Influence of receptor axons on the formation of olfactory glomeruli in a hemimetabolous insect, the cockroach Periplaneta americana.

The embryonic development of the hemimetabolous insect Periplaneta americana requires approximately 31 days. Deafferentation experiments were used to investigate the role of ingrowing receptor axons during embryogenesis, specifically their influence 1) on the subdivision of the antennal lobe neuropil into glomeruli, 2) on the morphology and number of glial cells, and 3) on the arborization pattern of central neurons. The flagellum of one antenna was removed from embryos at different developmental stages starting with day 10. Subsequently, they were raised in culture until a total age of 26 days. At day 10, the deutocerebrum has received only a very small number (ca. 0.4%) of antennal receptor axons; deafferentation at this stage allowed us to deprive the deutocerebrum of approximately 99% of its normal antennal input. Deafferentation has marked effects on the organization of the antennal lobe neuropil. The deafferented lobe is reduced in volume compared to the control side; the characteristic glomeruli are missing. During normal development glomeruli are formed between day 19 and 22, first in dorsal and then in ventral antennal lobe regions. By removing the antenna before day 20, their formation is disturbed in all parts of the antennal lobe. If deafferentation is performed after stage 20, glomeruli persist in dorsal regions, but are missing in ventral regions. On day 24 or later, glomeruli in both dorsal and ventral regions are unaffected by deafferentation. Glial cells continue to extend fine processes into the neuropil in the absence of ingrowing receptor axons. The number of glial cells is reduced compared to control lobes. Multiglomerular local interneurons and other gamma-amino butyric acid-immunoreactive neurons, as well as projection neurons, fail to develop glomerular arborization patterns in antennal lobes deprived of sensory axons.