The development of the olfactory mucosa in the mouse : light microscopy

Abstract

It has long been recognized that the primordium of the olfactory organ is the paired olfactory placode which can be distinguished at an early stage of embryonic development as a patch of thickened ectoderm situated on the ventrolateral aspect of the head. Each placode subsequently sinks below the surface to form an olfactory pit. In many vertebrate groups the olfactory organ primordium comes to lie close to the fore brain some time before the olfactory nerve begins to develop and, because of this, some of the earlier investigators assumed a measure of cellular continuity between the organ and the central nervous system (Beard, 1885; Cameron & Milligan, 1914; Frazer, 1953) although this was denied by others (Coghill, 1916; Burr, 1932). The precise origin of the olfactory nerve was also disputed by the early investigators (Balfour, 1877; Marshall, 1878; Beard, 1885; Chiarugi, 1891; Cameron & Milligan, 1914). With superior histological methods, Wilhelm His (1889) and Cajal (1890) demonstrated clearly that the olfactory axons arise from the cells of the placode and grow towards the fore brain. At about the same period of development, Schwann cells, which possibly arise from the placode (Bedford, 1904; Van Campenhout, 1956), appear either before the outgrowth of the axons, as in the chick (Van Campenhout, 1956), or after the axons have emerged, as in mammals (Bedford, 1904; Van Campenhout, 1956).