Synapse formation and the development of neuronal connections in the hippocampal formation

Postnatal development of the hippocampal formation and its connections has been described in detail in rats. There are but few available data in primates, and data pertaining to human development are particularly sparse. The lack of empirical study can be partially explained by the difficulties of tissue preservation due to the regulated postmortem delay between the time of death and autopsy. After a few hours' delay, the cellular ultrastructure is obscured in human tissue. In rats, synaptic connections and dendritic development of a few cell types have been described, such as those of the granule cells (Seress and Pokorny, 1981), mossy cells (Ribak, Seress, and Amaral, 1985), and the pyramidal cells of the CA1 area of Ammon's horn (Pokorny and Yamamoto, 1981). Light microscopic changes of the dendrites, such as spine density and spine morphology, correlate with the increasing number of afferent terminals that establish synapses with dendrites of hippocampal neurons. Studies in rats suggest that in the dentate gyrus an increasing number of mossy fiber axon terminals establish connections with the hilar mossy cells and the pyramidal cells of the CA3 area (Amaral and Dent, 1981; Ribak, Seress, and Amaral, 1985). First connections are established with dendritic shafts at birth (Amaral and Dent, 1981). The more complex terminals of mossy fibers induce the development of thorny excrescences on the dendrites of 2-week-old rats (Amaral and Dent, 1981). Maturation of this connection appears to terminate 2-3 weeks after the last granule cells have been formed at the end of the third week. The CA3 pyramidal cells send the Schaffer collaterals to the CA1 pyramidal cells. It is not known when the Schaffer collaterals reach the CA1 cells, nor when this connection is fully mature. However, the CA1 pyramidal cells show a delayed maturation relative to the devel opment of CA3 pyramidal cells; CA1 pyramidal cells show extensive dendritic growth and an increasing synaptic density up to the 90th postnatal day. This correlates with an increasing number of synapses in the stratum radiatum (Pokorny and Trojan, 1986; Pokorny and Yamamoto, 1981).

The circuitries of the hippocampal formation have been known since the time of Ramon y Csyal (1911). Andersen and colleagues (1971) used the term "hippocampal trisynaptic circuit" to refer to the unique unidirectional progression of excitatory pathways that link the subregions of the hippocampal formation (Andersen, Bliss, and Skrede, 1971). Within this circuit the granule cells are the first elements. They connect the dentate gyrus to the CA3 pyramidal neurons of Ammon's horn, which in turn project to the CA1 pyramidal cells. CA1 pyramidal neurons are the sole cortical ef-ferents of the hippocampus (for further discussion, see Johnston and Amaral, 1998).

Granule cells are the last to be formed among the neurons of the trisynaptic circuit. As a consequence, the synaptic connections between the dentate gyrus and Ammon's horn are established late in ontogenesis. This highlights the functional importance of this first link in the trisynaptic circuit, because its morphological maturation correlates with hippocampus-dependent behavioral maturation (Nadel and Willner, 1989).

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