Postnatal development of projections from hippocampal formation to entorhinal cortex in the rat
Master thesis
Permanent lenke
http://hdl.handle.net/11250/229208Utgivelsesdato
2010Metadata
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Sammendrag
Neuroanatomical studies on the postnatal development of connectivity in the entorhinal cortex and hippocampal formation are sparse in the scientific literature. Previous studies have established that all neurons in the entorhinal cortex and hippocampal formation are born prenatally, and described the perinatal development of the entorhinal-to-hippocampal projections (Bayer 1980a; Deng, Yu et al. 2007). In this study, retrograde and anterograde anatomical tracing was performed on newborn and young rats from birth until 16 days of age, with the aim to determine (i) whether the connections from hippocampal formation to entorhinal cortex are present at birth, and (ii) how the postnatally developing hippocampal-to-entorhinal connections are organized relative to the adult situation. To our knowledge, no one has previously undertaken a similar anatomical tracing study of the developing connectivity in the rat hippocampal formation and parahippocampal region, by adapting adult methodology to pups. In order to complete the experiments in this thesis, surgery and perfusion conditions were optimized. Further, choice of anterograde and retrograde tracers, as well as survival parameters, were tested and optimized. The final experiments reported here use Fast Blue and Fluorogold as retrograde tracers. For the anterograde tracing, a well established tracer such as biotinylated dextran amine (BDA) yielded the best results. All tracers show good transport with a 24 survival period. Injections of retrograde tracers into the medial entorhinal cortex show that already at birth extensive projections from CA1 and the subiculum are present. Anterograde tracing data confirm this finding for the subiculum. Interestingly, anterograde tracing revealed a terminal pattern in superficial layers of the entorhinal cortex not previously reported in adult anatomy. This superficial terminal pattern was seen in ages P0, P2, P7, P11, P14, and P16. Our retrograde tracer experiments further indicate that during early phases of development, CA3 may send projections to the medial entorhinal cortex. Such connections have been reported in the adult rat brain, but replications of the original claims have not provided support for adult projections from CA3 to medial entorhinal cortex. In view of possible confounds in retrograde data such as uptake of tracer by passing fibers the existence of these putative CA3 to MEA projections need to be confirmed by other methods, such as anterograde tracing. In conclusion, the present data taken together with previously published data on EC to hippocampal projection indicate that reciprocal hippocampal-entorhinal connectivity is present at birth. We suggest that postnatal development into a fully developed adult hippocampal-entorhinal cortex system involves extensive pruning of the circuitry.