Berdan R C, Shivers R R, Bulloch A G
Neuroscience Research Group, University of Calgary, Alberta, Canada.
Synapse. 1987;1(4):304-23. doi: 10.1002/syn.890010404.
The nervous system of the snail, Helisoma trivolvis, has been utilized for a wide range of studies of neuronal plasticity; however, the ultrastructural features of this tissue were previously unknown. The present study examined the nature of synaptic interactions of neurons and glia and considered several plasma membrane specializations of these cells. The symmetrical pair of buccal ganglia consisted of a ring of unipolar neurons surrounding a central neuropil. The neurons were separated by two morphologically distinct types of glia: type I were most numerous and possessed an electron-dense homogeneous cytoplasm, whereas type II glia were of lower electron density, possessed a heterogeneous cytoplasm, and appeared to be phagocytic. Gap junctions were abundant between glia and were occasionally found between neuronal processes, including those of neurons 19 injected with horseradish peroxidase (HRP). Comparison of neuron and glial gap junction widths (16.4 and 17.6 nm, respectively) in thin sections and their intramembrane particle diameters (13.1 and 13.7 nm, respectively) by freeze fracture, did not elucidate significant differences. A heterogeneous population of putative chemical synapses, similar to those reported in other molluscs, was also observed between axonal collaterals in the neuropil. Additionally, examination of freeze-fractured neuropil revealed rhombic arrays of particles localized on neuronal membranes; these arrays do not appear to form intercellular junctions but may represent postsynaptic receptor sites. Freeze fracture also revealed small, square arrays consisting of 7-9 nm diameter particles on glial membranes which may correspond to pentalaminar membrane contacts (pseudo-gap junctions) seen in thin sections between glia situated around dilated extracellular spaces (lacunae).
三旋椎实螺（Helisoma trivolvis）的神经系统已被广泛用于神经元可塑性的各类研究；然而，此前人们对该组织的超微结构特征并不了解。本研究考察了神经元与神经胶质细胞之间突触相互作用的性质，并研究了这些细胞的几种质膜特化结构。对称的一对口腔神经节由围绕中央神经纤维网的单极神经元环组成。神经元被两种形态不同的神经胶质细胞分隔开：I型神经胶质细胞数量最多，具有电子致密的均匀细胞质，而II型神经胶质细胞电子密度较低，细胞质异质，且似乎具有吞噬作用。神经胶质细胞之间存在丰富的缝隙连接，在神经元突起之间偶尔也能发现，包括注射了辣根过氧化物酶（HRP）的19号神经元的突起。通过超薄切片比较神经元和神经胶质细胞缝隙连接的宽度（分别为16.4纳米和17.6纳米），并通过冷冻断裂比较它们的膜内颗粒直径（分别为13.1纳米和13.7纳米），未发现显著差异。在神经纤维网的轴突侧支之间还观察到了一群异质性的假定化学突触，类似于其他软体动物中报道的突触。此外，对冷冻断裂的神经纤维网进行检查发现，神经元膜上有菱形颗粒阵列；这些阵列似乎并未形成细胞间连接，但可能代表突触后受体位点。冷冻断裂还揭示了神经胶质细胞膜上由直径7 - 9纳米的颗粒组成的小方形阵列，这可能与在扩张的细胞外间隙（腔隙）周围的神经胶质细胞之间的超薄切片中看到的五板层膜接触（假缝隙连接）相对应。
