DYNAMICS OF RETINOTECTAL SYNAPTOGENESIS IN NORMAL AND 3-EYED FROGS - EVIDENCE FOR THE POSTSYNAPTIC REGULATION OF SYNAPSE NUMBER

Quantitative stereological methods were used to determine if the numbe r, density, and types of synaptic connections formed during developmen t are regulated by presynaptic input or by postsynaptic target cells i n the optic tectum of normal and 3-eyed Rana pipiens tadpoles and frog s. Our analysis indicates that the number and size of synapses is appr oximately the same in both tecta of 3-eyed tadpoles and frogs, even th ough one tectal lobe is receiving input from twice the normal compleme nt of retinal ganglion cells. Moreover, the number and size of synapse s in the tectal lobes of 3-eyed animals did not differ significantly f rom values determined for normal tadpoles and frogs of the same develo pmental stage. These data suggest strongly that developing tectal cell s regulate the number of synaptic contacts they will form. Differences in several morphological features between singly and doubly innervate d tecta, however, including synapse density, distribution and complexi ty, amount of extracellular space, and number of myelin figures, sugge st that the presence of supernumerary input retards tectal maturation. We propose that the noncorrelated activity of retinal ganglion cell t erminals in the doubly innervated tectum results in fewer stabilized s ynapses per unit volume of neuropil and in the delayed maturation of t he tectal neuropil. Taken together, our data suggest a complex dynamic interaction between retina and tectum during development. (C) 1994 Wi ley-Liss, Inc.