Dorsal Shell Research Articles

The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan

The unique anatomical features of turtles have raised unanswered questions about the origin of their unique body plan. We generated and analyzed draft genomes of the soft-shell turtle (Pelodiscus sinensis) and the green sea turtle (Chelonia mydas); our results indicated the close relationship of the turtles to the bird-crocodilian lineage, from which they split ~267.9–248.3 million years ago (Upper Permian to Triassic). We also found extensive expansion of olfactory receptor genes in these turtles. Embryonic gene expression analysis identified an hourglass-like divergence of turtle and chicken embryogenesis, with maximal conservation around the vertebrate phylotypic period, rather than at later stages that show the amniote-common pattern. Wnt5a expression was found in the growth zone of the dorsal shell, supporting the possible co-option of limb-associated Wnt signaling in the acquisition of this turtle-specific novelty. Our results suggest that turtle evolution was accompanied by an unexpectedly conservative vertebrate phylotypic period, followed by turtle-specific repatterning of development to yield the novel structure of the shell.

Micro-structure and mechanical properties of the turtle carapace as a biological composite shield

Turtle shell is a multi-scale bio-composite in which the components are arranged in various spatial patterns, leading to an unusually strong and durable structure. The keratin-coated dorsal shell, termed the carapace, exhibits a flat bone, sandwich-like structure made up of two exterior cortices enclosing a cancellous interior. This unique structure was developed by nature to protect the reptile from predator attacks by sustaining impact loads and dissipating energy. In the present study we attempt to correlate the micro-scale architecture with the mechanical properties of the carapace sub-regions of the red-eared slider turtle. The microscopic structural features were examined by scanning electron microscopy and micro-computed tomography. Nanoindentation tests were performed under dry and wet conditions on orthogonal anatomical planes to evaluate the elastic modulus and hardness of the various carapace sub-regions. The mineral content was also measured in the different regions of the carapace. Consequently, we discuss the influence of hydration on the carapace sub-regions and the contribution of each sub-region to the overall mechanical resistance of the assemblage.

Inducible defenses in the eastern oyster Crassostrea virginica Gmelin in response to the presence of the predatory oyster drill Urosalpinx cinerea Say in Long Island Sound

The response of the eastern oyster C. virginica to the presence of the oyster drill Urosalpinx cinerea was examined from July to September 2011. Several aspects of oyster growth were measured, including wet weight, shell weight, and dorsal shell area for oysters collected near Groton, Connecticut (41.32036 N, −72.06330 W). Wet weight and shell weight growth were significantly higher in the presence of the predator U. cinerea, while tissue weight showed no difference from the control. The control group showed more shell area growth and a much lower ratio of shell weight growth to shell area growth. Differences in shell weight to area ratio indicated that C. virginica dramatically shifted from lateral shell growth to shell thickening in the presence of U. cinerea. This inducible defense has not been previously shown for C. virginica and could play an important role in the predator–prey interaction between these two species.

Shell ecophenotype in the blue mussel (Mytilus edulis) determines the spatial pattern in foraging behaviour of an oystercatcher (Haematopus ostralegus) population

When feeding on blue mussels (Mytilus edulis), oystercatchers (Haematopus ostralegus) either stab into the mollusc’s gaping valves or hammer through its dorsal or ventral shell. Whilst the selectivity of hammering and stabbing oystercatchers for specific prey morphologies has been well studied, the way in which the effects of environment on M. edulis morphology can in turn affect feeding methods of H. ostralegus is very poorly understood. Based on morphological analyses on randomly selected shells from three intertidal zones, this study failed to detect differences in morphology or distribution of dorsally and ventrally hammered shells but confirms the finding of previous authors that hammering oystercatchers select thinner mussels than stabbing birds. Additionally, we show that this difference in optimal prey morphology can lead to spatial patterns in oystercatcher feeding behaviour. Whilst at the low intertidal and higher mid intertidal zones, characterised by comparatively thick shells, most empty shells had apparently been stabbed, hammering was the dominant feeding behaviour at the lower mid intertidal zone, where shells were thinner. Preference of hammering birds for smaller mussels was not ubiquitous. Sagittal shell shape was predominantly influenced by allometric growth effects and had only minor effect on prey selection. All oystercatchers preferred less inflated mussels, with the degree of shell inflation gradually increasing with higher intertidal elevation. Our results illustrate the importance of small-scale patterns in prey ecophenotypes in determining the distribution and feeding dynamics of wading birds.

Resolving the evolutionary relationships of molluscs with phylogenomic tools

Molluscs (snails, octopuses, clams and their relatives) have a great disparity of body plans and, among the animals, only arthropods surpass them in species number. This diversity has made Mollusca one of the best-studied groups of animals, yet their evolutionary relationships remain poorly resolved. Open questions have important implications for the origin of Mollusca and for morphological evolution within the group. These questions include whether the shell-less, vermiform aplacophoran molluscs diverged before the origin of the shelled molluscs (Conchifera) or lost their shells secondarily. Monoplacophorans were not included in molecular studies until recently, when it was proposed that they constitute a clade named Serialia together with Polyplacophora (chitons), reflecting the serial repetition of body organs in both groups. Attempts to understand the early evolution of molluscs become even more complex when considering the large diversity of Cambrian fossils. These can have multiple dorsal shell plates and sclerites or can be shell-less but with a typical molluscan radula and serially repeated gills. To better resolve the relationships among molluscs, we generated transcriptome data for 15 species that, in combination with existing data, represent for the first time all major molluscan groups. We analysed multiple data sets containing up to 216,402 sites and 1,185 gene regions using multiple models and methods. Our results support the clade Aculifera, containing the three molluscan groups with spicules but without true shells, and they support the monophyly of Conchifera. Monoplacophora is not the sister group to other Conchifera but to Cephalopoda. Strong support is found for a clade that comprises Scaphopoda (tusk shells), Gastropoda and Bivalvia, with most analyses placing Scaphopoda and Gastropoda as sister groups. This well-resolved tree will constitute a framework for further studies of mollusc evolution, development and anatomy.

A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors.

Aplacophorans have long been argued to be basal molluscs. We present a molecular phylogeny, including the aplacophorans Neomeniomorpha (Solenogastres) and Chaetodermomorpha (Caudofoveata), which recovered instead the clade Aculifera (Aplacophora + Polyplacophora). Our relaxed Bayesian molecular clock estimates an Early Ordovician appearance of the aculiferan crown group consistent with the presence of chiton-like molluscs with seven or eight dorsal shell plates by the Late Cambrian (approx. 501-490 Ma). Molecular, embryological and palaeontological data indicate that aplacophorans, as well as chitons, evolved from a paraphyletic assemblage of chiton-like ancestors. The recovery of cephalopods as a sister group to aculiferans suggests that the plesiomorphic condition in molluscs might be a morphology similar to that found in monoplacophorans.

A Chiton Uses Aragonite Lenses to Form Images

Hundreds of ocelli are embedded in the dorsal shell plates of certain chitons. These ocelli each contain a pigment layer, retina, and lens, but it is unknown whether they provide chitons with spatial vision. It is also unclear whether chiton lenses are made from proteins, like nearly all biological lenses, or from some other material. Electron probe X-ray microanalysis and X-ray diffraction revealed that the chiton Acanthopleura granulata has the first aragonite lenses ever discovered. We found that these lenses allow A. granulata's ocelli to function as small camera eyes with an angular resolution of about 9°-12°. Animals responded to the sudden appearance of black, overhead circles with an angular size of 9°, but not to equivalent, uniform decreases in the downwelling irradiance. Our behavioral estimates of angular resolution were consistent with estimates derived from focal length and receptor spacing within the A. granulata eye. Behavioral trials further indicated that A. granulata's eyes provide the same angular resolution in both air and water. We propose that one of the two refractive indices of the birefringent chiton lens places a focused image on the retina in air, whereas the other does so in water.

Primitive soft-bodied cephalopods from the Cambrian

The exquisite preservation of soft-bodied animals in Burgess Shale-type deposits provides important clues into the early evolution of body plans that emerged during the Cambrian explosion. Until now, such deposits have remained silent regarding the early evolution of extant molluscan lineages-in particular the cephalopods. Nautiloids, traditionally considered basal within the cephalopods, are generally depicted as evolving from a creeping Cambrian ancestor whose dorsal shell afforded protection and buoyancy. Although nautiloid-like shells occur from the Late Cambrian onwards, the fossil record provides little constraint on this model, or indeed on the early evolution of cephalopods. Here, we reinterpret the problematic Middle Cambrian animal Nectocaris pteryx as a primitive (that is, stem-group), non-mineralized cephalopod, based on new material from the Burgess Shale. Together with Nectocaris, the problematic Lower Cambrian taxa Petalilium and (probably) Vetustovermis form a distinctive clade, Nectocarididae, characterized by an open axial cavity with paired gills, wide lateral fins, a single pair of long, prehensile tentacles, a pair of non-faceted eyes on short stalks, and a large, flexible anterior funnel. This clade extends the cephalopods' fossil record by over 30 million years, and indicates that primitive cephalopods lacked a mineralized shell, were hyperbenthic, and were presumably carnivorous. The presence of a funnel suggests that jet propulsion evolved in cephalopods before the acquisition of a shell. The explosive diversification of mineralized cephalopods in the Ordovician may have an understated Cambrian 'fuse'.

Electrophysiological evidence of mediolateral functional dichotomy in the rat nucleus accumbens during cocaine self‐administration II: phasic firing patterns

In the cocaine self-administering rat, individual nucleus accumbens (NAcc) neurons exhibit phasic changes in firing rate within minutes and/or seconds of lever presses (i.e. slow phasic and rapid phasic changes, respectively). To determine whether neurons that demonstrate these changes during self-administration sessions are differentially distributed in the NAcc, rats were implanted with jugular catheters and microwire arrays in different NAcc subregions (core, dorsal shell, ventromedial shell, ventrolateral shell, or rostral pole). Neural recording sessions were typically conducted on days 13-17 of cocaine self-administration (0.77 mg/kg per 0.2-mL infusion; fixed-ratio 1 schedule of reinforcement; 6-h daily sessions). Pre-press rapid phasic firing rate changes were greater in lateral accumbal (core and ventrolateral shell) than in medial accumbal (dorsal shell and rostral pole shell) subregions. Slow phasic pattern analysis revealed that reversal latencies of neurons that exhibited change + reversal patterns differed mediolaterally: medial NAcc neurons exhibited more early reversals and fewer progressive/late reversals than lateral NAcc neurons. Comparisons of firing patterns within individual neurons across time bases indicated that lateral NAcc pre-press rapid phasic increases were correlated with tonic increases. Tonic decreases were correlated with slow phasic patterns in individual medial NAcc neurons, indicative of greater pharmacological sensitivity of neurons in this region. On the other hand, the bias of the lateral NAcc towards increased pre-press rapid phasic activity, coupled with a greater prevalence of tonic increase firing, may reflect particular sensitivity of these neurons to excitatory afferent signaling and perhaps differential pharmacological influences on firing rates between regions.

Electrophysiological evidence of mediolateral functional dichotomy in the rat accumbens during cocaine self-administration: tonic firing patterns.

Given the increasing research emphasis on putative accumbal functional compartmentation, we sought to determine whether neurons that demonstrate changes in tonic firing rate during cocaine self-administration are differentially distributed across subregions of the NAcc. Rats were implanted with jugular catheters and microwire arrays targeting NAcc subregions (core, dorsal shell, ventromedial shell, ventrolateral shell and rostral pole shell). Recordings were obtained after acquisition of stable cocaine self-administration (0.77 mg/kg/0.2mL infusion; fixed-ratio 1 schedule of reinforcement; 6-h daily sessions). During the self-administration phase of the experiment, neurons demonstrated either: (i) tonic suppression (or decrease); (ii) tonic activation (or increase); or (iii) no tonic change in firing rate with respect to rates of firing during pre- and post-drug phases. Consistent with earlier observations, tonic decrease was the predominant firing pattern observed. Differences in the prevalence of tonic increase firing were observed between the core and the dorsal shell and dorsal shell-core border regions, with the latter two areas exhibiting a virtual absence of tonic increases. Tonic suppression was exhibited to a greater extent by the dorsal shell-core border region relative to the core. These differences could reflect distinct subregional afferent processing and/or differential sensitivity of subpopulations of NAcc neurons to cocaine. Ventrolateral shell firing topographies resembled those of core neurons. Taken together, these observations are consistent with an emerging body of literature that differentiates the accumbens mediolaterally and further advances the likelihood that distinct functions are subserved by NAcc subregions in appetitive processing.

Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny

Popov, L.E., Bassett, M.G., Holmer, L.E., Skovsted, C.B. & Zuykov, M.A. 2010: Earliest ontogeny of Early Palaeozoic Craniiformea: implications for brachiopod phylogeny. Lethaia, Vol. 43, pp. 323–333. Well preserved specimens of the Early Palaeozoic craniiform brachiopods Orthisocrania and Craniops retain clear evidence of a lecithotrophic larval stage, indicating the loss of planktotrophy early in their phylogeny. The size of the earliest mineralized dorsal shell was <100 μm across, and the well preserved shell structure in these fossil craniiforms allows their earliest ontogeny to be compared directly with that of living Novocrania, in which the first mineralized dorsal shell (metamorphic shell) is secreted only after settlement of the lecithotrophic larvae. Immediately outside this earliest shell (early post-metamorphic or brephic shell) and in the rest of the dorsal valve the primary layer in both fossil and living craniiforms has characteristic radially arranged laths, which are invariably lacking in the earliest dorsal shell. The ventral valve of the fossil specimens commonly preserves traces of an early attachment scar (cicatrix), which is equal in size to the dorsal metamorphic shell, and the brephic post-metamorphic ventral valve also has a primary shell with radially arranged laths. However, a primary shell with radial laths is completely lacking in the ventral valve of living Novocrania, indicating that heterochrony may have been involved in the origin of the encrusting mode of life in living craniids; the entire ventral valve of Recent craniids (with the possible exception of Neoancistrocrania) may correspond to the earliest attachment scar of some fossil taxa such as Orthisocrania. It is also probable that the unique absence of an inner mantle lobe as well as the absence of lobate cells in Novocrania could be the result of heterochronic changes. The dorsal valve of both fossil and living craniiforms has a marked outer growth ring, around 500 μm across, marking the transition to the adult, and a significant change in regime of shell secretion. The earliest craniiform attachment is considered to be homologous to the unique attachment structures described recently in polytoechioids (e.g. Antigonambonites) and other members of the strophomenate clade. However, unlike the craniiforms, polytoechioids and strophomenates all have planktotrophic larvae, and planktotrophy is most probably a plesiomorphic character for all Brachiopoda. □Brachiopoda, Craniiformea, Early Palaeozoic, ontogeny, phylogeny.

Turtles all the way down: loggerheads at the root of the chelonian tree

The recent discovery of the oldest and most primitive stem turtle, Odontochelys semitestacea (Li et al. 2009), from the lower Upper Triassic of China reopens the debate regarding the origin of the turtle shell, the paleoecology of basal turtles (terrestrial or aquatic), and the placement of turtles among amniotes. The debate surrounding the origin of the shell dates back to Cuvier (1800–1805) with two competing hypotheses. The de novo hypothesis, argues that the carapace (dorsal shell) formed from the outgrowth of intramembranous bone from the periost of the ribs and neurals, and this view is largely supported by developmental data on current turtles (Gilbert et al. 2001; Cebra-Thomas et al. 2005). The composite origin hypothesis argues that the shell formed largely by the fusion of overlying osteoderms with the underlying ribs, and this perspective has some histological support (Scheyer et al. 2008).

A novel role for dpp in the shaping of bivalve shells revealed in a conserved molluscan developmental program

During the molluscan evolution leading to the bivalves, the single dorsal shell was doubled. To elucidate the molecular developmental basis underlying this prominent morphological transition, we described the cell cleavage and expression patterns of three genes, brachyury, engrailed, and dpp in the Japanese spiny oyster Saccostrea kegaki, and examined the function of dpp in this species. The cleavage pattern of the S. kegaki embryo was nearly the same as the previously described pattern of other bivalve species, suggesting that the pattern itself is highly important for the establishment or the maintenance of the bivalve body plan. The expression pattern of a brachyury homolog in S. kegaki ( SkBra) was similar to the pattern in gastopods even at the single cell level despite the deep divergence of gastropods and bivalves. Engrailed and dpp were previously found to be expressed around the shell anlagen in gastropods. Like that of gastropods, an engrailed homolog in S. kegaki ( SkEn) was found to be expressed around the shell anlagen. However, the dpp homolog in S. kegaki ( SkDpp) was expressed only in the cells along the dorsal midline. ZfBMP4 treatment experiments revealed the importance of dpp in establishing the characteristic shape of the bivalve shell anlagen.

New data on Kimberella , the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications

Abstract The taphonomic varieties of over 800 specimens of Kimberella (collected from the Vendian rocks of the White Sea region) provide new evidence of the animal's anatomy such as: shell morphology, proboscis, mantle, possibly respiratory folds and possibly musculature, stomach and glands. Feeding tracks, crawling trails and, presumably, escape structures preserved along with the body imprint provide insights on the mode of locomotion and feeding of this animal. The shield-like dorsal shell reached up to 15 cm in length, 5–7 cm in width, and 3–4 cm in height. The shell was stiff but flexible. Evidence of dorso-ventral musculature and fine transverse ventral musculature suggests arrangement in a metameric pattern. Locomotion may have been by means of peristaltic waves, both within the sediment and over the surface of the sea floor, by means of a foot resembling that of monoplacophorans. Respiration may have been through a circumpedal folded strip (possibly an extension of the mantle). Feeding was accomplished by a retractable proboscis bearing terminal hook-like organs and provided with a pair of structures interpreted here as glands. Whilst feeding, Kimberella moved backwards. The structural complexity of Kimberella poses questions about the time of origin of the triploblastic metazoans.

The Characterization of Tortoise Shell and its Imitations

TORTOISE SHELL AND ITS IMITATIONS GEMS & GEMOLOGY SPRING 2006 he term “tortoise shell” generally refers to the carapacial (dorsal shell) and plastron (belly) plates of the hawksbill sea turtle (Eretmochelys imbricata). The use of tortoise shell dates at least to pre-dynastic Egypt (3500–3100 BC), from which period dishes, combs, bracelets, and the like are known (“Animal products...,” 2005). Tortoise shell objects were popular with both the ancient Greeks and wealthy citizens of ancient Rome (Bariand and Poirot, 1998). The commercial exploitation of this material in Europe began as early as the 15th century in Spain (“Natural plastics,” 2005), and many types of tortoise shell objects have been produced since then. These include furniture inlays, eyeglass frames, decorative boxes, rings, bracelets, and earrings (see, e.g., figure 1). In Japan, tortoise shell crafting, or bekko, has been an important industry since at least the 17th century, most of it centered in Nagasaki (Pedersen, 2004). Bekko objects such as hair ornaments are still being created today from stockpiled material. The vast majority of worked tortoise shell material comes from the shells of two species of sea turtles: the hawksbill and, more rarely, the green turtle (see box A). The shells of these turtles exhibit attractive patterns that normally consist of light to dark brown patches, though a homogeneous “cream”-colored variety called blond tortoise shell also exists. The popularity of tortoise shell during the 18th through early 20th centuries caused these animals to be hunted almost to extinction, leading to a near-worldwide ban on collection in the 1970s, as well as a ban on international trade in tortoise shell products (Spotila, 2004; box B). For the modern gemologist, tortoise shell has become a rather exotic material, for which a thorough gemological study is lacking.

Hox code in embryos of Chinese soft-shelled turtlePelodiscus sinensis correlates with the evolutionary innovation in the turtle

Turtles have the most unusual body plan of the amniotes, with a dorsal shell consisting of modified ribs. Because this morphological change in the ribs can be described as an axial-level specific alteration, the evolution of the turtle carapace should depend on changes in the Hox code. To identify turtle-specific changes in developmental patterns, we cloned several Hox genes from the Chinese soft-shelled turtle, Pelodiscus sinensis, examined their expression patterns during embryogenesis, and compared them with those of chicken and mouse embryos. We detected possibly turtle-specific derived traits in Hoxc-6 expression, which is restricted to the paraxial part of the embryo; in the expression of Hoxa-5 and Hoxb-5, the transcripts of which were detected only at the cervical level; and in Hoxc-8 and Hoxa-7 expression, which is shifted anteriorly relative to that of the other two amniote groups. From the known functions of the Hox orthologs in model animals, these P. sinensis-specific changes apparently correlate with specializations in the turtle-specific body plan.

Msx genes are expressed in the carapacial ridge of turtle shell: a study of the European pond turtle, Emys orbicularis.

The turtle shell forms by extensive ossification of dermis ventrally and dorsally. The carapacial ridge (CR) controls early dorsal shell formation and is thought to play a similar role in shell growth as the apical ectodermal ridge during limb development. However, the molecular mechanisms underlying carapace development are still unknown. Msx genes are involved in the development of limb mesenchyme and of various skeletal structures. In particular, precocious Msx expression is recorded in skeletal precursors that develop close to the ectoderm, such as vertebral spinous processes or skull. Here, we have studied the embryonic expression of Msx genes in the European pond turtle, Emys orbicularis. The overall Msx expression in head, limb, and trunk is similar to what is observed in other vertebrates. We have focused on the CR area and pre-skeletal shell condensations. The CR expresses Msx genes transiently, in a pattern similar to that of fgf10. In the future carapace domain, the dermis located dorsal to the spinal cord expresses Msx genes, as in other vertebrates, but we did not see expansion of this expression in the dermis located more laterally, on top of the dermomyotomes. In the ventral plastron, although the dermal osseous condensations form in the embryonic Msx-positive somatopleura, we did not observe enhanced Msx expression around these elements. These observations may indicate that common mechanisms participate in limb bud and CR early development, but that pre-differentiation steps differ between shell and other skeletal structures and involve other gene activities than that of Msx genes.

Lower Cretaceous ammonites from the Neuquén Basin, Argentina: the Hauterivian neocomitid genusHoplitocrioceras (Giovine, 1950)

Hoplitocrioceras is a poorly-known ammonite genus of late Early Hauterivian age that may be endemic to the Neuquén Basin, Argentina, where it occurs at a clearly defined horizon in the Agrio Formation. It is an unusual genus with uncoiling (crioceratitid) whorls that remain linked by a thick dorsal shell that drapes over the tips of sharp, upwardly-pointing ventro-lateral spines. It has been interpreted as either a necomitid or a crioceratitid, but is here regarded as one of the last members of the Family Neocomitidae. One new species is proposed: H. giovinei.

Theoretical morphology of hinge and shell form in Bivalvia: geometric constraints derived from space conflict between umbones

Geometric properties of the shells of 123 species of extant Bivalvia were analyzed from the viewpoint of theoretical morphology. The effects of shell form and the structure of ligament on the interumbonal space and the maximum shell opening received particular attention. The results of computer simulation and morphospace analysis indicate that possessing both prosogyrous shell form and an extended hinge without the parivincular ligament tends to cause space conflict between umbones or dorsal shell margins of right and left valves. To a large degree, a prosogyrous shell form with a long parivincular ligament helps shell opening without umbonal conflict, if the shell is flat enough to avoid the mutual interference between dorsal shell margins of closed valves. Extension of the ligament and plunging of the anterior part of the coiling axis into the ventral side provide enough space along the dorsal shell margins in which a parivincular ligament and its substrata are developed.

Suprachiasmatic pacemaker organization analyzed by viral transynaptic transport

The suprachiasmatic nucleus (SCN) of the hypothalamus, the principal circadian pacemaker, is a paired structure with two subdivisions, a ventral core receiving photic input and a dorsal shell receiving non-photic input. Rhythmicity is thought to be generated by individual SCN neurons which are coupled to achieve synchrony [D.K. Welsh, D.E. Logothetis, M. Meister, S.M. Reppert, Individual neurons dissociated from rat suprachiasmatic nucleus express independently phased circadian firing patterns, Neuron, 14 (1995) 697–706]. Normally, the core and shell, and the nuclei on each side, act in unison to transmit rhythmicity to effector systems. It is not known how coupling between neurons in the two subdivisions, and between the two SCNs, takes place. In the present study, we analyze the intrinsic, commissural, and efferent projections of the SCN using the swine herpesvirus (pseudorabies virus, PRV) as a tool for transynaptic analysis of circuits and small iontophoretic injections of the conventional tracer horseradish peroxidase (HRP) conjugated to fluorescein. We find that the core and shell each project through commissural efferents to homologous contralateral areas. The core projects densely to shell but we find little reciprocal innervation. The two subdivisions project to different hypothalamic areas, with the core projecting to the lateral subparaventricular zone and shell to the dorsomedial hypothalamic nucleus and medial subparaventricular zone. These data are the first demonstration that connections within the SCN, and from the SCN to effector regions, are topographically organized and lend insight into the flow of information through and out of the pacemaker.