Farris S M, Strausfeld N J
Division of Neurobiology, Arizona Research Laboratories, University of Arizona, Tucson, 85721, USA.
J Comp Neurol. 2001 Oct 22;439(3):331-51. doi: 10.1002/cne.1354.
The mushroom bodies of the insect brain are lobed integration centers made up of tens of thousands of parallel-projecting axons of intrinsic (Kenyon) cells. Most of the axons in the medial and vertical lobes of adult cockroach mushroom bodies derive from class I Kenyon cells and are organized into regular, alternating pairs (doublets) of pale and dark laminae. Organization of Kenyon cell axons into the adult pattern of laminae occurs gradually over the course of nymphal development. Newly hatched nymphs possess tiny mushroom bodies with lobes containing a posterior lamina of ingrowing axons, followed by a single doublet, which is flanked anteriorly by a gamma layer composed of class II Kenyon cells. Golgi impregnations show that throughout nymphal development, regardless of the number of doublets present, the most posterior lamina serves as the "ingrowth lamina" for axons of newborn Kenyon cells. Axons of the ingrowth lamina are taurine- and synaptotagmin-immunonegative. They produce fine growth cone tipped filaments and long perpendicularly oriented collaterals along their length. The maturation of these Kenyon cells and the formation of a new lamina are marked by the loss of filaments and collaterals, as well as the onset of taurine and synaptotagmin expression. Class I Kenyon cells thus show plasticity in both morphology and transmitter expression during development. In a hemimetabolous insect such as the cockroach, juvenile stages are morphologically and behaviorally similar to the adult. The mushroom bodies of these insects must be functional from hatching onward, while thousands of new neurons are added to the existing structure. The observed developmental plasticity may serve as a mechanism by which extensive postembryonic development of the mushroom bodies can occur without disrupting function. This contrasts with the more evolutionarily derived holometabolous insects, such as the honey bee and the fruit fly, in which nervous system development is accomplished in a behaviorally simple larval stage and a quiescent pupal stage.
昆虫大脑的蘑菇体是叶状整合中心，由数万个内在（肯扬）细胞的平行投射轴突组成。成年蟑螂蘑菇体内侧叶和垂直叶中的大多数轴突来自I类肯扬细胞，并被组织成规则的、交替的浅色和深色薄片对（双联体）。肯扬细胞轴突组织成成年薄片模式是在若虫发育过程中逐渐形成的。刚孵化的若虫拥有微小的蘑菇体，其叶中包含向内生长轴突的后薄片，接着是单个双联体，其前方两侧是由II类肯扬细胞组成的γ层。高尔基浸染显示，在整个若虫发育过程中，无论存在多少双联体，最靠后的薄片都作为新生肯扬细胞轴突的“向内生长薄片”。向内生长薄片的轴突对牛磺酸和突触结合蛋白免疫阴性。它们沿着长度产生细的生长锥尖端细丝和长的垂直定向侧支。这些肯扬细胞的成熟和新薄片的形成以细丝和侧支的消失以及牛磺酸和突触结合蛋白表达的开始为标志。因此，I类肯扬细胞在发育过程中在形态和递质表达上都表现出可塑性。在蟑螂这样的半变态昆虫中，幼虫阶段在形态和行为上与成虫相似。这些昆虫的蘑菇体从孵化起就必须发挥功能，同时数千个新神经元被添加到现有结构中。观察到的发育可塑性可能是一种机制，通过该机制蘑菇体的广泛胚后发育可以在不破坏功能的情况下发生。这与进化上更高级的全变态昆虫，如蜜蜂和果蝇形成对比，在全变态昆虫中，神经系统发育在行为简单的幼虫阶段和静止的蛹阶段完成。
