Electron microscopy and morphometric analyses of microtubules in two differently sized types of axons in the protocerebral tract of a crustacean

Abstract

Despite several reports on the morphology and functions associated with the morphometry of the vertebrate axoplasm cytoskeleton, the subject has not been thoroughly explored in invertebrates. In vertebrates, among many other functions, microtubules (MTs) serve as scaffolding for axon assembly, and neurofilaments (NFs) as the elements that determine the axon caliber. Intermediate filaments have never been described by electron microscopy in arthropods, although NF proteins have been revealed in the MT side-arms of the axoplasm of certain species, such as the crab Ucides cordatus. Thus, it is not known which elements of the cytoskeleton of invertebrates are responsible for determination of the axon caliber. We studied, by electron microscopy and morphometric analyses, the MT and axon area variability in differently sized axons of the protocerebral tract of the crab Ucides cordatus. Our results revealed differences in the distance between MTs, in MT density and number, and in the areas of differently sized axons. The number of MTs increases with the axon area, but this relationship is not directly proportional. Therefore, MT density is greater in smaller axons than in medium axons, similar to the morphometry of the vertebrate axon MT. The distance between MTs is, however, directly related to the axonal area. On the basis of the results shown here, and on previous reports by us and others, we suggest that MTs may be involved in the determination of the axon caliber, possibly due to the presence of NF proteins found in the side-arms. Microsc. Res. Tech. 71:214-219, 2008. V

Hierarchical Microtubule Organization Controls Axon Caliber and Transport and Determines Synaptic Structure and Stability

“…First, we show that MTs are organized into a striking grid-like pattern in motoneuron axons. A similar stereotypic MT organization has been reported for adult Drosophila motoneurons and for different arthropod species (Benshalom and Reese, 1985; Corrêa et al, 2008; Egger et al, 1997; Nadelhaft, 1974). Second, in the absence of Ank2-XL and Futsch, we observe a dramatic reduction in inter-MT spacing, a loss of MT membrane association, and a severe decrease of axonal diameter.…”

Culture of neural cells of the eyestalk of a mangrove crab is optimized on poly-l-ornithine substrate

“…For this purpose, we used the visual system of the adult mangrove crab Ucides cordatus (Crustacea, Decapoda), because not only we have been using the visual system of this species as a model for studies of neuronal and glial cells (Allodi et al 1999;Chaves da Silva et al 2010Corrêa et al 2008;da Silva and Allodi 2000, 2001Fusco et al 2014;Hollmann et al 2015;Miguel et al 2002Miguel et al , 2005Miguel et al , 2007, but because in adult decapods neurogenesis occurs in the optic lobe (Schmidt 1997). The visual system of U. cordatus consists of a retina with photoreceptors projecting to the optic lobe, which is composed of neurons and glial cells constituting the lamina ganglionaris (La), the external medulla (EM) and the internal medulla (IM) (Corrêa et al 2004;da Silva et al 2003).…”