Petrak Lara J, Harris Kristen M, Kirov Sergei A
Department of Biology, Boston University, Boston, Massachusetts 02215, USA.
J Comp Neurol. 2005 Apr 4;484(2):183-90. doi: 10.1002/cne.20468.
During development, dendritic spines emerge as stubby protrusions from single synapses on dendritic shafts or from retracting filopodia, many of which have more than one synapse. These structures are rarely encountered in the mature brain. Recently, confocal and two-photon microscopy have revealed a proliferation of new filopodia-like protrusions in mature hippocampal slices, especially when synaptic transmission was blocked. It was not known whether these protrusions have synapses nor whether they are accompanied by the other immature spine forms. Here, reconstruction from serial section electron microscopy (ssEM) was used to answer these questions. Acute hippocampal slices from mature male rats, ages 56 and 63 days, were maintained in vitro in control medium or in a nominally calcium-free medium with high magnesium, glutamate receptor antagonists, and sodium and calcium channel blockers. At the end of each 8-hour experiment, all slices were fixed, coded, and processed for ssEM. In agreement with light microscopy, there were more filopodia along dendrites in slices with blocked synaptic transmission. These filopodia were identified by their pointy tips and either the absence of synapses or presence of multiple synapses along them. There was also a proliferation of stubby spines. Filopodia along mature dendrites were typically shorter than developmental filopodia, with outgrowth likely being constrained by reduced extracellular space and compact neuropil, providing numerous candidate presynaptic partners in the vicinity of the mature dendrites. These findings suggest that synaptogenesis and spine formation are readily initiated under conditions of reduced activity in the mature brain.
在发育过程中，树突棘作为从树突干上的单个突触或从回缩的丝状伪足伸出的短粗突起出现，其中许多有不止一个突触。这些结构在成熟大脑中很少见。最近，共聚焦显微镜和双光子显微镜揭示了成熟海马切片中出现了大量新的丝状伪足样突起，尤其是在突触传递被阻断时。尚不清楚这些突起是否有突触，也不清楚它们是否伴有其他不成熟的棘突形式。在此，利用连续切片电子显微镜（ssEM）重建来回答这些问题。将56天和63天龄成熟雄性大鼠的急性海马切片在对照培养基或含有高镁、谷氨酸受体拮抗剂以及钠和钙通道阻滞剂的无钙培养基中进行体外培养。在每个8小时实验结束时，所有切片均固定、编码并进行ssEM处理。与光学显微镜观察结果一致，突触传递被阻断的切片中，树突上的丝状伪足更多。这些丝状伪足通过其尖状末端以及沿其无突触或有多个突触来识别。短粗棘突也有增多。成熟树突上的丝状伪足通常比发育过程中的丝状伪足短，其生长可能受到细胞外空间减少和神经毡致密化的限制，这在成熟树突附近提供了众多潜在的突触前伙伴。这些发现表明，在成熟大脑活动减少的情况下，突触形成和棘突形成很容易启动。
