FEATURES OF THE STRUCTURAL DEVELOPMENT OF THE INFERIOR COLLICULUS IN RELATION TO THE ONSET OF HEARING IN A MARSUPIAL - THE NORTHERN QUOLL, DASYURUS-HALLUCATUS

The time course of synaptogenesis and the arrival and myelination of a fferent 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. Synap tic terminals in the IC at 9 days after arrival in the pouch were extr emely rare and were associated with very low vesicle numbers. The numb er of synapses increased smoothly during pouch life, whereas the numbe r of neurons with nucleoli fell over the same time period. The ratio o f synapses to cells steadily increased from day 9 to day 63, then rapi dly accelerated to day 73; a similar high ratio was observed in adults . Retrograde labeling from the IC of fibers projected from the medulla ry auditory nuclei, first observed on day 36, became progressively den ser during pouch life. Myelination of lateral lemniscal fibers was abs ent on day 45, extremely sparse on day 54, and clear on day 63. Myelin sheaths were not observed within the IC electron microscopically unti l day 73. Examination of the peripheral auditory system revealed that until about day 40 the middle ear was fluid-filled and middle ear stru ctures 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 be gins (67 days), and the last major anatomical change preceding hearing appears to be the opening of the external ear canal. (C) 1996 Wiley-L iss, Inc.