The comparative morphology of pit organs in elasmobranchs

The pit organs of elasmobranchs (sharks, skates and rays) are free neuromasts of the mechanosensory lateral line system. Pit organs, however, appear to have some structural differences from the free neuromasts of bony fishes and amphibians. In this study, the morphology of pit organs was investigated by scanning electron microscopy in six shark and three ray species. In each species, pit organs contained typical lateral line hair cells with apical stereovilli of different lengths arranged in an “organ‐pipe” configuration. Supporting cells also bore numerous apical microvilli taller than those observed in other vertebrate lateral line organs. Pit organs were either covered by overlapping denticles, located in open grooves bordered by denticles, or in grooves without associated denticles. The possible functional implications of these morphological features, including modification of water flow and sensory filtering properties, are discussed. J. Morphol. 2009. © 2009 Wiley‐Liss, Inc.     

The pit organs of elasmobranchs: a review

Elasmobranchs have hundreds of tiny sensory organs, called pit organs, scattered over the skin surface. The pit organs were noted in many early studies of the lateral line, but their exact nature has long remained a mystery. Although pit organs were known to be innervated by the lateral line nerves, and light micrographs suggested that they were free neuromasts, speculation that they may be external taste buds or chemoreceptors has persisted until recently. Electron micrographs have now revealed that the pit organs are indeed free neuromasts. Their functional and behavioural role(s), however, are yet to be investigated.     

Immunocytochemical localization of somatostatin and vasotocin in the brain of the Pacific hagfish,Eptatretus stouti

Summary The brain of the Pacific hagfish, Eptatretus stouti, was studied immunocytochemically using antisera against somatostatin (SRIH), arginine vasopressin (AVP), and adrenocorticotropic hormone (ACTH). SRIH-immunoreactive perikarya were distributed bilaterally in the postoptic nucleus and in the hypothalamic nucleus. Although several short, stained fibers were observed in the vicinity of the perikarya, SRIH-immunoreactivity was not found in the neurohypophysis, nor in other parts of the brain. On the other hand, presumed arginine vasotocin (AVT) perikarya were distributed in an arc-shaped region extending from the posterior part of the preoptic nucleus to the anterior-most end of the hypothalamic nucleus and projected their fibers to the neurohypophysis. Most presumptive AVT perikarya were located close to the paired prehypophysial arteries near the anterior end of the postoptic nucleus. In the neurohypophysis, abundant presumptive AVT-fibers terminated in the posterior dorsal wall, although some fibers terminated in the anterior dorsal wall and only a few fiber endings were found in the ventral wall. No ACTH-positive cells were detected in the hagfish brain or in the pituitary gland.Supported from a grant from the National Science Foundation PCM 8141393     

Lateral line morphology and cranial osteology of the Rubynose Brotula,Cataetyx rubrirostris

Cranial osteology, canal neuromast distribution, superficial neuromast distribution and innervation, and cephalic pore structure were studied in cleared and stained specimens of the deep sea brotulid Cataetyx rubrirostris. The cranial bone structure of C. rubrirostris is similar to other brotulids (Dicrolene sp.) and zoarcids (Zoarces sp.), except for an unusual amount of overlapping of the bones surrounding the cranial vault. The superficial neuromasts are innervated by the anterodorsal, anteroventral, middle and posterior lateral line nerves and are organized similarly to those of the blind ophidioid cave fish Typhliasina pearsei. The cephalic pores open into a widened lateral line canal system. The canal is compartmentalized into a series of neuromast‐containing chambers that probably amplify signals received by the system. J. Morphol. 241:265–274, 1999. © 1999 Wiley‐Liss, Inc.     

版纳鱼螈侧线系统的结构

版纳鱼螈(Ichthyophis bannanica)是我国无足目的仅有代表,应用光镜和扫描电镜对版纳鱼螈的侧线系统进行形态学和组织学观察的研究表明:版纳鱼螈幼体表皮中的侧线器官有接受机械刺激的神经丘和电接受壶腹器官两种,神经丘包括表面神经丘和陷神经丘。侧线分布主要包括:头部的鼻侧线、眶上线、眶下线、眶后线、口侧线、下颌线、咽侧线、鳃孔上线和身体上的背侧线。侧线器官的分布密度、大小和凹陷深度明显与周围表皮的厚度和不同部位有关。幼体的侧线器官退化与鳃孔的退化同步,亚成体以后不保留侧线系统。版纳鱼螈的侧线分布和器官结构与其它无足类的大致相似,仅在眶上线和眶下线的器官分布上存在微小的差别     

The lateral line system in larvalIchthyophis (Amphibia: Gymnophiona)

Summary The lateral line systems of larval caecilians of the genusIchthyophis possess two types of elements, free neuromasts and ampullary organs. Free mechanoreceptive neuromasts are typical of those found in other vertebrates, and are arranged in series roughly homologous to neuromast groups in many other fishes and amphibians. In contrast to other amphibians,Ichthyophis larvae possess only one paired, dorsal body series of neuromasts. Regional specialization of neuromasts is evident inIchthyophis. Premaxillary and anterior head neuromasts are the largest in size and total cell number. Overall, size and total cell numbers are correlated with depth of epidermis. Neuromasts on the anterior sides of the head occur in slight grooves and have apical tips situated farther below the level of the epidermis and with greater apical indentation. These features probably provide increased protection against abrasion. Apparently abnormal neuromasts are frequently found among the neuromast series. Such neuromasts contain fewer cells that lack normal apical extension, producing a sunken effect similar to that of the ampullary organ elements. The ampullary organs ofIchthyophis are morphologically similar to those found in various freshwater fishes and known to function as electroreceptors. These organs are not observed in the lateral line systems of members of other amphibian orders (Urodela and Anura), and we suggest that they function as electroreceptors. The sunken neuromasts of theIchthyophis lateral line system may parallel the possible evolutionary development of pit organs from normal neuromasts.     

The wors of I.I. Schmalhousen on comparative morphology of vertebrates: Significance for paleontological reconstructions (Data on the seismo-sensory system of agnathans)

The morphology and function of the seismo-sensory system of agnathans (Agnatha: Heterostraci) are analyzed. This system has been reconstructed in Cyathaspidiformes and Amphiaspidiformes. The seismo-sensory system of agnathans was compared with the lateral line system of fishes: the difference between these systems in the density of seismo-sensory lines and the contribution level of these systems to orientation in the water was detected. The possibility of reconstruction of the length and direction of heterostracans cranial nerves has been shown using the position of seismo-sensory lines and departments of the brain.     

Diversification of brain and sense organ morphology in Antarctic dragonfishes (Perciformes: Notothenioidei: Bathydraconidae)

In the subzero shelf waters of Antarctica, fishes of the perciform suborder Notothenioidei dominate the fish fauna and constitute an adaptive radiation and a species flock. The 16 species of dragonfishes of the family Bathydraconidae live from surface waters to nearly 3,000 m and have the greatest overall depth range among notothenioid families. We examined the anatomy and histology of the brain, retina, and cephalic lateral line system of nine bathydraconid species representing 8 of the 11 known genera. We evaluate these data against a cladogram identifying three clades in the family. We provide a detailed drawing of the brain and cranial nerves of Gymnodraco acuticeps and Akarotaxis nudiceps. Bathydraconid brain morphology falls into two categories. Brains of most species are similar to those of generalized perciforms and some basal notothenioids (Class I). However, brains of deep-living bathydraconids (members of the tribe Bathydraconini minus Prionodraco) have a reduced telencephalon and tectum that renders the neural axis visible - the stalked brain morphology (Class II). All bathydraconids have duplex (rod and cone) retinae but there is considerable interspecific variation in the ratio of cones:rods and in the number of cells in the internal nuclear layer. Retinal histology reflects habitat depth but is not tightly coupled to phylogeny. Although the deep-living species of Bathydraconini have rod-dominated retinae, the retinae of some sister species are photopic. An expanded cephalic lateral line system is also characteristic of all members of the Bathydraconini as exemplified by Akarotaxis. This morphology includes large lateral line pores, wide membranous canals, hypertrophied canal neuromasts, and large anterodorsal lateral line nerves, eminentia granulares, and crista cerebellares. The saccular otoliths are also enlarged in members of this tribe. Neural diversification among bathydraconids on the Antarctic shelf has not involved the evolution of sensory specialists. Brain and sense organ morphologies do not approach the specialized condition seen in primary deep-sea fishes or even that of some secondary deep-sea fishes including sympatric non-notothenioids such as liparids (snailfishes) and muraenolepidids (eel cods). The brains and sense organs of bathydraconids, including the deep-living species, reflect their heritage as perciform shorefishes.     

Electroreception in the Guiana dolphin (Sotalia guianensis)

Passive electroreception is a widespread sense in fishes and amphibians, but in mammals this sensory ability has previously only been shown in monotremes. While the electroreceptors in fish and amphibians evolved from mechanosensory lateral line organs, those of monotremes are based on cutaneous glands innervated by trigeminal nerves. Electroreceptors evolved from other structures or in other taxa were unknown to date. Here we show that the hairless vibrissal crypts on the rostrum of the Guiana dolphin (Sotalia guianensis), structures originally associated with the mammalian whiskers, serve as electroreceptors. Histological investigations revealed that the vibrissal crypts possess a well-innervated ampullary structure reminiscent of ampullary electroreceptors in other species. Psychophysical experiments with a male Guiana dolphin determined a sensory detection threshold for weak electric fields of 4.6 μV cm(-1), which is comparable to the sensitivity of electroreceptors in platypuses. Our results show that electroreceptors can evolve from a mechanosensory organ that nearly all mammals possess and suggest the discovery of this kind of electroreception in more species, especially those with an aquatic or semi-aquatic lifestyle.     

Neurons of the medulla oblongata and hypothalamus projecting on the sympathetic neurons of the ventral horn of the spinal cord]

Connections of the neurons of the spinal cord ventral horn with the structures, situating above have been investigated. After injection of uranyl acetate into the TIII segment of the spinal cord, labelled neurons are found in various reticular nuclei of the medulla oblongata. At the level of the roots of the XII pair of the cranial nerves they are revealed in the reticular paramedian, ventral, parvocellular and lateral nuclei. The formations mentioned participate in regulation of the cardio-vascular system. More rostral (2 and 4 mm relatively to the roots of the XII pair of the cranial nerves) the neurons are observed in the reticular giant cellular nucleus, in nuclei of the raphe and in the group of the P-substance reactive neurons. Besides, labelled neurons are revealed in the posterior, lateral fields and in the dorso- and ventromedial nuclei of the hypothalamus.     

Anatomy and histology of the brain and sense organs of the Antarctic eel cod Muraenolepis microps (Gadiformes; Muraenolepididae)

Brain regions, cranial nerves, and sense organs in Muraenolepis microps, an Antarctic gadiform fish, were examined to determine which features could be attributed to a gadiform ancestry and which to habitation of Antarctic waters. We found that the central nervous system and sense organs are well developed, showing neither substantial regression nor hypertrophy. A detailed drawing of the brain and cranial nerves is provided. The rostral position of the olfactory bulbs and telencephalic size and lobation are common for the order. The optic tectum and corpus cerebelli are smaller than in most other gadiforms. The shape of the corpus cerebelli is not distinctive among gadiforms. The lateral line region is moderately well-developed, but not hypertrophied to the extent seen in deep-sea gadiforms. As is the case in gadids possessing barbels and elongated pelvic rays, Muraenolepis has well-developed facial lobes, although these are smaller and more laterally positioned. The vagal lobes are deeply placed in the rhombencephalon and project into the fourth ventricle. The brain of Muraenolepis resembles that of a phyletically derived gadoid, especially a phycid, more than it resembles the brain of a phyletically basal macrourid. Two histological features of the diencephalon of Muraenolepis appear to be unique among gadiforms: a well-organized thalamic central medial nucleus and subependymal expansions. Muraenolepis has a pure rod retina like many deep-sea species but lacks the superimposed layers of rod outer segments. The histology of the nonvisual sense organs, especially the olfactory and external taste systems, are well-developed in Muraenolepis but not hypertrophied. We relate our findings to what is known about neural morphology in other gadiforms and in phyletically distant notothenioids and liparids that are sympatric with Muraenolepis on the Antarctic shelf. The only feature that reflects an Antarctic existence is the diencephalic subependymal expansions, which within notothenioids mirror the habitation of cold waters and have been found in every Antarctic species examined to date. Although the waters of the Antarctic shelf are cold, dark, and deep, brain and sense organ morphology in Muraenolepis are remarkably free of extreme specialization.     

Dual origins of the prechordal cranium in the chicken embryo

The prechordal cranium, or the anterior half of the neurocranial base, is a key structure for understanding the development and evolution of the vertebrate cranium, but its embryonic configuration is not well understood. It arises initially as a pair of cartilaginous rods, the trabeculae, which have been thought to fuse later into a single central stem called the trabecula communis (TC). Involvement of another element, the intertrabecula, has also been suggested to occur rostral to the trabecular rods and form the medial region of the prechordal cranium. Here, we examined the origin of the avian prechordal cranium, especially the TC, by observing the craniogenic and precraniogenic stages of chicken embryos using molecular markers, and by focal labeling of the ectomesenchyme forming the prechordal cranium. Subsequent to formation of the paired trabeculae, a cartilaginous mass appeared at the midline to connect their anterior ends. During this midline cartilage formation, we did not observe any progressive medial expansion of the trabeculae. The cartilages consisted of premandibular ectomesenchyme derived from the cranial neural crest. This was further divided anteroposteriorly into two portions, derived from two neural crest cell streams rostral and caudal to the optic vesicle, called preoptic and postoptic neural crest cells, respectively. Fate-mapping analysis elucidated that the postoptic neural crest cells were distributed exclusively in the lateroposterior part of the prechordal cranium corresponding to the trabeculae, whereas the preoptic stream of cells occupied the middle anterior part, differentiating into a cartilage mass corresponding to the intertrabecula. These results suggest that the central stem of the prechordal cranium of gnathostomes is composed of two kinds of distinct cartilaginous modules: a pair of trabeculae and a median intertrabecula, each derived from neural crest cells populating distinct places of the craniofacial primordia through specific migratory pathways.     

Cranial nerves in the Australian lungfish,Neoceratodus forsteri,and in fossil relatives (Osteichthyes: Dipnoi)

Three systems, two sensory and one protective, are present in the skin of the living Australian lungfish, Neoceratodus forsteri, and in fossil lungfish, and the arrangement and innervation of the sense organs is peculiar to lungfish. Peripheral branches of nerves that innervate the sense organs are slender and unprotected, and form before any skeletal structures appear. When the olfactory capsule develops, it traps some of the anterior branches of cranial nerve V, which emerged from the chondrocranium from the lateral sphenotic foramen. Cranial nerve I innervates the olfactory organ enclosed within the olfactory capsule and cranial nerve II innervates the eye. Cranial nerve V innervates the sense organs of the snout and upper lip, and, in conjunction with nerve IX and X, the sense organs of the posterior and lateral head. Cranial nerve VII is primarily a motor nerve, and a single branch innervates sense organs in the mandible. There are no connections between nerves V and VII, although both emerge from the brain close to each other. The third associated system consists of lymphatic vessels covered by an extracellular matrix of collagen, mineralised as tubules in fossils. Innervation of the sensory organs is separate from the lymphatic system and from the tubule system of fossil lungfish.     

Cadherin-1, -2 and -4 expression in the cranial ganglia and lateral line system of developing zebrafish

Cadherins are cell adhesion molecules that play important roles in development of a variety of tissues and organs including the nervous system. In this study we analyzed expression patterns of three zebrafish classical (type I) cadherins (cadherin-1, -2, and -4) in the embryonic zebrafish cranial ganglia and lateral line system using in situ hybridization and immunohistochemical methods. All three cadherins exhibit distinct spatiotemporal patterns of expression during cranial ganglia and lateral line system development. cadherin-1 message was detected in the trigeminal and facial ganglia, in the lateral line ganglia, and in most of neuromasts in the lateral lines. Cadherin-2 mRNA and protein were expressed by the majority of the cranial ganglia and lateral line system. Both cadherins were found in embryos younger than 24 hours post fertilization as well as in 2-3-day old embryos and larvae. In contrast, cadherin-4 mRNA and protein expression was detected in embryos older than 30 hours post fertilization and limited to the trigeminal, statoacoustic, and vagal cranial ganglia, and the lateral line ganglia of older embryos and larvae.     

Functional anatomy of the canine mediastinal cardiac nerves located at the base of the heart

The major canine cardiopulmonary nerves which arise from the middle cervical and stellate ganglia and the vagi course toward the heart in the dorsal mediastinum where they form, at the base of the heart dorsal to the pulmonary artery and aorta, the dorsal mediastinal cardiac nerves. In addition, the left caudal pole and interganglionic nerves project onto the left lateral side of the heart as the left lateral cardiac nerve. These nerves contain afferent and (or) efferent axons which, upon stimulation, modify specific cardiac regions and (or) systemic pressure. In addition, with the exception of the left lateral cardiac nerve, stimulation of each of these nerves produces compound action potentials in the cranial ends of the majority of the major cardiopulmonary nerves demonstrating that axons in each dorsal mediastinal cardiac nerve interconnect with axons in the majority of the cardiopulmonary nerves. Axons in the left lateral cardiac nerve connect primarily with axons in the left caudal pole and left interganglionic nerves. The dorsal mediastinal nerves project distally onto the heart as coronary nerves accompanying the right or left coronary arteries. These innervated the ventricular myocardium which is supplied by their respective vessels. The left lateral cardiac nerve projects directly onto the lateral epicardium of the left ventricle. The dorsal mediastinal and left lateral cardiac nerves are the major sympathetic cardiac nerves. Thus, the cardiac nerves located in the mediastinum at the base of the heart are not simple extensions of cardiopulmonary nerves, but rather have a unique anatomy and function of their own.     

Confocal imaging and phylogenetic considerations of the subcutaneous neurons in the Atlantic hagfish Myxine glutinosa

We used confocal microscopy and immunohistochemistry to characterize the morphology of the subcutaneous neurons and the innervation of the slime glands and striated muscles in the hagfish Myxine glutinosa. A rich plexus of 5HT‐, ChAT‐ and TH‐positive neurons is described in the capsule of the slime glands. These neurons, like those of the subcutaneous plexus, receive pericellular terminations from the axons of central cells. Capsular neurons receive innervation from 5HT‐positive and nNOS‐positive nerve fibres. Other nerve endings belonging to two separate nerve populations are identified in the striated muscles. They contain TH and nNOS immunoreactivity. Due to the lack of any topographical labelling, the cell origin and the projections of the neurons into the cranial and spinal nerves are unknown. This study provides anatomical evidence of multiple (5HT and nNOS) peripheral innervation of the neurons. However, it does not provide information about the function of these neurons in the hagfish. We suggest that hagfish neurons have a phylogenetic relationship with the spinal group of the dorsal cells of lampreys and the supramedullary cells of teleosts.     

Expression of zebrafish fkd6 in neural crest-derived glia

The zebrafish fkd6 gene is a marker for premigratory neural crest. In this study, we analyze later expression in putative glia of the peripheral nervous system. Prior to neural crest migration, fkd6 expression is downregulated in crest cells. Subsequently, expression appears initially in loose clusters of cells in positions corresponding to cranial ganglia. Double labelling with a neuronal marker shows that fkd6-expressing cells are not differentiated neurones and generally lie peripheral to neurones in ganglia. Later, expression appears associated with the posterior lateral line and other cranial nerves. For the posterior lateral line nerve, we show that fkd6-labeling extends caudally along this nerve in tight correlation with lateral line primordium migration and axon elongation. Expression in colourless mutant embryos is consistent with these cells being satellite glia and Schwann cells.