FEATURES OF THE STRUCTURAL DEVELOPMENT OF THE INFERIOR COLLICULUS IN RELATION TO THE ONSET OF HEARING IN A MARSUPIAL - THE NORTHERN QUOLL, DASYURUS-HALLUCATUS
L. Aitkin et al., FEATURES OF THE STRUCTURAL DEVELOPMENT OF THE INFERIOR COLLICULUS IN RELATION TO THE ONSET OF HEARING IN A MARSUPIAL - THE NORTHERN QUOLL, DASYURUS-HALLUCATUS, Journal of comparative neurology, 375(1), 1996, pp. 77-88
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.