König N, Roch G, Marty R
Anat Embryol (Berl). 1975 Nov 6;148(1):73-87. doi: 10.1007/BF00315564.
The onset of synaptogenesis was studied in the temporal cortex of rat fetuses whose age ranged between 15 and 19 days of gestation. First synapses were found at a surprisingly early stage of cortical development: on day 16. These contacts showed relatively few vesicles and very inconspicuous membrane-thickenings. They were located in the marginal layer, above as well as below the narrow band formed by the newly arrived first neuroblasts of the prospective corticle plate. The postsynaptic structures were probably dendrites of the horizontally or obliquely orientated neurons scattered throughout the marginal layer (such neurons were seen even within the cell-dense band). On day 17, the cortical plate separated the differentiated cells definitely into a superficial and a deep population. As on the following days, synapses were found above and below the cortical plate but not within it. In addition to contacts showing the same features as those described on day 16, there were already synapses with numerous vesicles and clearly asymmetric membrane thickenings. On days 18 and 19 the borders of the cortical plate became more clear-cut. The well-differentiated neurons situated above and below this plate could now be identified as Retzius-Cajal cells of the prospective molecular layer and as polymorphous cells of the layer VI b respectively. The presence of axo-somatic contacts on these neurons provided direct evidence that both cell types are targets for synapses. Desmosome-like junctions were found even in the youngest fetuses studied. Their roughly symmetric membrane thickenings were clearly more conspicious than those of earliest synapses. Desmosome-like junctions occurred very frequently between structures which subsequently were never seen to become synaptically linked. During the entire period studied, numerous coated vesicles fused with cell membranes were noted. Such "open" vesicles were seen on neurons (sometimes in the immediate vicinity of synapses) but also on non-nervous, extracortical as well as intracortical structures. Thus there does not seem to be a specific relationship between desmosome-like junctions and coated vesicles on the one hand and synapse formation on the other.
在妊娠15至19天的大鼠胎儿颞叶皮质中研究了突触发生的起始。最早的突触在皮质发育的一个惊人早期阶段被发现：在第16天。这些突触联系显示出相对较少的囊泡和非常不明显的膜增厚。它们位于边缘层，在前瞻性皮质板新到达的第一批神经母细胞形成的窄带之上和之下。突触后结构可能是散布在整个边缘层的水平或倾斜定向神经元的树突（即使在细胞密集带内也能看到这样的神经元）。在第17天，皮质板将分化的细胞明确地分为浅层和深层群体。与接下来的几天一样，在皮质板上方和下方发现了突触，但在皮质板内未发现。除了具有与第16天描述的特征相同的突触联系外，还已经有了具有大量囊泡和明显不对称膜增厚的突触。在第18天和第19天，皮质板的边界变得更加清晰。位于该板上方和下方的分化良好的神经元现在可以分别被鉴定为前瞻性分子层的雷兹乌斯 - 卡哈尔细胞和VI b层的多形细胞。这些神经元上轴体突触联系的存在提供了直接证据，表明这两种细胞类型都是突触的靶标。即使在研究的最年轻胎儿中也发现了桥粒样连接。它们大致对称的膜增厚明显比最早突触的膜增厚更明显。桥粒样连接在随后从未被观察到形成突触连接的结构之间非常频繁地出现。在整个研究期间，注意到许多有被小泡与细胞膜融合。这种“开放”小泡在神经元上可见（有时在突触附近），但也在非神经的皮质外以及皮质内结构上可见。因此，一方面桥粒样连接和有被小泡与另一方面突触形成之间似乎没有特定关系。
