Braubach, O.R., Miyasaka, N., Koide, T., Yoshihara, Y., Croll, R.P., and Fine, A. (2013) Experience-Dependent versus Experience-Independent Postembryonic Development of Distinct Groups of Zebrafish Olfactory Glomeruli. The Journal of neuroscience : the official journal of the Society for Neuroscience. 33(16):6905-6916.
Olfactory glomeruli are innervated with great precision by the axons of different olfactory sensory neuron types and act as
functional units in odor information processing. Approximately 140 glomeruli are present in each olfactory bulb of adult zebrafish;
these units consist of either highly stereotypic large glomeruli or smaller anatomically indistinguishable glomeruli. In the
present study, we investigated developmental differences among these types of glomeruli. We observed that 10 large and individually
identifiable glomeruli already developed before hatching, at 72 h after fertilization, in configurations that resembled their
mature organization. However, the cross-sectional area of these glomeruli increased throughout larval development, and they
eventually comprised the largest units in postlarval olfactory bulbs. In contrast, small and anatomically indistinguishable
glomeruli formed only after hatching, apparently by segregating from five larger precursors that were identifiable during
embryonic development. The differentiation of these small glomeruli proceeded with conspicuous variation in number and arrangement,
both among larvae and between olfactory bulbs of the same individuals. To determine factors that might contribute to this
variability, we investigated the effects of olfactory enrichment on the development of amino acid-responsive lateral glomeruli,
which include both large and small units. Larvae reared in an amino acid-enriched environment had normal large lateral glomeruli,
but the small lateral glomeruli were more numerous and displayed reduced cross-sectional areas compared with glomeruli in
control animals. Our results suggest that large and small glomeruli mature via distinct developmental processes that may be
differentially influenced by sensory experience.