Two classes of factors contributing to spinogenesis have been described in the literature, based on whether they can be considered extrinsic or intrinsic to the dendrite (my classification). Here is a short list of evidence in favor of either:
A proposed mechanism extrinsic to the dendrite that can lead to spinogenesis is glutamate spill-over. Conceivable mechanisms intrinsic to the dendrite that can control spinogenesis can be the competition for resources (structural proteins, mRNA etc).
Dendrites of pyramidal cells in some areas of the cortex and the hippocampus exhibit a relatively high turn-over of spine formation and elimination (Holtmaat et al. 2005, Attardo et al. 2005), at least as compared to axonal boutons (e.g. de Paola 2006). Whereas some of these new spines stabilize, many disappear soon after their formation. This indicates that, at least in part, their creation is not fully specified by the existence of a pre-synaptic partner, if we assume that the pre-synaptic partner continues to "attract" them (and that's a big if). The relative independence of spinogenesis from presynaptic activity is further corroborated by the fact that most new spines lack a synapse (Knott et al., 2006).
It appears, then, that dendrites over-produce spines in order to sample their environment for potential synaptic partners. The advantage of such a mechanism can be seen when considering the potential wiring diagram changes that neurons can achieve with such relatively cost-less microscopic changes (Stepanyants et al. 2002). It is unclear at the moment at what point between spinogenesis and synapse formation presynaptic activity becomes an influencing factor in the intact adult brain. A theory incorporating the existing information on spinogenesis is forthcoming.
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