Atlas of olfactory organs of Drosophila melanogaster 2. Internal organization and cellular architecture of olfactory sensilla

Antennae and maxillary palps of Drosophila melanogaster were studied with t he electron microscope on serial sections of cryofixed specimens. The numbe r of epidermal cells roughly equals the number of sensilla, except for regi ons where the latter are scarce or absent. Each epidermal cell forms about two non-innervated spinules, a prominent subcuticular space and a conspicuo us basal labyrinth, suggesting a high rate of fluid transport through the s ensory epithelium. The internal organization and fine structure of trichoid , intermediate and basiconic sensilla is very similar. Receptor cell somata are invested by thin glial sheaths extending distad to the inner dendritic segments. Further distally, the the cogen cell forms a sleeve around the d endrites, but an extracellular dendrite sheath is absent. At the base of th e cuticular apparatus, the inner sensillum-lymph space around the ciliary a nd outer dendritic segments is confluent with the large outer sensillum-lym ph space formed by the trichogen and tormogen cells. All three auxiliary ce lls exhibit many features of secretory and transport cells but extend only thin basal processes towards the haemolymph sinus. The bauplan and fine str ucture of coeloconic sensilla differs in the following aspects: (1) the cil iary segment of the dendrites is located deeper below the base of the cutic ular apparatus than in the other sensillum types; (2) a prominent dendrite sheath is always present, separating inner and outer sensillum-lymph spaces completely; (3) the apical microlamellae of the auxiliary cells are more e laborate, but free sensillum-lymph spaces are almost absent; (4) there are always four not three auxiliary cells. Morphometric data are presented on t he diameter of inner and outer dendritic segments and on the size of recept or cells, as well as of the receptor and auxiliary cell nuclei. The special fine structural features of Drosophila olfactory sensilla are discussed un der the aspects of sensillar function and the localization of proteins rele vant for stimulus transduction. (C) 2001 Elsevier Science Ltd. All rights r eserved.