The Gustatory System of Lampreys

The present is a review of the gustatory system of lampreys, which are representative of the earliest vertebrates. They are the oldest extant vertebrates that possess taste buds. Because of the phylogenetic position of lampreys, the study of their gustatory system will provide important information to help understand the early evolution of this system in vertebrates. The taste buds of larval lampreys, which are papillae located on the first six pairs of gill arches facing the water current, respond to classical taste substances. They consist of two types of tall differentiated cells, serotonergic biciliated taste receptors (‘light’ cells) and microvillous sustentacular cells (‘dark cells’). The taste buds also contain basal proliferative cells. Afferent gustatory fibers of the glossopharyngeal and vagal nerves innervate the taste buds of lampreys and contact the basal surface of the biciliated cells without entering the bud. Central processes of the glossopharyngeal and vagal cranial nerves terminate in a caudal rhombencephalic region that may correspond to the nucleus of the solitary tract of gnathostomes. To date, most studies in lampreys have focused on characterizing taste buds; future Received: March 15, 2010 Returned for revision: April 16, 2010 Accepted after revision: May 12, 2010 Published online: July 24, 2010 María Celina Rodicio Departmento de Biología Celular y Ecología, CIBUS Campus Sur, Universidad de Santiago de Compostela ES–15782 Santiago de Compostela (Spain) Tel. +34 981 563 100, ext. 16946, Fax +34 981 596 904, E-Mail mcelina.rodicio @ usc.es © 2010 S. Karger AG, Basel 0006–8977/10/0754–0241$26.00/0 Accessible online at: www.karger.com/bbe D ow nl oa de d by : 54 .7 0. 40 .1 1 10 /6 /2 01 7 4: 41 :0 9 A M Barreiro-Iglesias/Anadón/Rodicio Brain Behav Evol 2010;75:241–250 242 oxus and tunicates) and gnathostome vertebrates. It is therefore generally accepted that lampreys represent the closest living forms to early vertebrates and that they may be good models for studying the evolution of the chemosensory systems. Since the first studies by Baatrup in the 1980s on the ultrastructure and physiology of the taste buds of lampreys [Baatrup, 1983a, b, 1985a, b], the gustatory system of lampreys has not received much attention. However, recent neurochemical data on the taste buds of sea lampreys have revealed some interesting new aspects of the early evolution of taste buds in vertebrates [Barreiro-Iglesias et al., 2008c]. In the present review, we aim to summarize the current knowledge of the organization and physiology of the gustatory system of lampreys. This will provide a basis for further studies of the gustatory system of lampreys, which in turn will provide valuable information to help understand how these animals have adapted to their habitat as well as to help understand the early evolution of the gustatory system in vertebrates. Lampreys: The Earliest Vertebrates with Taste Buds Hagfishes are chordates, and perhaps also vertebrates [Ota et al., 2007], which have a system of sensory organs resembling taste buds, the Schreiner organs [Braun, 1998]. Schreiner organs are located extensively – on the skin, the prenasal sinus, the nasopharyngeal duct, the pharynx and, at lower densities, in the oral and velar cavities of hagfishes [Braun, 1998]. These structures were initially described on the tentacles and the skin of the head and body of hagfishes [Retzius, 1892; Schreiner, 1919; Georgieva et al., 1979]. In these early studies, Schreiner organs were initially identified as end buds. End buds are found in large numbers on the skin of many fishes and they are, in most cases, external taste buds similar in structure and innervation to taste buds within the oropharyngeal cavities [Lane and Whitear, 1982]. Out-group analysis has indicated that taste buds were primitively restricted to the oropharynx, and that external taste buds (end buds), distributed over the head and, in some cases, even the trunk, evolved independently a number of times [Northcutt, 2004]. Recent studies have revealed important systematic differences between the organization of the Schreiner organs system of hagfishes and the end bud/taste bud system of vertebrates: (1) Schreiner organs are innervated by sensory trigeminal branches, the glossopharyngeal/ vagal nerve and by the cutaneous branches of spinal nerves [Braun, 1998], while gnathostome taste buds are innervated by the facial/glossopharyngeal/vagal nerves [Northcutt, 2004]. (2) The central projections of the nerves innervating Schreiner organs of hagfishes form a continuous tract in the trigeminal sensory zone and the dorsolateral funiculus of the spinal cord, but only some Schreiner organs may be represented in the nucleus of the solitary tract [Braun, 1998], whose rostral part (the gustatory nucleus) is the primary recipient of the gustatory afferents in gnathostomes [see Smith and Davis, 2000]. (3) Supporting cells of Schreiner organs are not associated with high levels of ecto-ATPase [Finger, 2006], which is a key feature of the structurally similar type I cells of vertebrate taste buds in which ATP serves as a neurotransmitter [Finger et al., 2005]. For these reasons, it appears that Schreiner organs are not homologous to end buds/ taste buds [Braun, 1998; Finger, 2006]. This sensory modality of hagfishes has no direct homologue in vertebrates, and appears to be a specialization of hagfishes [Braun, 1998]. It is therefore generally accepted that lampreys are the oldest extant vertebrates that possess true taste buds [Finger and Simon, 2000; Northcutt, 2004] and, therefore, that taste buds evolved in the common ancestor of lampreys and gnathostome vertebrates. General Organization Lampreys have clearly differentiated larval and adult stages. Larval lampreys spend most of their time buried within the sand of freshwater streams. They are filter feeders and feed exclusively on microorganisms in suspension, which become embedded in mucus in the pharynx [Moore and Mallat, 1980]. The mouth of larval lampreys is divided into buccal and oral cavities ( fig. 2 a). The anterior opening of the mouth leads into a large buccal cavity through a narrow oral aperture ( fig. 2 a). Posterior oral cirri form a ring of branched projections around and across the oral aperture, and prevent the passage of large objects into the oral cavity [Mallat, 1979, 1981]. The muscles used for feeding are innervated by the trigeminal nerve [Homma, 1975]. In larvae, a muscular velum that is also innervated by the trigeminal nerve pumps water through the pharynx and gills [Johnston, 1905] ( fig. 2 a). The water current used for respiration and feeding flows into the pharynx by the coordinated action of the velum movements and by the sequential contraction and expansion of the branchial region. The water comes out through the seven pairs of branchiopores. The pharyngeal region is also the site where food particles are trapped before passing into the esophagus. Tracts of ciliated cells present D ow nl oa de d by : 54 .7 0. 40 .1 1 10 /6 /2 01 7 4: 41 :0 9 A M Gustation in Lampreys Brain Behav Evol 2010;75:241–250 243 in the pharynx help to move the food cord, which is formed by mucus secretion and trapped particles, toward the esophagus. These ciliated tracts are prominent in the bilateral pseudobranchial grooves, in the hypobranchial groove, and in the anterolateral surfaces of the gill seams [Mallat, 1979]. The first six pairs of gill arches bear volcano-like papillae that face the water current ( fig. 1 , 2 a). These gill structures were identified in larval lampreys in early light [Schreiner, 1879; Retzius, 1893; Schaffer, 1895; Alcock, 1899] and electron microscopic studies [Mallat, 1979] as taste buds. According to Alcock [1899] and Pietschmann [1929], these organs of larval lampreys are innervated by branches of the glossopharyngeal and vagal nerves. The anatomy of adult lampreys differs considerably from that of the larvae because of the changes that take place during metamorphosis. In the adult, a secondarily formed esophagus begins as a dorsal orifice of the oral cavity at the level of the velum and communicates directly with the oral cavity ( fig. 2 b), also called ‘pharynx’ by early authors. During transformation, the larval pharynx becomes a respiratory region, formed by the water tube with the associated branchial chambers. The secondarily formed esophagus is separated from the more ventrally located water tube by cartilaginous processes [Weissenberg, 1926] ( fig. 2 b). After metamorphosis, lampreys are carnivorous and feed on a variety of bottom fauna, including worms and crustacea, in addition to fish tissues. Half of all living species of lampreys [38] are parasitic and feed on fishes by sucking their blood and tissues [Hardisty, 2006]. In adult lampreys, taste buds have also been described in the inner branchiopores joining the water tube and branchial chambers of the pharynx ( fig. 2 b) [Retzius, 1893; Fahrenholz, 1936]. Like larval taste buds, the buds of adult lampreys are innervated by branches of the glossopharyngeal and vagal nerves [Alcock, 1899; Johnston, 1905; Pietschmann, 1929]. Experimental studies in gnathostomes have shown that taste buds are innervated by branches of the facial, glossopharyngeal and vagal nerves [see Northcutt, 2004]. If the taste buds of lampreys lack facial nerve innervation, this would indicate that this is an evolutionary novelty of gnathostome vertebrates. However, it is now known that in lampreys the rostral part of the first gill, which bears taste buds [Barreiro-Iglesias et al., 2008c], is innervated by the facial nerve [Guimond et al., 2003]. Therefore, the possible facial nerve innervation of the taste buds located on the first gill of lampreys should be determined by use of modern tract-tracing techniques. Taste Bud Structure Transmission electron microscopic studies have revealed that the structure of the taste buds of larval and adult lampreys is very similar [Baatrup, 1983a, b]. Taste buds are located close to the origin of the gill filaments on the surface, which turns towards the inside of the pharyngeal cavity. Each papilla appears as a volcano-like elevation (100–150 m in diameter) extending between 50 and 100  m beyond the surrounding general surface [Baatrup, 1983a]. Fig. 1. Schematic drawing showing the location of taste buds in relation to the gill support elements, the afferent innervation (at the left) and the location of the nucleus of the solitary tract in relation to other rhombencephalic nuclei (at the right) in larval lampreys. Top schematic drawing: transverse section at the level of the caudal rhombencephalon where the nucleus of the solitary tract is located (at the right). Bottom schematic drawing: oblique, more caudal section of the branchial region in which rostral is at the top. Modified from Barreiro-Iglesias et al. [2008c]. D ow nl oa de d by : 54 .7 0. 40 .1 1 10 /6 /2 01 7 4: 41 :0 9 A M Barreiro-Iglesias/Anadón/Rodicio Brain Behav Evol 2010;75:241–250 244 The taste buds consist of tall elongated cells that extend through the entire length of the epithelium. With light microscopy, it is possible to distinguish between ‘light’ and ‘dark’ elongated cells in the taste buds of lampreys. These cell types can also be distinguished on the basis of their apical processes by electron microscopy: those with electron-dense cytoplasm end apically in a tuft of microvilli, while the more electron-lucent cells are ciliated. The ciliated cells are separated by the microvillar cells that surround them [Baatrup, 1983a]. Each ciliated cell bears two 2to 5m-long cilia (biciliated cells) with a 9 + 2 microtubular arrangement; they originate from basal bodies with 2–5 basal feet in larvae [Baatrup, 1983a] and 2–4 basal feet in adults [Baatrup, 1983b]. Neither accessory centrioles nor rootlets are associated with the basal bodies [Baatrup, 1983a]. The basal region of these cells is filled with vesicles with electron-lucent contents and also with a small number of dense-cored vesicles (100–120 nm). The microvillar cells extend 1.5  m beyond the surface of the neighboring ciliated cells. The microvilli are circular in cross section and have a central core of microfilaments, which continues proximally into the apical cytoplasm [Baatrup, 1983a]. Afferent nerve fibers do not enter the taste buds of lampreys [Baatrup, 1983a, b], unlike mammals and other jawed vertebrates [Finger, 2006]. However, nerve fibers are numerous in the connective tissue below the taste buds. They are 1.3–2  m in diameter. Contacts, which may be interpreted as chemical synapses, are only found between biciliated cells and fiber varicosities. These connections are established through holes in the basal lamina between the ciliated cell and the subjacent afferent fiber. There is some degree of membrane thickening at these sites. A third cell type has occasionally been observed by electron microscopy in taste buds of lampreys. These are spherical ‘basal cells’ with a thin layer of cytoplasm enveloping a cell nucleus showing deep invaginations [Baatrup, 1983a]. They have been considered as Merkel cells, because they are structurally similar to those on the skin [Whitear and Lane, 1981], since they have microvilli and spur synapses [Baatrup, 1985a]. These basal ‘Merkel cells’ are only observed in some buds and are absent from others. No mitotic activity has been observed in this type of cell [Baatrup, 1983a]. a