The development of hippocampal structure and how it is influenced by hypoxia

When analysing the function of a neuronal system, it is important to know how the connections of the various neuronal elements are organized. One way in which the structure of nervous tissue can be studied is to become acquainted with the basic principles of its development. This can be achieved by studying the process of normal development, or else by experimental means aimed at inducing changes which help to uncover the laws of the interrelationships of the various elements forming the neuronal system. We studied pyramidal cell structure in the CA1 region of the rat hippocampus during normal postnatal development and after repeated exposure to altitude hypoxia (8 h a day from birth to 17 days at a simulated altitude of 7000 m). At 18 and 90 days the brains of the experimental and control animals were impregnated by the Golgi-Cox method for light microscopy. The brains of 5-, 10-, 15-, 24-, 48- and 90-day-old animals which had developed normally were treated similarly and subjected to a parallel analysis including quantitative methods of electron microscopy. The various parts of the dendritic system are not formed simultaneously. Up to the 15th day, the basal dendrites and the shaft of the apical dendrite, together with its terminal branches, develop. Between the 15th and the 24th day development continues with the proliferation and ramification of the lateral and preterminal branches of the apical dendrite. The number of dendrites is established the first, followed by growth and branching. Development of the dendritic spines involves a change in their shape as well as an increase in their number. From being short and thick the spines develop into structures with a thin neck and a claviform and sometimes branched head. Development of the receptive component of neuronal structure is accompanied by an increase in the number of afferent fibre terminals. Type I synapses are differentiated earlier and in larger numbers than type II synapses. The pyramidal cells of the hippocampus of rats exposed to hypoxia in early life have fewer basal dendrites and fewer terminal fibres on the apical dendrites at 18 days. Hypoxia also leads to lower dendritic spine density and to changes in the shape of the spines reminiscent of less mature forms.(ABSTRACT TRUNCATED AT 400 WORDS)