscal fibers was absent on day 45, extremely sparse on day 54, and clear on day 63. Myelin sheaths were not observed within the IC electron microscopically until day 73. Examination of the peripheral auditory system revealed that until about day 40 the middle ear was fluid-filled, and middle ear structures were spongy. Between days 51 and 63 the middle ear cleared, the eardrum became shiny, and the ear canal became patent. The structural development of the IC is therefore very mature at the time hearing begins (67 days), and the last major anatomical change preceding hearing appears to be the opening of the external ear canal.

Synaptogenesis in the inferior colliculus of the pre-hearing postnatal ferret

“…In agreement with the present study, synapses in the IC of the Northern quoll, a marsupial cat, first can be detected in the IC prior to the onset of hearing and gradually increase in number until shortly before hearing begins (Aitkin et al, 1996). Synapse formation then accelerates rapidly suggesting that an analogous increase in synaptogenesis may occur in the ferret early in the second postnatal month.…”

Functional and structural changes throughout the auditory system following congenital and early-onset deafness: implications for hearing restoration

“…Moreover, early-onset hearing loss leads to a sharp reduction in synaptic density relative to normal hearing animals, and an apparent decrease in the number of presynaptic vesicles in many of the remaining synapses (Hardie et al, 1998). Developmental studies have shown that dramatic increases in synaptic density in the IC follow the onset of hearing, suggesting a role for stimulus-evoked neural activity in shaping connections in this nucleus (Aitkin et al, 1996, 1997). …”