ELECTRON-MICROSCOPIC STUDY OF SYNAPTOGENESIS AND MYELINATION OF THE OLFACTORY CENTERS IN DEVELOPING RATS

Development of the central olfactory system was studied in the rat wit h;an electron microscope at three main structures: the olfactory bulb, the lateral olfactory tract, and the primary olfactory cortex (the pi riform cortex). As a parameter of development, the synaptic density wa s examined quantitatively in the bulbar glomerulus and layer Ia (termi nation of bulbofugal fibers) of the piriform cortex, which are the key stations of the olfactory pathway. The synaptic densities in the glom erulus and those in layer Ia were 5.7% and 4.6% on embryonic day 19, 1 5.8% and 12.5% on postnatal day (P) 0, and 57.3% and 37.2% on P10, as compared with the adult (100%). As another parameter of development, t he density of myelinated axons in the lateral olfactory tract was-exam ined quantitatively. The densities of myelinated axons in the tract we re O% on P5, 15.1% on P10, and 73.5% on P21 of the adult density. Matu ration in the tract was still progressing, even at P21, in terms of bu ndle formation and the thickness of myelin sheaths. The results show t hat synaptogenesis in the bulbar glomerulus is followed by synaptogene sis in layer Ia of the piriform cortex, and that myelination in the la teral olfactory tract occurs over a prolonged period, even in the stag es after P21.