In the results of this study, we obtained morphological findings on the expression patterns of synaptic marker (synaptophysin) between the layers of the hippocampal region and also according to the developmental stages of embryonic, neonatal, and adult. As described in the introduction, this study was a continuation of previous morphological results in cerebral cortex. It is well known that the hippocampus is connected to the septum, mammillary body of hypothalamus, and the anterior nucleus of thalamus which are involved with emotional behavior. However, the hippocampus is an elaboration of the edge of the cerebral cortex (limbic system) anatomically. The entorhinal cortex which is located in the parahippocampal gyrus is also a part of the hippocampal region because of its anatomical connections and its reciprocal connections with many other parts of the cerebral cortex. Therefore based on the morphological findings in cerebral cortex and hippocampal region, we characterize morphological patterns of neural connections and synaptogenesis according to the developmental stages for the next step.
In the embryonic and neonatal hippocampal region, synaptophysin immunofluorescence was observed mainly in the molecular and oriens layers. Furthermore, the synaptophysin immunoreactivities following neural processes extended perpendicular between pyramidal cell layer and lacunosum molecular layer through radiatum layer. These results seem to be associated with a dendritic structure of pyramidal cells. In the rodent hippocampus, apical dendrites of pyramidal cells extend from the cell body in pyramidal cell layer through the radiatum layer and into the lacunosum molecular layer. The basal dendrites of pyramidal cells also produce another dense tuft in oriens layer. Therefore the differences of synaptophysin immunoreactivities between the layers could be explained by different synaptogenic activities between apical and basal dendrites of pyramidal cells according to the developmental stages.
Unlike CA in hippocampus, synaptophysin immunoreactivities were slight or negligible from the dentate gyrus even in the embryonic and neonatal group. The major input to the dentate gyrus is from entorhinal cortex and there's no direct input from other cortical structures. In the layers of dentate gyrus, granule cells in the middle layer are most prominent and known to project mostly to the interneurons within CA3 subfield. More than 80% of the granule cells are generated after birth in rodents and in humans, it is estimated that granule cells continue being generated not only after birth but also all the way into adulthood [18
]. In the study of granule cell migration during rat brain development, the granule cells begin to migrate and settle into the developing dentate gyrus around embryonic day 17 or 18 [19
]. Therefore considering numerous nuclei stained with DAPI within the dentate gyrus in all groups, synaptogenic activities from and to the dentate gyrus seem focused on the periods between neonatal and adulthood. Because we used only neonatal day 1 mice for neonatal group at this time, further experiments with the developing stages between neonatal and adulthood will reveal certain morphological synaptogenic activities in dentate gyrus. In the results showing changes of synaptophysin expressions according to the developmental stages, the adult group also possessed significant or even higher intensities than embryonic or neonatal. As shown in the picture of result, only homogeneous background immunofluorescence was detected with no specific synaptophysin immunoreactivities in the hippocampal region of adult group. Therefore it seems to be more reasonable to ignore the values obtained from the adult group although the background intensities in adult group were included for the morphological calculations and statistical analyses. Along with the morphological and many other electrophysiological findings of synaptogenic activities, various factors known to be involved in the neural network formation and plasticity should be also weighed for more detailed and precise interpretation of synaptogenic activities in the cortical and hippocampal region throughout the developmental stages. For example, progesterone, which is known to have a protective effect on damage to the central nervous system, also increased synaptophysin expression in the CA1 region of hippocampus [20
], and the amount of synaptic protein expression was reported to differ between male and female through the postnatal periods [21
In conclusion, we delineated differential synaptophysin expressions between molecular and oriens layers of embryonic and neonatal hippocampus. We also demonstrated decreased synaptophysin expressions from the neonatal group compared to embryonic in both layers. It is not enough to explain synaptogenic findings with only one synaptic marker. However, synaptophysin is known as the most abundant [22
] and is expressed ubiquitously in almost all types of synapse forming neurons. Therefore the results from this study along with the previous work from the cortical region will contribute not only to analyzing regional characteristics of synaptogenic activities through developmental stages but also to understanding pathological physiologies of brain development in association with time and regional distinctions of synaptogenic activities.