Early scale development in Heterodontus (Heterodontiformes; Chondrichthyes): a novel chondrichthyan scale pattern

In two species of Heterodontus, H. portusjacksoni and H. galeatus, the first scales to develop form two opposing rows along the caudal fin axis on both the left and right sides of the fin. The opposing rows originate from an initial scale located on either side of the posterior tip of the caudal fin, with subsequent scales erupting in a posterior to anterior direction along the tail axis. These scale rows may strengthen tail movements, providing aeration in the egg case, but are lost later in ontogeny. Development of subsequent body scales shows a more irregular origin and arrangement, from anterior to posterior, to cover the dorsal and ventral lobes of the caudal fin. Although the early developmental pattern of the scale associated with the Heterodontus caudal fin has not been previously described, several chondrichthyan taxa, including chimeroids, likewise possess ordered rows of flank scales early in ontogeny that are subsequently lost. These ordered scales contrast with previous suggestions that chondrichthyan scale development is entirely random. Instead, regulated and sequential development of scales may be a plesiomorphic character for both chondrichthyans and osteichthyans, with the less organized arrangement in later ontogenetic stages being a derived condition within Chondrichthyes.     

Upper Ordovician chondrichthyan‐like scales from North America

Studies of Ordovician micromeric fish scales from the Sandbian of North America have identified a number of scale‐based taxa potentially referable to the chondrichthyans and therefore can be among the stratigraphically oldest representatives of the clade described to date. Two of these, Tezakia hardingensis gen. et sp. nov. and Canyonlepis smithae gen. et sp. nov., are formally described herein. Tezakia gen. nov. scales are composed exclusively of tubular dentine and possess polyodontocomplex crowns with a characteristically large primordial odontode. Similar scale crown architecture has been reported only in the reputed chondrichthyan Altholepis composita (Lower Devonian of Podolia, Ukraine), and on these grounds, the two are united within the newly erected Altholepidiformes ordo nov. Multiple odontocomplexes are also a feature of Canyonlepis gen. nov. scale crowns; however, the latter do not demonstrate prominent primordial odontodes and are supported by a base composed of acellular bone. Additional data suggest that both taxa possess a combination of characteristics (areal crown growth, scale symmetry, linear odontocomplex architecture and absence of enamel, osteons, cancellous bone and hard‐tissue resorption) previously documented to occur only in chondrichthyan scales. This study contributes to a growing body of evidence that reveals the presence of diverse tissue types (bone, tubular and atubular dentine) and morphogenetic patterns (odontocomplex and non‐odontocomplex type of scale crown growth) in the dermal skeleton of putative Ordovician chondrichthyans.     

An enigmatic actinopterygian (Pisces: Osteichthyes) from the Upper Permian of China

A new actinopterygian fish Yaomoshania minutosquama gen. & sp. nov., from the Upper Permian of the Dzungaria [Junggar] Basin in China is described. The material consists of two very fragmentary specimens showing almost exclusively scale rows. The arrangement of the scale rows of the holotype resembles the reversion lines of acanthodian caudal fins and of cheirolepids. The size and shape of the scales, and the diminishing size of the scales along the rows, is similar to those of acanthodians and those actinopterygians with small scales such as Cheirolepis. However, the histology of the scales demonstrates features that are found in more advanced actinopterygians: superimposed ganoine lamellae, the arrangement of dentine tubules and pulp canals, buried odontodes stacked on each other to make the ornamental ridges, and an asymmetric pattern of growth achieved by extensive remodelling (resorption and redeposition) of all layers of the scale. Yaomoshania minutosquama is proposed as a small-scale bearing, basal actinopterygian, different from Cheirolepis , but characterized by advanced histological characters.     

Organization of the superficial neuromast system in goldfish, Carassius auratus

Distribution, morphology, and orientation of superficial neuromasts and polarization of the hair cells within superficial neuromasts of the goldfish (Carassius auratus) were examined using fluorescence labeling and scanning electron microscopy. On each body side, goldfish have 1,800-2,000 superficial neuromasts distributed across the head, trunk and tail fin. Each superficial neuromast had about 14-32 hair cells that were arranged in the sensory epithelium with the axis of best sensitivity aligned perpendicular to the long axis of the neuromast. Hair cell polarization was rostro-caudal in most superficial neuromasts on trunk scales (with the exception of those on the lateral line scales), or on the tail fin. On lateral line scales, the most frequent hair cell polarization was dorso-ventral in 45% and rostro-caudal in 20% of the superficial neuromasts. On individual trunk scales, superficial neuromasts were organized in rows which in most scales showed similar orientations with angle deviations smaller than 45 degrees . In about 16% of all trunk scales, groups of superficial neuromasts in the dorsal and ventral half of the scale were oriented orthogonal to each other. On the head, most superficial neuromasts were arranged in rows or groups of similar orientation with angle deviations smaller than 45 degrees . Neighboring groups of superficial neuromasts could differ with respect to their orientation. The most frequent hair cell polarization was dorso-ventral in front of the eyes and on the ventral mandible and rostro-caudal below the eye and on the operculum.     

Evolution of developmental pattern for vertebrate dentitions: an oro-pharyngeal specific mechanism

Classically the oral dentition with teeth regulated into a successional iterative order was thought to have evolved from the superficial skin denticles migrating into the mouth at the stage when jaws evolved. The canonical view is that the initiation of a pattern order for teeth at the mouth margin required development of a sub-epithelial, permanent dental lamina. This provided regulated tooth production in advance of functional need, as exemplified by the Chondrichthyes. It had been assumed that teeth in the Osteichthyes form in this way as in tetrapods. However, this has been shown not to be true for many osteichthyan fish where a dental lamina of this kind does not form, but teeth are regularly patterned and replaced. We question the evolutionary origin of pattern information for the dentition driven by new morphological data on spatial initiation of skin denticles in the catshark. We review recent gene expression data for spatio-temporal order of tooth initiation for Scyliorhinus canicula, selected teleosts in both oral and pharyngeal dentitions, and Neoceratodus forsteri. Although denticles in the chondrichthyan skin appear not to follow a strict pattern order in space and time, tooth replacement in a functional system occurs with precise timing and spatial order. We suggest that the patterning mechanism observed for the oral and pharyngeal dentition is unique to the vertebrate oro-pharynx and independent of the skin system. Therefore, co-option of a successional iterative pattern occurred in evolution not from the skin but from mechanisms existing in the oro-pharynx of now extinct agnathans.     

BMP-2 functions independently of SHH signaling and triggers cell condensation and apoptosis in regenerating axolotl limbs

Background  

Axolotls have the unique ability, among vertebrates, to perfectly regenerate complex body parts, such as limbs, after amputation. In addition, axolotls pattern developing and regenerating autopods from the anterior to posterior axis instead of posterior to anterior like all tetrapods studied to date. Sonic hedgehog is important in establishing this anterior-posterior axis of limbs in all tetrapods including axolotls. Interestingly, its expression is conserved (to the posterior side of limb buds and blastemas) in axolotl limbs as in other tetrapods. It has been suggested that BMP-2 may be the secondary mediator of sonic hedgehog, although there is mounting evidence to the contrary in mice. Since BMP-2 expression is on the anterior portion of developing and regenerating limbs prior to digit patterning, opposite to the expression of sonic hedgehog, we examined whether BMP-2 expression was dependent on sonic hedgehog signaling and whether it affects patterning of the autopod during regeneration.     

Sonic hedgehog expression during early tooth development in Suncus murinus

Tooth development is a highly organized process characterized by reciprocal interactions between epithelium and mesenchyme. However, the expression patterns and functions of molecules involved in mouse tooth development are unclear from the viewpoint of explaining human dental malformations and anomalies. Here, we show the expression of sonic hedgehog (Shh), a potent initiator of morphogenesis, during the early stages of tooth development in Suncus murinus. Initially, symmetrical, elongated expression of suncus Shh (sShh) was observed in the thin layer of dental epithelial cells along the mesial-distal axis of both jaws. As the dental epithelium continued to develop, sShh was strictly restricted to the predicted leading parts of the growing, invaginating epithelium corresponding to tooth primordia and enamel knots. We propose that some aspects of Shh function in tooth development are widely conserved in mammalian phylogeny.     

Teeth before jaws? Comparative analysis of the structure and development of the external and internal scales in the extinct jawless vertebrate Loganellia scotica

Traditional hypotheses posit that teeth evolved from dermal scales, through the expansion of odontogenetically competent ectoderm into the mouth of jawless vertebrates. The discovery of tooth‐like scales inside thelodonts, an extinct group of jawless vertebrates, led to the alternative hypothesis that teeth evolved from endodermal derivatives and that there exists a fundamental developmental and phylogenetic distinction between oral/pharyngeal and external odontodes. We set out a test of this latter hypothesis, examining the development of scales of the thelodont Loganellia scotica using synchrotron radiation X‐ray tomographic microscopy (SRXTM). We reveal that the internal scales are organized into fused patches and rows, a key distinction from the discrete dermal scales. The pattern of growth of oral scale patches is polarized, but not along a particular vector, whereas pharyngeal scale rows grew along a vector. Our test of the phylogenetic distribution of oral and pharyngeal scales and teeth in vertebrates indicates that odontodes are first expressed in an external position. Internal scales, where present, are always located near to external orifices; the sequential development of pharyngeal scales in Loganellia is peculiar among thelodonts and other stem gnathostomes. It represents a convergence on, rather than the establishment of, the developmental pattern underpinning tooth replacement in jawed vertebrates. The available evidence suggests that internal odontodes evolved through the expansion of odontogenic competence from external to internal epithelia.     

Two sympatric new species of woodlizards (Hoplocercinae, Enyalioides) from Cordillera Azul National Park in northeastern Peru

We report the discovery of two sympatric new species of Enyalioides from a montane rainforest of the Río Huallaga basin in northeastern Peru. Among other characters, the first new species is distinguishable from other Enyalioides by the combination of the following characters: strongly keeled ventral scales, more than 37 longitudinal rows of dorsals in a transverse line between the dorsolateral crests at midbody, low vertebral crest on the neck with vertebrals on neck similar in size to those between hind limbs, projecting scales on body or limbs absent, 96 mm maximum SVL in both sexes, and caudals increasing in size posteriorly within each autotomic segment. The second new species differs from other species of Enyalioides in having strongly keeled ventral scales, scales posterior to the superciliaries forming a longitudinal row of strongly projecting scales across the lateral edge of the skull roof in adults of both sexes, 31 or fewer longitudinal rows of strongly keeled dorsals in a transverse line between the dorsolateral crests at midbody, vertebrals on neck more than five times the size of vertebrals between hind limbs in adult males, projecting scales on body or limbs absent, and caudals increasing in size posteriorly within each autotomic segment. We also present an updated molecular phylogenetic tree of hoplocercines including new samples of Enyalioides rudolfarndti, Enyalioides rubrigularis, both species described in this paper, as well as an updated identification key for species of Hoplocercinae.     

Scale structure of putative chondrichthyan Gladbachus adentatus Heidtke & Krätschmer, 2001 from the Middle Devonian Rheinisches Schiefergebirge,Germany

Gladbachus adentatus is a putative chondrichthyan, known only from the holotype specimen, which comprises an articulated endoskeleton complete from head to pelvic region with the squamation also preserved. The scales superficially resemble those of placoderms more than sharks, in having a similar gross morphology, lamellar cellular bone forming the base and upright dentinous tubercles comprising the crown. The odontocytic mesodentine in the tubercles is comparable to that in the Osteostraci and in some acanthodian taxa, known only from isolated scales, and is probably the plesiomorphic form of dentine for Gnathostomata.     

Specification of motoneurons from human embryonic stem cells

An understanding of how mammalian stem cells produce specific neuronal subtypes remains elusive. Here we show that human embryonic stem cells generated early neuroectodermal cells, which organized into rosettes and expressed Pax6 but not Sox1, and then late neuroectodermal cells, which formed neural tube-like structures and expressed both Pax6 and Sox1. Only the early, but not the late, neuroectodermal cells were efficiently posteriorized by retinoic acid and, in the presence of sonic hedgehog, differentiated into spinal motoneurons. The in vitro-generated motoneurons expressed HB9, HoxC8, choline acetyltransferase and vesicular acetylcholine transporter, induced clustering of acetylcholine receptors in myotubes, and were electrophysiologically active. These findings indicate that retinoic acid action is required during neuroectoderm induction for motoneuron specification and suggest that stem cells have restricted capacity to generate region-specific projection neurons even at an early developmental stage.     

Sonic hedgehog regulates proliferation and differentiation of mesenchymal cells in the mouse metanephric kidney

Signaling by the ureteric bud epithelium is essential for survival, proliferation and differentiation of the metanephric mesenchyme during kidney development. Most studies that have addressed ureteric signaling have focused on the proximal, branching, ureteric epithelium. We demonstrate that sonic hedgehog is expressed in the ureteric epithelium of the distal, non-branching medullary collecting ducts and continues into the epithelium of the ureter -- the urinary outflow tract that connects the kidney with the bladder. Upregulation of patched 1, the sonic hedgehog receptor and a downstream target gene of the signaling pathway in the mesenchyme surrounding the distal collecting ducts and the ureter suggests that sonic hedgehog acts as a paracrine signal. In vivo and in vitro analyses demonstrate that sonic hedgehog promotes mesenchymal cell proliferation, regulates the timing of differentiation of smooth muscle progenitor cells, and sets the pattern of mesenchymal differentiation through its dose-dependent inhibition of smooth muscle formation. In addition, we also show that bone morphogenetic protein 4 is a downstream target gene of sonic hedgehog signaling in kidney stroma and ureteral mesenchyme, but does not mediate the effects of sonic hedgehog in the control of mesenchymal proliferation.     

Descriptions of Four New Species of Criconematoidea (Tylenchina: Nemata) from Southern Chile

Four new species of Criconematoidea are described from Hoste Island, Chile. Criconema certesi n. sp. is distinguished by the fine, spine-like, cuticular extensions on body annuli; projection of annuli into rows of scales on posterior part of body; single, smooth, labial annulus set off by short collar from second (first body) annulus which is about same diameter as first (labial) annulus. Male with prominent caudal alae, slender curved spicules, and four incisures in lateral field. Ogma terrestris n. sp. is distinguished by small scales with rounded tips bearing minute, short bristles; scales number 21 at mid-body; and first (labial) annulus rounded, not retrorse, not set off from succeeding annuli, narrower in diameter from second (first body) annulus. Hemicycliophora macrodorata n. sp. is distinguished by its large size (L = 1.52 [1.28-1.72] mm); large stylet (146 [127-161] μm); annuli = 297 (280-315); tail slightly spicate, lateral field with or without interruptions of incisures, occasional anastomoses; and males with U-shaped spicules. Paratylenchus fueguensis n. sp. is distinguished by its prominent stylet with large, rounded knobs (4-5 μm across); cephalic region rounded not at all set off; lateral field with four incisures; lateral vulvar membranes present; and male tail short, strongly curved (almost 180°) ventrad.     

Chondrichthyan‐like scales from the Middle Ordovician of Australia

Abstract: Microvertebrate sampling of the Stairway Sandstone (Darriwilian, Middle Ordovician, central Australia) has yielded scales that are chondrichthyan‐like in their overall construction, and Tantalepis gatehousei gen. et sp. nov. is erected here to describe these specimens. Tantalepis gatehousei gen. et sp. nov. is the stratigraphically oldest microsquamous taxon described thus far, and the ‘shark‐like’ appearance of the scales may extend the chondrichthyan lineage back into the Middle Ordovician. The presence of ‘shark‐like’ scales in the fossil record some 50 myr prior to the first articulated chondrichthyan body fossils and 44 myr before the first clearly identifiable chondrichthyan teeth suggests there is a considerable scope for the recovery of articulated specimens with which to document the early history of crown gnathostomes. Traditional hypotheses of phylogenetic relationships among early jawed vertebrates were recently challenged by the proposal of a radically different tree topology. However, the development of a new data set specifically addressing scale‐based characters is required before taxa such as Tantalepis, that are based upon disarticulated remains alone, can be firmly placed within the emerging, revised, evolutionary narrative.     

Nematodes of the Order Rhabditida from Andalucía Oriental,Spain. The Genera Nothacrobeles Allen &Noffsinger, 1971 and Zeldia Thorne, 1937

A new species of the genus Nothacrobeles is described from natural areas (a salt lake) in the Southeast Iberian Peninsula. Nothacrobeles lanceolatus sp. n. is characterized by its body length, two rows of cuticular punctations per annulus, labial probolae bifurcate with divergent prongs, pharyngeal corpus 2.4 to 3.5 times isthmus length, spermatheca length, postuterine sac 0.5 to 1.1 times the corresponding body diameter ratio, female tail conical and bearing a spindle-shaped or conical mucro with acute terminus, phasmid at 8 to 17 µm posterior to the anus, male tail conical with acute mucro, spicules length, and gubernaculum length. In addition, Nothacrobeles cf. lunensis and Zeldia punctata are studied. Cervidellus capricornis is transferred to genus Nothacrobeles. A key to species of Nothacrobeles is also provided.     

Postembryonic Morphology in Epsilonematidae, with a Discussion on the Variability of Caudal Gland Outlets

A new species of Akanthepsilonema and the first-stage juvenile of Glochinema trispinatum are described. Furthermore, additional morphological information is provided for Triepsilonema tripapillata. Animals originate from a cold-water coral degradation zone in the Porcupine Seabight area (North-East Atlantic Ocean). Akanthepsilonema sinecornibus sp. n. differs from A. heUeouetae in number of body annules, sexual dimorphism in amphid size, absence of copulatory thorns in males, absence of large spines and horns, shape of the copulatory apparatus, and position of ambulatory setae relative to vulva in females. The genus diagnosis for Akanthepsilonema is adjusted to incorporate the new species. Akanthepsilonema mainly differs from every other genus in the family by the combination of six rows of ambulatory setae situated around the vulva in females and eight subcephalic setae not displaced toward the anterior part of the head capsule. Small differences between the Papua New Guinea and the Porcupine Seabight populations of T. tripapillata indicate minimal intraspecific variability. Second-stage juveniles from Papua New Guinea have two rows of three ambulatory setae, whereas Porcupine Seabight specimens have two rows of four ambulatory setae. First- and fourth-stage juveniles of T. tripapillata are described for the first time. Literature data and personal observations showed that the molting of first-stage juveniles into second-stage juveniles and of third-stage juveniles into fourth-stage juveniles involves a decrease in the number of body rings, resulting in a loss of flexibility which is possibly compensated for by the development (I-II) or the doubling of the number of rows (III-IV) of ambulatory setae. This decrease is also linked with the formation of the head capsule and the smooth tail tip, although intergeneric variability is evident. The molting of second-stage juveniles into third-stage juveniles and of fourth-stage juveniles into adults is also subject to intergeneric variability. The variability in the number and orientation of caudal gland outlets among different nematode taxa is discussed. The presence of separate outlets for the caudal glands seems to be widespread within the family Epsilonematidae and has also been observed in various other, unrelated taxa of free-living aquatic nematodes, although their arrangement in Epsilonematidae is opposite. This aberrant arrangement is probably related to the aberrant locomotory pattern in this family.     

Ausgangsform und Entwicklung der rhombischen Schuppen der Osteichthyes (Pisces)

Ganoid and cosmoid scales, the two types of rhombic scales within the osteichthyans, can be traced back to a primitive scale similar to the scales ofLophosteus. The primitive rhombic scale did not have a peg-and-socket articulation, it is composed of lamellar bone superposed by many layers of spongy bone + dentine. That kind of superposition of layers of spongy bone + dentine (+ enamel) has been retained in the cosmoid scale; in contrast in the ganoid scale the growth of the dentine has become restricted to the lateral surface, the growth of ganoin to the outer surface and the growth of bone to the inner surface of the scale. The scales ofAndreolepis have a position between the primitive rhombic scale and the ganoid scale. — Scales from the Gedinnian (Lower Devonian) of New Sibirian Islands, USSR, are described asDialipina markae n. sp. The morphological features are very typical for the genus, but the histology is different from the type speciesD. salgueiroensis. Within the two Devonian palaeoniscoid generaDialipina andOrvikuina acellular bone with irregular non-vascular canals of Williamson has developed twice from cellular bone.