Eckenhoff M F, Rakic P
Yale University School of Medicine, Section of Neurobiology, New Haven, CT 06510.
Brain Res Dev Brain Res. 1991 Dec 17;64(1-2):129-35. doi: 10.1016/0165-3806(91)90216-6.
Quantitative electron microscopy was used to study synapse formation in the molecular layer of the dentate gyrus in rhesus monkeys ranging in age from embryonic day 62 to adult. Four to eight radial probes, consisting of a series of overlapping electronmicrographs and extending across the full thickness of the molecular layer were made in each specimen. Synaptic density (normalized to volume of neuropil) increased significantly during the last half of gestation, reaching adult levels at the time of birth. However, new synapses were added during infancy, resulting in an apparent peak in density at between 4 and 5 months of age. This increase was followed by a decline in the synaptic density over the next 5 months, to levels comparable to that of the newborn. In addition to synaptic density, synapse type (symmetric, asymmetric), location (on dendritic shafts or spines), and laminar distribution in the developing molecular layer was determined. The decrease in synaptic density is unlikely to be due to 'dilution' caused by an increase in molecular layer volume since no increase in the volume of the dentate gyrus could be detected during this period. Our calculations suggest that a selective overproduction of asymmetrical, axo-spinous synapses occurs during infancy. Finally, synaptic density was significantly greater in the middle third of the molecular layer suggesting that synaptic exuberance may be related to entorhinal input.
运用定量电子显微镜技术，对年龄从胚胎第62天至成年的恒河猴齿状回分子层中的突触形成进行了研究。在每个标本中制作了4至8个径向探针，每个探针由一系列重叠的电子显微镜照片组成，且横跨分子层的全层厚度。突触密度（以神经毡体积进行归一化）在妊娠后半期显著增加，在出生时达到成年水平。然而，在婴儿期会添加新的突触，导致密度在4至5个月龄时出现明显峰值。在此之后，突触密度在接下来的5个月中下降，降至与新生儿相当的水平。除了突触密度外，还确定了突触类型（对称型、不对称型）、位置（在树突干或棘上）以及在发育中的分子层中的层状分布。突触密度的下降不太可能是由于分子层体积增加导致的“稀释”，因为在此期间未检测到齿状回体积增加。我们的计算表明，在婴儿期会出现不对称的轴-棘突触选择性过度产生的情况。最后，分子层中间三分之一区域的突触密度显著更高，这表明突触的过度生长可能与内嗅输入有关。
