It is well-established that synapse formation involves highly selective chemospecific mechanisms, but how neuron arbors are positioned before synapse formation remains unclear. Using 3D reconstructions of 298 neocortical cells of different types (including nest basket, small basket, large basket, bitufted, pyramidal, and Martinotti cells), we constructed a structural model of a cortical microcircuit, in which cells of different types were independently and randomly placed. We compared the positions of physical appositions resulting from the incidental overlap of axonal and dendritic arbors in the model (statistical structural connectivity) with the positions of putative functional synapses (functional synaptic connectivity) in 90 synaptic connections reconstructed from cortical slice preparations. Overall, we found that statistical connectivity predicted an average of 74 ± 2.7% (mean ± SEM) synapse location distributions for nine types of cortical connections. This finding suggests that chemospecific attractive and repulsive mechanisms generally do not result in pairwise-specific connectivity. In some cases, however, the predicted distributions do not match precisely, indicating that chemospecific steering and aligning of the arbors may occur for some types of connections. This finding suggests that random alignment of axonal and dendritic arbors provides a sufficient foundation for specific functional connectivity to emerge in local neural microcircuits. connectome | microcircuitry | neocortex | neuronal connectivity D uring development, a broad range of molecular signaling mechanisms act to form the neocortical layers (1, 2), lay down guidance tracts for neurons to position themselves and grow their axonal and dendritic arbors (3, 4), shape the morphologies of different types of neurons (5), guide input axons to specific layers (6), trigger differentiation and clustered arborizations of axonal arbors (7), prune axonal arbors in an activitydependent manner (8), guide axonal growth along the axon initial segment (9), and selectively form synapses between specific types of neurons (10-12). Molecular mechanisms also contribute to the activity-triggered formation of synapses between specific pairs of neurons (13). The mechanisms determining this selective connectivity have been a subject of active debate for over a century (3, (14) (15) (16) (17) , with proposals ranging from the idea of "chemical relations" between connected neurons in the work by Langley (18) to the "chemoaffinity hypothesis" (the idea that precise synaptic positioning is specified by "highly specific cytochemical affinities" between neurons) in the work by Sperry (19). The result is an intriguing map of synaptic connections between all neurons and specific types of neurons, commonly called the connectome. How the connectome arises is still debated (20) . A question that is at the core of the debate is to what extent do chemospecific mechanisms specifically align neuronal arbors between neurons before synaptic connections are formed. Spines are dynamic structures that form, grow, shrink, or even disappear in response to activity and experience (21), and neocortical pyramidal neurons connect and disconnect from each other dynamically and in response to stimulation (13). Local dendritic and axonal specializations, such as the formation of a spine and synaptic bouton, are sufficient to reconfigure neural circuits without the need for axons to grow to a target cell. Therefore, all neurons that can potentially form connections must already be aligned before synapse formation to ensure cell type-specific synaptic patterning. Even the strongest proponents of statistical connectivity or neurogeometry (22, 23) believe that arbor alignment requires extracellular chemical repulsion (24) and attraction (25). However, it has been proposed that the unique alignment of thousands of individual fibers of specific pairs of neurons would require "millions and perhaps billions" of cell-specific molecular isoforms (19). To address the question of the extent to which the alignment of arbors between specific types of neurons is caused by pairwise chemospecific signaling as opposed to the incidental overlap of independent neural morphologies, we constructed a model neocortical circuit containing a subset of the neuron types found in neocortex. By assembling the circuit from morphologies that were from the same brain area and age range but different animals and positioned randomly, any possible pairwise chemospecific alignment was excluded. This assembly allowed us to test the extent to which statistical connectivity, as measured in terms of appositions between arbors of independently positioned and grown morphologies, could predict measurements of functional synaptic connectivity found in cortical slice preparations. We found that, with only a few exceptions, statistical connectivity was a good predictor of functional synaptic connectivity. This finding suggests that the connectome of cortical microcircuitry is largely formed from the nonspecific alignment of neuron morphologies. In only a few cases are chemospecific mechanisms required to explain specific fiber alignments. Although many studies have examined the extent that geometrical overlap predicts the number of synapses between neurons (23), it has been more difficult to test whether random geometrical mechanisms can account for the distribution of
The paper REF also studies the real biological brain and in particular, the chemospecific steering and aligning process for synaptic connections.
Statistical connectivity provides a sufficient foundation for specific functional connectivity in neocortical neural microcircuits.
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Biology Medicine