Klußmann-Fricke et al. Frontiers in Zoology 2014, 11:28 http://www.frontiersinzoology.com/content/11/1/28 RESEARCH Open Access A wonderful network unraveled - Detailed description of capillaries in the prosomal ganglion of scorpions Bastian J Klußmann-Fricke*, Sebastian W Pomrehn and Christian S Wirkner Abstract Introduction: Though it has long been known that the prosomal ganglion of scorpions is supplied by a dense system of arteries, the pattern of this network has never been described and analyzed in detail. Using MicroCT in combination with computer aided 3D-reconstruction we provide the first detailed description of the pattern of arteries in the prosomal ganglion of Brotheas granulatus (Scorpiones, Chactidae) and other scorpion species. Results: The entire prosomal ganglion in scorpions is supplied by a network of arteries that branch off the major arteries of the anterior aorta system. The most prominent of these are the nine transganglionic arteries which run through the nerve mass along the midline of the body and branch terminally, i.e. below the neuropils, into smaller arteries. These arteries reticulate into a dense network between the surrounding somata and the centrally located neuropil structures of the ganglion. Conclusions: We demonstrate the presence in the prosomal ganglion of scorpions of a capillary system made up of afferent arteries which deliver hemolymph into the ganglion and efferent arteries which transport the hemolymph out of the ganglion. Adopting the structural definition used for vertebrate circulatory systems, this capillary network can also be termed a bipolar rete mirabile (located as it is between afferent and efferent arteries) analogous to those found in vertebrates and some echinoderms. Within the rete mirabile of the scorpion prosomal ganglion, some regions (i.e. neuropils) are better supplied than others. The structural information provided here can now be used in functional neuronal studies to determine the physiological and computational significance of the various neuropils in the complex scorpion nervous system. Introduction distinguishable in arachnids [2], where the supraeso- The central nervous system (CNS) is one of the most phageal ganglion sits broadly on the anterior part of the important organ systems in animals, so ensuring that it subesophageal ganglion. Only in lateral view can the is supplied with nutrients and oxygen is a crucial matter. rather thin esophagus be taken as a fictive border between In arthropods, the central nervous system consists of a the two major ganglionic masses. 'syncerebrum' and a number of free segmental ganglia The prosomal ganglion is supplied by the hemolymph which are situated ventrally in the body and therefore vascular system (HVS), which in scorpions consists of a known as the ventral nerve cord [1]. In arachnids, the dorsal tubular heart located in the midline of the meso- syncerebrum is fused with a taxon-specific number of soma (Figure 1; H) and surrounded by a sac-like pericar- ventral nerve cord ganglia to form the condensed proso- dium [2,4-6]. Hearts are equipped with seven pairs of ostia mal ganglion (Pg; [2]). A prosomal ganglion combines a and six or eight pairs of cardiac arteries which supply the dorsal supra- and a ventral subesophageal ganglia. Clas- organs of the mesosoma. Hearts are connected posteriorly sically, supra- and subesophageal ganglia are connected to the posterior aorta, which runs dorsally through the by circumesophageal connectives [3], but these are not metasoma into the telson (Figure 1; Po), and anteriorly to the anterior aorta, which runs into the prosoma to the * Correspondence: [email protected] level of the supraesophageal ganglion and goes on to Allgemeine & Spezielle Zoologie, Institut für Biowissenschaften, Universität Rostock, Universitätsplatz 2, 18055 Rostock, Germany © 2014 Klußmann-Fricke et al.; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Klußmann-Fricke et al. Frontiers in Zoology 2014, 11:28 Page 2 of 15 http://www.frontiersinzoology.com/content/11/1/28 Results Gross morphology of the prosomal ganglion and the vascular system which supplies it In the following, morphology of the prosomal ganglion and the vascular system which supplies it is described Po for Brotheas granulatus. Comparative morphological aspects within scorpions are described in a separate chapter below. The terminology used in the following H description of the prosomal ganglion follows the works IV Vv of Babu [14] and Horn and Achaval [3]. As stated above, the prosomal ganglion comprises a supra- and a subeso- III phageal ganglion. Its nervous tissue consists of densely II packed peripheral somata (Figure 2, space between neuri- I lemma – Nl and neuropil) and centrally located neuropil Ao Pp structures (e.g. Figure 2; NpI) made up of the axons and synapses of the neurons. The supraesophageal ganglion consists of the posterior arcuate body (see below), with Ch the remaining protocerebrum receiving the optic nerves and the deutocerebrum, i.e. the cheliceral neuromere Figure 1 Schematic drawing of major vascular system of Scorpiones (modified from [6] with permission from Wiley and (Figure 2A, B; Ptc + Dtc). The arcuate body (sensu Loesel Son). Abbreviations: Ao, anterior aorta; Ch, cheliceral arteries; H, et al. [15]) (Figures 2A, C and 3B; Ac) is an unpaired heart; I-IV, leg arteries; Po, posterior aorta; Pp, pedipalpal artery; double-layered midline neuropil which is located at the Vv, ventral vessel. posterior border of the protocerebrum. The subesophageal ganglion is formed by the neuromeres of the pedipalps (i.e. the tritocerebrum) (Figure 2; Ppn), the neuromeres supply, via a complex branching pattern, the various organ of the four pairs of limbs (Figure 2; NpI-IV), and four systems and appendages (Figure 1; Ao). incorporated ganglia of the ventral nerve cord (Figure 2; It has long been known that the prosomal ganglion in Pnp). The latter are known as the posterior neuropils scorpions is supplied by a number of vessels [4,7-9], and (sensu Babu, [14]) and include the pectinate neuropils (see even that these vessels form a dense network [2,6]. How- [16] for details). In the anteroventral midline of the sube- ever, the structure and pattern of the network has never sophageal ganglion laterally to the first five transganglionic been studied in detail. At first glance, the fine arteries in arteries (see below), the paired central ganglion (sensu question could be interpreted as capillaries, which in Babu, [14]) can be found. The whole prosomal ganglion is vertebrates are defined as networks of small-diameter surrounded by the multilayered neurilemma (Figures 2A, vessels (5 – 10 μm) that connect arteries and veins. Ca- B, and 7A; Nl). pillaries are characterized by the presence of exclusively From its origin at the heart, the anterior aorta endothelial layers and connective tissue, and are where (Figure 1; Ao) runs in an anteroventral direction to the exchange of oxygen and nutrients takes place [10]. the posterodorsal part of the supraesophageal gan- As veins are not present in arthropods [11], the older lit- glion (Figure 3A-C; Ao) where the cheliceral arteries erature on arthropods (or more precisely on crustaceans) branch off and run around the supraesophageal ganglion defines capillaries either as small-caliber vessels continu- laterally (Figure 3B, C; Ch). Anterior to the supraesopha- ous with the efferent arteries and without visible intima geal ganglion these arteries bend upwards and run into [12], or as vessel networks that link afferent and efferent the chelicerae (Figure 3C). Shortly after this bend, a su- vessels [13]. pracerebral artery (Figure 3B; Sc) emanates either from This paper provides the first detailed description of the the left or the right cheliceral artery (see [2] for details morphology of the cerebral vasculature in scorpions. It on variation). also identifies parts of the prosomal ganglion that are After the cheliceral arteries have branched off dorsally better supplied than others and discusses possible reasons to the thin esophagus, the anterior aorta bifurcates into for this pattern. We discuss the terminology commonly two lateral trunks from which subsequently the arteries used to describe capillaries and retia mirabilia and the of the pedipalps and the four pairs of walking legs branch possible adoption of these terms to arthropod circulatory off (Figure 3C; Pp, I-IV). On top of the subesophageal systems in general. Furthermore, we hypothesize on how ganglion, the lateral trunks curve backwards, reunite, and the pattern of arteries described might be established give rise to the ventral vessel (Figure 3C; Vv). This artery during ontogeny. runs atop the ventral nerve cord in a posterior direction Klußmann-Fricke et al. Frontiers in Zoology 2014, 11:28 Page 3 of 15 http://www.frontiersinzoology.com/content/11/1/28 Nl Ac Ptc Nl Dtc Pnp Ppn NpIV NpI NpII A 500µm Ppn NpII NpIII 400µm B NpI Ac Cn Ptc Cn Pa Pnp Tga Pc Ppn Dtc Ppn Ca NpI C 300µm D 300µm Figure 2 Gross morphology of the prosomal ganglion in B. granulatus.A: Surface renderings of the neurilemma (Nl; transparent) and the neuropil of the prosomal ganglion showing the general organization of the ganglion; note that space between neurilemma and neuropils is filled with somata (not shown); in lateral view. B: Surface renderings of the neurilemma (Nl) and the neuropil of the prosomal ganglion showing pedipalpal (Pnp) and leg neuropils (NpI-IV); in ventral view. C: Surface renderings of the neurilemma and the neuropil of the prosomal ganglion showing neuropils of the proto- (Ptc), deuto- (Dtc), and tritocerebrum (i.e.