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Development of the central olfactory system was studied in the rat with an electron microscope at three main structures: the olfactory bulb, the lateral olfactory tract, and the primary olfactory cortex (the piriform cortex). As a parameter of development, the synaptic density was examined quantitatively in the bulbar glomerulus and layer Ia (termination of bulbofugal fibers) of the piriform cortex. which are the key stations of the olfactory pathway. The synaptic densities in the glomerulus and those in layer Ia were 5.7% and 4.6% on embryonic day 19, 15.8% 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, the density of myelinated axons in the lateral olfactory tract was examined quantitatively. The densities of myelinated axons in the tract were 0% on P5, 15.1% on P10, and 73.5% on P21 of the adult density. Maturation in the tract was still progressing, even at P21, in terms of bundle formation and the thickness of myelin sheaths. The results show that synaptogenesis in the bulbar glomerulus is followed by synaptogenesis in layer Ia of the piriform cortex, and that myelination in the lateral olfactory tract occurs over a prolonged period, even in the stages after P21.

We present an extremely rare case of fourth branchial fistula in a 23-year-old male. The characteristic clinical feature was a recurrent left lower neck abscess which did not respond to appropriate medical and surgical therapy. Radiography and a computed tomographic scan with contrast material revealed a fistula running from the apex of the left pyriform sinus (internal opening) to the left lower neck abscess. The fistula tract was excised surgically. Histological examination of the excised fistula revealed a squamous epithelial lining and subepithelial lymphoid tissue. This pyriform sinus fistula is thought to be of fourth pharyngeal pouch origin, because of its surgical aspects and the histological findings of the excised fistula.

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