Gray E G
J Neurocytol. 1975 Jun;4(3):315-39. doi: 10.1007/BF01102116.
When certain intracellular and extracellular localities known to be rich in protein complexes are fixed and processed for electron microscopy, they show a reticulate precipitate which represents three-dimensional framework of material that forms the wall of polygonal lacunae. This is referred to as a stereoframework. Examples of a stereoframework described her include the presynaptic dense projections, cleft substance, postsynaptic density, the cytonet, coats of coated vesicles, reticulosomes, 'microfilamentous' network of growth cones, the glycocalyx of gut microvilli, blood plasma, precipitates of the Golgi apparatus, the chromatin of nuclei and the nuclear pore complex. The stereoframeworkappears most electron-dense when it has a very close mesh, e.g. as in the case of the dense projections. The stereoframework is assumed to have no direct relationship with themolecular architecture of the protein complexes in vivo and so can be regarded as a denaturization and precipitation artifact. This being so, attempts to elucidate the substructure of the above entities simply by inspection are fruitless. Furthermore, evidence is given that stereoframework precipitation can distort or completely obliterate organelles occupying the same locality, for example this could apply to structures such as actin filaments (perhaps running into the locality marked by a dense projection), microtubules(running into the presynaptic bag), smooth ER, tenuous connections between synaptic vesicles and the presynaptic membrane, structures within the nuclear pore complex and chromosome substructures in the nucleus. Finally it is suggested that the flat shape of synaptic vesicles (at inhibitory synapses) may be a distortion effect imposed upon the synaptic vesicles not as a result of osmotic effects, but as a conformation to the shape of a stereoframework which has been precipitated from protein complexes in the vicinity ofthe synaptic vesicles.
当某些已知富含蛋白质复合物的细胞内和细胞外区域被固定并进行电子显微镜处理时，它们会显示出一种网状沉淀物，该沉淀物代表形成多边形腔壁的物质的三维框架。这被称为立体框架。本文所述的立体框架的例子包括突触前致密突起、缝隙物质、突触后致密区、细胞网络、被膜小泡的被膜、网状体、生长锥的“微丝状”网络、肠道微绒毛的糖萼、血浆、高尔基体沉淀物、细胞核染色质和核孔复合体。当立体框架具有非常紧密的网孔时，例如在致密突起的情况下，它看起来电子密度最高。立体框架被认为与体内蛋白质复合物的分子结构没有直接关系，因此可以被视为一种变性和沉淀假象。既然如此，仅仅通过观察来阐明上述实体的亚结构的尝试是徒劳的。此外，有证据表明立体框架沉淀会扭曲或完全掩盖占据同一位置的细胞器，例如这可能适用于肌动蛋白丝（可能延伸到由致密突起标记的位置）、微管（延伸到突触前囊泡）、光滑内质网、突触小泡与突触前膜之间的细微连接、核孔复合体内的结构以及细胞核中的染色体亚结构等结构。最后有人提出，（抑制性突触处的）突触小泡的扁平形状可能是施加在突触小泡上的一种扭曲效应，不是由于渗透作用，而是由于与从突触小泡附近的蛋白质复合物中沉淀出来的立体框架的形状相适应。
