Olfactory pit

The chemosensory stem cells give rise to several types of neuronal and non-neuronal cell lines under the influence of multiple organisers. From a ventro-lateral infolding, the olfactory pit is produced and this invagination soon becomes separated into two areas which will produce the main and accessory olfactory neurones [Figs. 4.2(a)-(d)]. 

Fig. 4.2 Differentiation from olfactory placode of main and VN primordia (a) to (b), embryonic stages in reptiles (Squamates and Crocodylia). (a), Early invagination of placode (a ), late separation of primordia for AOS/MOS and (b), agenesis of presumptive VN(JO) cells in crocodile (NPT, nasal pit) (from Parsons, 1970).

Kent K. S., Harrow I. D., Quartararo P. and Hildebrand J. G. (1986) An accessory olfactory pathway in Lepidoptera the labial pit organ and its central projections in Manduca sexta and certain other sphinx moths and silk moths. Cell Tissue Res. 245, 237-245. 

The diverse morphology of the olfactory organ among insect species provides a basis upon which classification has been based. Basically, the olfactory organ in the adult insect comprises two components on the head the antenna and the maxillary palp (Fig. la). Numerous sensilla cover the surface of the antennae and prevent direct contact of ORNs with the external environment. Each sensillum is filled with a potassium- and protein-rich fluid called sensillum lymph and houses one to four ORN dendrites. The small pits on the cuticle surfaces of sensilla allow contact of the ORN dendrite with volatile odorants that dissolve in the lymph. In Drosophila, the third segment of the antenna and of the maxillary palp possess approximately 1,200 and 120 ORNs, respectively. 

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