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In this paper qualitative results are presented from GOLGI preparations by light microscopic evaluation about the appearance and the ontogenetic changes on GOLGI-II-type neurons of the rat hippocampus from 15 stages of age between the 20th embryonic and the 20th postnatal day. Interneurons are already detectable in all the hippocampal regions in the prenatal period. Their postnatal growth expresses above all in lengthening rather than in multiplication of the amount of dendrites. Cells predetermined for developing spines show already early in the postnatal period a rather large amount of spines. The further multiplication of the spine density remains minimal that never values of adult pyramids will be reached. The form of the dendritic and axonic fields of neurons with short axons is very variable. Nevertheless three main categories of GOLGI-II-type neurons can be distinguished: 1. neurons, the axonic and dendritic ramifications of which strongly remain within the layer (do not exist within the stratum pyramidale), 2. neurons, which are able to contact afferent fibres in other layers by means of ascending and/or descending dendrites, 3. basket cells, the axons of which have direct influence on the perikarya of pyramidal neurons. Within the region CA4b interneurons of the 2nd group could be found, dendrites of which penetrate into layers of the fascia dentata. Such a fact does not exist at the multipolare, efferent neurons of the same region and could never be seen at any axonic ramification. In the presented paper aspects of influencing the maturation of pyramides by interneurons are discussed. Also has been tried to summarize the results about the neuronal structure of the rat hippocampus by schemes of frontal sections from four selected stages of age.

In this paper qualitative investigations by means of light microscopy (Golgi impregnation) considering the ontogenesis of efferent neurons of the CA3, CA4a, CA4b regions in the rat hippocampus are presented following the already described development of CA1 pyramids (MINKWITZ and HOLZ 1975). After the migration of neuroblasts the types of projective neurons can be distinguished in the mentioned regions at the end of the prenatal period. During the first postnatal time particularly the basal dendritic tree develops expressing more distinctly the typical forms of pyramids (CA3, CA4a) and the of the multipolare neurons of CA4b. No distinct qualitative jumps at the transition of the regions in contrast to the CA3--CA1 boundary can be observed. Temporal retardations in the appearance of postsynaptic structures (microdendrites, excrescenses, spines) at pyramidal neurons from CA4 to CA3 and CA1 are regarded as signs of earlier development of the phylogenetic older pool of cells. During the early postnatal developmental period (until stage P10) proceedings of the growth are predominant. Later on, during the late postnatal period, the maturation of the neurons takes place with a considerable multiplication of spines distributed in similar ways as known from other pyramids. At stage P20 neurons of all hippocampal regions cannot be distinguished from adult ones without using morphometric procedures. The development of parts of the intrahippocampal network including mossy fibre system and SCHAFFER collaterals is discussed with regard to in this way appearing boundaries between fascia dentata and regio inferior CA4, CA3) and regio superior (CA1). These fibre systems establish by their stepwise construction one of the basic elements to hippocampal functioning.