Ramaekers Ariane, Magnenat Edwige, Marin Elizabeth C, Gendre Nanaë, Jefferis Gregory S X E, Luo Liqun, Stocker Reinhard F
Department of Biology, University of Fribourg, Chemin du Musée 10, CH-1700 Fribourg, Switzerland.
Curr Biol. 2005 Jun 7;15(11):982-92. doi: 10.1016/j.cub.2005.04.032.
Drosophila larvae possess only 21 odorant-receptor neurons (ORNs), whereas adults have 1,300. Does this suggest that the larval olfactory system is built according to a different design than its adult counterpart, or is it just a miniature version thereof?
By genetically labeling single neurons with FLP-out and MARCM techniques, we analyze the connectivity of the larval olfactory circuit. We show that each of the 21 ORNs is unique and projects to one of 21 morphologically identifiable antennal-lobe glomeruli. Each glomerulus seems to be innervated by a single projection neuron. Each projection neuron sends its axon to one or two of about 28 glomeruli in the mushroom-body calyx. We have discovered at least seven types of projection neurons that stereotypically link an identified antennal-lobe glomerulus with an identified calycal glomerulus and thus create an olfactory map in a higher brain center.
The basic design of the larval olfactory system is similar to the adult one. However, ORNs and projection neurons lack cellular redundancy and do not exhibit any convergent or divergent connectivity; 21 ORNs confront essentially similar numbers of antennal-lobe glomeruli, projection neurons, and calycal glomeruli. Hence, we propose the Drosophila larva as an "elementary" olfactory model system.
果蝇幼虫仅拥有21个嗅觉受体神经元（ORN），而成虫有1300个。这是否表明幼虫嗅觉系统的构建设计与其成虫的不同，或者它只是成虫嗅觉系统的一个微型版本？
通过利用FLP-out和MARCM技术对单个神经元进行基因标记，我们分析了幼虫嗅觉回路的连接性。我们发现21个ORN中的每一个都是独特的，并投射到21个形态上可识别的触角叶神经球之一。每个神经球似乎由单个投射神经元支配。每个投射神经元将其轴突发送到蘑菇体花萼中约28个神经球中的一个或两个。我们发现了至少七种类型的投射神经元，它们以刻板的方式将一个已识别的触角叶神经球与一个已识别的花萼神经球相连，从而在更高的脑中枢中创建了一个嗅觉图谱。
幼虫嗅觉系统的基本设计与成虫相似。然而，ORN和投射神经元缺乏细胞冗余，并且不表现出任何汇聚或发散连接；21个ORN面对的触角叶神经球、投射神经元和花萼神经球数量基本相似。因此，我们提出将果蝇幼虫作为一个“基本的”嗅觉模型系统。
