ABSTRACT
The descriptive anatomy of olfactory system development had been done by nineteenthcentury anatomists, who had shown that in vertebrates the paired nasal (olfactory) placodes on the anterolateral region of the embryonic head were the precursors of the nasal cavity, which contained the olfactory sensory epithelium. The first successful experimental study on development of the olfactory system was done in 1916 by Burr, who surgically removed one or both placodes from the larval form of Amblystoma, a salamander, and showed that the olfactory bulb(s) on the operated side(s) did not develop. A previous effort to do the same experiment in the frog, Hyla esculenta, had failed because the placode regenerated and development of the bulb was not compromised (Bell, 1907). Burr’s success in his experiments was dependent on the fact that in Amblystoma the olfactory placode was sharply outlined and easily distinguishable from the surrounding epidermis, whereas in the frog, the experimental animal used by Bell, the outline of the placode was difficult to discern. Indeed, Burr showed that partial removal of the placode in the frog permitted regeneration of the placode, as Bell (1907) had reported, but complete removal resulted in failure of the bulb to develop. Burr concluded: “The removal of the nasal epithelium deprives the developing forebrain of a stimulus necessary for its complete development. This is evidenced by the fact that the forebrain of the six months old larva from which one placode had been removed, showed considerable differences in the size of the two hemispheres, the operated side being the smaller.” Thus, Burr (1916) was the first to demonstrate that development of the olfactory bulb was, in some way, dependent on the nerves growing out of the olfactory placode and reaching the forebrain. In this chapter we review current knowledge about the major steps in development of the sensory epithelium and the bulb and describe some of the experimental studies used to examine the developing olfactory system.
