J. Kil et al., DEVELOPMENT OF VENTRAL COCHLEAR NUCLEUS PROJECTIONS TO THE SUPERIOR OLIVARY COMPLEX IN GERBIL, Journal of comparative neurology, 353(3), 1995, pp. 317-340
The postnatal development of the projection from the ventral cochlear
nucleus to the principal nuclei in the superior olivary complex in ger
bil (Meriones unguiculatus) was studied in an age-graded series of pup
s ranging from 0 to 18 days old. Small crystals of -dioctadecyl-3,3,3'
;3'-tetramethylindocarbocyanine perchlorate (DiI) were inserted into t
he ventral cochlear nucleus of aldehyde-fixed brains, and the labeled
projections were examined with epifluorescence microscopy, Selected se
ctions were photooxidized in a solution of diaminobenzidine and subseq
uently processed for electron microscopy to examine the development of
labeled synapses in the target nuclei. Horseradish peroxidase was inj
ected into the ventral cochlear nucleus of adult gerbils to assess the
form and persistence of projections observed in the neonatal animals.
In addition, electrophysiological responses to acoustic stimuli of si
ngle units in the adult auditory brainstem were analyzed to confirm th
e functionality of the novel projection from the ventral cochlear nucl
eus to the contralateral lateral superior olive. By the day of birth (
PO), developing axons from the ventral cochlear nucleus have already e
stablished highly ordered pathways to the three primary nuclei of the
superior olivary complex: the ipsilateral lateral superior olive, the
contralateral medial nucleus of the trapezoid body, and at the lateral
and medial dendrites of the ipsilateral and contralateral medial supe
rior olive, respectively. Developing axons from the ventral cochlear n
ucleus that innervated the contralateral medial nucleus of the trapezo
id body lacked the terminal morphology characteristic of the calyx of
Held, but began to adopt a more characteristic form on P5. The mature
calyx appeared around P14-16, Exuberant developmental projections to t
opographically inappropriate areas of the superior olivary complex wer
e not observed at the postnatal ages studied. In addition to the proje
ctions of the ventral cochlear nucleus to the superior olivary complex
described in other species, we observed the development and maintenan
ce of a major direct projection from the ventral cochlear nucleus to t
he contralateral lateral superior olive. On PO, ventral cochlear nucle
us axons decussate;in the dorsal trapezoid body, form a plexus at the
dorsal edge of the contralateral medial superior olive, and enter the
ventrolateral limb of the contralateral lateral superior olive. Over t
he next 2 weeks, fascicles of fibers form on the hilar and ventral asp
ects of the ventrolateral limb. Fibers arising from these fascicles fo
rm converging, but nonoverlapping, arborizations within the ventrolate
ral limb at right angles-to the curvature of the nucleus. The medial r
egion was devoid of labeled axons. The direct innervation of the contr
alateral lateral superior olive was confirmed in the adult gerbil with
anterograde horseradish peroxidase histochemistry and by the recordin
g of excitatory responses in the innervated region to acoustic stimula
tion of the contralateral ear. (C) 1995 Wiley-Liss, Inc.