Meinertzhagen I A
Neuroscience Institute, Dalhousie University, Halifax, Nova Scotia, Canada.
J Neurogenet. 1994 Jul;9(3):157-76. doi: 10.3109/01677069409167277.
The number of synaptic contacts formed by a neuron is known to vary with its surface area. This could be because large neurons are able to establish more synaptic sites, or because those neurons that are able to establish more sites are subsequently able to enlarge. To test between these two possibilities clones of enlarged ommatidia were generated in the retina of the Drosophila mutant gigas, by mitotic recombination following gamma-irradiation in the third-instar larva. The numbers of afferent synaptic contacts formed by the photoreceptor terminals in the first optic neuropil, or lamina, were then counted in the adult. The terminals of mutant photoreceptors were also enlarged, but by varying degrees. The sizes of their profiles in single sections merged with the size distribution of terminals having a wild-type phenotype, lying outside the clone in the same lamina. A perimeter of 6.0 microns for the profiles of receptor terminal in cross section was established as a criterion for distinguishing between normal and mutant phenotypes. The mutant terminals had more presynaptic sites. Because only the gigas terminals are mutant and because they enlarged at a time long before synapse formation occurred in the lamina we may conclude that cell enlargement preceded elevated synaptic number. The increase in synaptic number roughly matched the increased membrane surface of the terminals, so as nearly to preserve a constant areal density of synaptic sites over a 5-fold range in synaptic frequency.
已知神经元形成的突触接触数量会随其表面积而变化。这可能是因为大型神经元能够建立更多的突触位点，或者是因为那些能够建立更多位点的神经元随后能够增大。为了在这两种可能性之间进行测试，在果蝇突变体“巨眼”（gigas）的视网膜中，通过在三龄幼虫期进行γ射线照射后的有丝分裂重组，产生了放大的小眼克隆。然后在成虫中计数第一视觉神经节（即神经层）中光感受器终端形成的传入突触接触数量。突变光感受器的终端也增大了，但程度不同。它们在单个切片中的轮廓大小与同一神经层中克隆外具有野生型表型的终端大小分布相融合。将横截面中受体终端轮廓的周长6.0微米确定为区分正常和突变表型的标准。突变终端具有更多的突触前位点。由于只有“巨眼”终端发生突变，并且它们在神经层中突触形成之前很久就增大了，所以我们可以得出结论，细胞增大先于突触数量增加。突触数量的增加大致与终端增加的膜表面积相匹配，从而在突触频率相差5倍的范围内，几乎保持突触位点的面密度恒定。
