Distribution of growth cones and synapses in developing laminar and interlaminar regions of the dorsal lateral geniculate nucleus

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Abstract
In the present study, we quantified the distribution of growth cones and synapses in 2 developing layers, as well as in the intervening interlaminar space of the dorsal lateral geniculate nucleus (dLGN) in tree shrews. Our goal was to gain insight into mechanisms involved in the segregation of dLGN cells into layers during development. We sacrificed tree shrews before (P0), during (P4 and P7), and after (P15) laminar segregation as well as at maturity (P90). The dLGN from each animal was sectioned horizontally, and all tissue for analysis was blocked from the middle third of the nucleus along the dorsoventral axis. Each micrograph was coded and blindly scored for the number of growth cones and synapses in layers 4 and 5 and the intervening interlaminar space. We also measured each growth cone and classified synapse type. Statistical analyses of these data reveal that neither growth cones nor synapses are significantly more common in the interlaminar space early in the period of laminar segregation (P4). By nearly a week after the interlaminar space can first be distinguished (P7), there are more growth cones in the interlaminar space than in the layers, but this difference is no longer present at (P15). These results suggest that, although neuropil development at the laminar borders may not play a role in the onset of laminar segregation, it may contribute to the widening of the interlaminar spaces once this process has begun. In addition, growth cones continuously decrease in number and become less bulbous and more linear in shape with development. Synapses, on the other hand, continuously increase in number with age and pass through a transient period characterized by heavy spinous terminations.