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Abstract:
The present paper is based on the question, to what extent the cortical structure is determined by genetic factors and how far it is dependent on environmental stimuli. Some deprivation experiment in the literature have supported the assumption tha excitation coming from the sense organs contributes to the formation of synaptic connections in the cortex. This made it possible to invoke the formation of synapses (or dendritic spines) as a substrate of learning processes. Results of experiments on the influence of artificial environments on the formation of synapses have been, however, somewhat contradictory. On this background it was interesting to investigate the cortical development of the guinea-pig, an animal which is highly developed at birth. This percocity separates in time the process of genetically determined development from the changes due to environmental stimuli, which are amply overlapping in altricial animals such as mouse, rat, and cat. A comparison between the cortices of prenatal and adult guinea-pigs showed that the density of dendritic spines has reached adult values already before birth (12/10 micrometers dendritic length before birth, 11,5/10 micrometers in adult animals, fig. 5-8, and 17). The counts have been made on basal dendrites of Golgi-impregnated pyramidal cells in the upper third of the cortex. Also, the difference in the density of synapses on electronmicrographs in animals just before birth (8,9 x 10(8)/mm(3)) and in adult animals (9,4 x 10(8)/mm(3)) was not significant (figs. 13, 14, and 16). The samples have been taken from the second cortical layer. The two areas investigated showed small but significant differences in the time course of spine formation. In both areas the density of spines reached a maximum first and then decreased slightly toward the adult values. However, in the postcallosal area the maximum was reached earlier than in the precallosal area (fig. 9). The decrease in spine density after the maximum, about 18% in both areas, may be partly explained by the growth of dendrites. From the increase in brain volume between birth and adult age and from the density of synapses at different stages, one can conclude that the total number of synapses at birth is about two thirds of that in adult animals. Similarly, the proportion of spines present at birth was at least the same or even higher (fig. 12). Thus, most of the connections in the cortex of the guniea pig are formed without the influence of environmental stimuli. This puts strong doubts on the idea of the formation of synapses or dendritic spines as memory traces. Synapses and spines seem to be the prerequisites of learning rather than the result of it. In part II the question will be examined, if postnatal changes in the cortex of the guinea-pig, especially on spines and synapses, are possible candidates for memory traces.
