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Normal Synaptic Ultrastructure of the Mouse and Rat Utricular Macula and the Mouse Crista Ampullaris. M. Ahmad and A. Lysakowski, Dept. of Anatomy, Univ. of Illinois at Chicago, 60612.

   The synaptic ultrastructure of the vestibular periphery consists of synaptic ribbons and calyceal invaginations.  Synaptic ribbons mediate neurotransmission from hair cells to afferent terminals.  Calyceal invaginations present in type I hair cells are suspected to play a role in synaptic transmission by facilitating vesicle recycling.  Others have suggested calyceal invaginations as a mode of ephaptic transmission between type I hair cells and calyx endings. (Goldberg, 1996).  We have shown that the numbers of synaptic ribbons and calyceal invaginations are modified under conditions of controlled hypergravity (Ahmad and Lysakowski, ASGSB Abst, 2002). The crista exhibited regional variations in the distribution of both synaptic ribbons and calyceal invaginations (Lysakowski and Goldberg, 1997), which were found to be more numerous in the central zone compared to the peripheral zone.

Synaptic ultrastructure of the mouse utricular macula and crista ampullaris was studied in 4 and 3 samples, respectively. The disector method was used. The results are preliminary, based on 113 hair cell equivalents (73.5 in the invaginations data). In the utricular macula, the average number of synaptic ribbons was 10.3 ± 1.1 in type I hair cells and 12.9 ± 0.8 in type II hair cells. The number of calyceal invaginations in type I hair cells was 32.7 ± 3.4, which varied by region with more invaginations (49.6 ± 11.7) per type I hair cell in the striola and fewer (32.5 ± 3.2) in the extrastriola.  In the crista, there were more synaptic ribbons and calyceal invaginations in type I hair cells peripherally (6.8 ± 2.8 and 34.9 ± 4.2, respectively) than in the central region (5.5 ± 1.5 and 20.5 ±7.6, respectively), based on 34 hair cell equivalents (23.5 in the invaginations data).  Type II hair cells had more synaptic ribbons centrally (13.9 ± 7) than peripherally (8.6 ± 3). Preliminary results from normal rat utricular macula (based on 19.5 hair cell equivalents) show that the number of ribbon synapses in type II (10) is higher than in type I hair cells (3.9).  The number of calyceal invaginations in the extrastriolar region (53.9) is similar so far to the striolar region (54.8).

Supported by NASA NAG2-1589 and NIH R01 DC2521.