Optic synapse number but not density is constrained during regeneration onto surgically halved tectum in goldfish: HRP‐EM evidence that optic fibers compete for fixed numbers of postsynaptic sites on the tectum

The number of optic synapses in the half tectum of goldfish was counted by using an improved HRP‐labeling protocol and a columnar sampling method that spanned the entire optic innervation layer, S‐SO‐SFGS. It was previously found by using this procedure in intact tectum that the normal number of optic synapses was regenerated by 30 days and maintained thereafter even in the absence of impulse activity. This suggested that the number of synapses in this system was intrinsically fixed. In order to examine whether this limit was imposed by optic fibers or by target cells, optic synapses were counted in surgically halved tecta which received compressed optic projections consisting of regenerating optic fibers from the entire retina. We reasoned that if synapse number is a function of the number of afferents, then there should be twice the normal number of optic synapses per column; on the other hand, if their number is fixed by target, then their number per column should be normal.