Hinds J W, Hinds P L
J Comp Neurol. 1976 Sep 1;169(1):15-40. doi: 10.1002/cne.901690103.
A quantitative study of synapse formation in the mouse olfactory bulb has been carried out using serial sections. Volumetric synaptic density as well as absolute number of synapses per olfactory bulb for eight distinct synaptic types have been determined at 15 different ages, from the beginning of synapse formation at embryonic day 14 (E14) to postnatal day 44 (P44). Synapses are first found in appreciable numbers at E15 when both axo-dendritic and a few dendro-dendritic synapses occur in the presumptive glomerular layer. Initial synapse formation correlates closely with the reorientation of mitral cells from a primitive tangenital to a definitive radial direction. Synapse formation by mitral cell dendrites occurs after mitral cell axons have grown into the future olfactory cortical areas, either simultaneous with or before synapse formation by these axons. Virtually all synaptic types detected in adults have been found on the day of birth, consistent with the idea that olfaction is an important sensory modality for newborn mice. Volumeric density of a given synaptic type generally increases 50--100 times during development while the absolute number increases about 1,000 times. Synapses in glomeruli develop more precociously than those in the time of origin and differentiation of the principal postsynaptic elements of these two divisions (mitral cells and internal granule cells). Correlation of the time of synapse formation of various synaptic types with their putative excitary or inhibitory role determined in adult studies suggests that excitatory synapses generally form somewhat earlier, although throughout nearly all of synaptic development, both excitatory and inhibitory synapses are present. Reciprocal dendro-dendritic synapses in the external plexiform layer appear to have a special mode of formation. It is suggested that a granule-to-mitral dendro-dendritic synapse only forms next to an already existing mitral-to-granule synapse on the same gemmule.
利用连续切片对小鼠嗅球中突触形成进行了定量研究。在从胚胎第14天（E14）突触形成开始到出生后第44天（P44）的15个不同年龄阶段，测定了8种不同突触类型的体积突触密度以及每个嗅球的突触绝对数量。在E15时首次发现大量突触，此时在假定的肾小球层中出现了轴突 - 树突突触和一些树突 - 树突突触。初始突触形成与二尖瓣细胞从原始切线方向到确定的径向方向的重新定向密切相关。二尖瓣细胞树突的突触形成发生在二尖瓣细胞轴突生长到未来的嗅觉皮质区域之后，与这些轴突的突触形成同时或之前。在出生当天就发现了几乎所有在成体中检测到的突触类型，这与嗅觉是新生小鼠重要的感觉方式这一观点一致。在发育过程中，给定突触类型的体积密度通常增加50 - 100倍，而绝对数量增加约1000倍。肾小球中的突触比这两个部分（二尖瓣细胞和内颗粒细胞）主要突触后元件的起源和分化时期的突触发育得更早。各种突触类型的突触形成时间与其在成体研究中确定的假定兴奋或抑制作用之间的相关性表明，兴奋性突触通常形成得稍早一些，尽管在几乎所有的突触发育过程中，兴奋性和抑制性突触都存在。外丛状层中的相互树突 - 树突突触似乎有一种特殊的形成方式。有人提出，颗粒 - 二尖瓣树突 - 树突突触仅在同一小球上已经存在的二尖瓣 - 颗粒突触旁边形成。
