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By examining development at the level of tissues and processes, rather than focusing on gene expression, we have formulated a general hypothesis to explain the dorso-ventral and anterior-posterior placement of paired appendage initiation sites in vertebrates. According to our model, the number and position of paired appendages are due to a commonality of embryonic tissue environments determined by the global interactions involving the two separated layers (somatic and visceral) of lateral plate mesoderm along the dorso-ventral and anterior-posterior axes of the embryo. We identify this distribution of developmental conditions, as modulated by the separation/contact of the two LPM layers and their interactions with somitic mesoderm, ectoderm, and endoderm as a dynamic developmental entity which we have termed the lateral mesodermal divide (LMD). Where the divide results in a certain tissue environment, fin bud initiation can occur. According to our hypothesis, the influence of the developing gut suppresses limb initiation along the midgut region and the ventral body wall owing to an "endodermal predominance." From an evolutionary perspective, the lack of gut regionalization in agnathans reflects the ancestral absence of these conditions, and the elaboration of the gut together with the concomitant changes to the LMD in the gnathostomes could have led to the origin of paired fins.

Inside the ‘cavernous sinus’ or ‘parasellar region’ the human internal carotid artery takes the shape of a siphon that is twisted and torqued in three dimensions and surrounded by a network of veins. The parasellar section of the internal carotid artery is of broad biological and medical interest, as its peculiar shape is associated with temperature regulation in the brain and correlated with the occurrence of vascular pathologies. The present study aims to provide anatomical descriptions and objective mathematical characterizations of the shape of the parasellar section of the internal carotid artery in human infants and its modifications during ontogeny. Three-dimensional (3D) computer models of the parasellar section of the internal carotid artery of infants were generated with a state-of-the-art 3D reconstruction method and analysed using both traditional morphometric methods and novel mathematical algorithms. We show that four constant, demarcated bends can be described along the infant parasellar section of the internal carotid artery, and we provide measurements of their angles. We further provide calculations of the curvature and torsion energy, and the total complexity of the 3D skeleton of the parasellar section of the internal carotid artery, and compare the complexity of this in infants and adults. Finally, we examine the relationship between shape parameters of the parasellar section of the internal carotid artery in infants, and the occurrence of intima cushions, and evaluate the reliability of subjective angle measurements for characterizing the complexity of the parasellar section of the internal carotid artery in infants. The results can serve as objective reference data for comparative studies and for medical imaging diagnostics. They also form the basis for a new hypothesis that explains the mechanisms responsible for the ontogenetic transformation in the shape of the parasellar section of the internal carotid artery.

Features in the endocranium, as revealed by computed tomography (CT) scans of largely complete mid-Pleistocene crania, have helped elucidate unexpected affinities in the genus Homo. Because of its extensive encrustations and deformations, it has been difficult to repeat such analyses with the Steinheim cranium. Here, we present several advances in the analysis of this Homo heidelbergensis cranium by applying filter algorithms and image editing techniques to its CT scan. First, we show how the encrustations have been removed electronically, revealing interesting peculiarities, particularly the many directions of the deformations. Second, we point out similarities and differences between the frontal and sphenoidal sinuses of the Steinheim, Petralona, and Broken Hill (Kabwe) crania. Third, we assess the extent of the endocranial deformations and, fourth, their implications for our estimation of the braincase volume. Anat Rec (Part B: New Anat) 273B:132–142, 2003. © 2003 Wiley-Liss, Inc.

Advances in imaging and modeling facilitate the calculation of biomechanical forces in biological specimens. These factors play a significant role during ontogenetic development of cichlid pharyngeal jaws, a key innovation responsible for one of the most prolific species diversifications in recent times. MicroCT imaging of radiopaque-stained vertebrate embryos were used to accurately capture the spatial relationships of the pharyngeal jaw apparatus in two cichlid species (Haplochromis elegans and Amatitlania nigrofasciata) for the purpose of creating a time series of developmental stages using finite element models, which can be used to assess the effects of biomechanical forces present in a system at multiple points of its ontogeny. Changes in muscle vector orientations, bite forces, force on the neurocranium where cartilage originates, and stress on upper pharyngeal jaws are analyzed in a comparative context. In addition, microCT scanning revealed the presence of previously unreported cement glands in A. nigrofasciata. The data obtained provide an underrepresented dimension of information on physical forces present in developmental processes and assist in interpreting the role of developmental dynamics in evolution.

Snake lungs show a remarkable diversity of organ asymmetries. The right lung is always fully developed, while the left lung is either absent, vestigial, or well-developed (but smaller than the right). A 'tracheal lung' is present in some taxa. These asymmetries are reflected in the pulmonary arteries. Lung asymmetry is known to appear at early stages of development in Thamnophis radix and Natrix natrix. Unfortunately, there is no developmental data on snakes with a well-developed or absent left lung. We examine the adult and developmental morphology of the lung and pulmonary arteries in the snakes Python curtus breitensteini, Pantherophis guttata guttata, Elaphe obsoleta spiloides, Calloselasma rhodostoma and Causus rhombeatus using gross dissection, MicroCT scanning and 3D reconstruction. We find that the right and tracheal lung develop similarly in these species. By contrast, the left lung either: (1) fails to develop; (2) elongates more slowly and aborts early without (2a) or with (2b) subsequent development of faveoli; (3) or develops normally. A right pulmonary artery always develops, but the left develops only if the left lung develops. No pulmonary artery develops in relation to the tracheal lung. We conclude that heterochrony in lung bud development contributes to lung asymmetry in several snake taxa. Secondly, the development of the pulmonary arteries is asymmetric at early stages, possibly because the splanchnic plexus fails to develop when the left lung is reduced. Finally, some changes in the topography of the pulmonary arteries are consequent on ontogenetic displacement of the heart down the body. Our findings show that the left-right asymmetry in the cardiorespiratory system of snakes is expressed early in development and may become phenotypically expressed through heterochronic shifts in growth, and changes in axial relations of organs and vessels. We propose a step-wise model for reduction of the left lung during snake evolution.

This study provides a quantitative analysis of the active movements of the chick embryo and of the contractions of the amnion over the entire developmental period of 21 days. Four types of embryo movements are distinguished. The motor activity of the embryo shows two characteristic peaks, with maximum contraction frequencies on the 12th and on the 16th day. In contrast, the amnion activity is higher at earlier stages and decreases as the body activity in- creases. The amnion activity is largely independent of the body activity. Illumination has a strong influence on embryo movements. It is shown that increases of light intensity affect the patterns of activity of both the embryo and the amnion. While the effect of light on the embryo can be inter- preted as being transmitted via the optic system, the mechanism of the amniotic response is unclear. The results suggest that the amnion itself may be sensitive to light. J. Exp. Zool. (Mol. Dev. Evol.) 291:186-194, 2001. © 2001 Wiley-Liss, Inc. Movements of the embryo represent an impor- tant epigenetic factor of vertebrate development and evolution (Muller, '89; Muller and Streicher, '89; Hall and Herring, '90; Streicher, '91). The em- bryonic and fetal movements indicate not only the capacity of cell contraction but also reflect the de- velopmental establishment of neuromuscular in- teraction (Drachman, '63; Murray and Smiles, '65; Bradley and Bekoff, '90, '92; Bekoff, '92). Correct formation of tissues and anatomical structures of the locomotor system depends on the establish- ment of specific patterns of embryo movements (Drachman and Sokoloff, '66). It is also a prereq- uisite for normal development of other structures. Disturbances of the motor activity can have dra- matic effects, primarily on the skeletal system and on functionally associated structures, such as muscles and nerves. Even minor deviations from the normal patterns of embryonic activity can have long lasting functional consequences in post- natal life (Sival et al., '92a,b, '93; Visser et al., '92; Sival, '93). Thus, next to the molecular and cellular development of its components, a causal understanding of dysfunction and dysplasia of the vertebrate locomotion system needs to include the development, and the morphogenetic role, of mo- tor activity in the embryo. A traditional model system for the study of the development of vertebrate motor activity is the chick embryo (Hamburger, '63; Hamburger and Balaban, '63; Hamburger et al., '65; Singer et al., '78; Provine, '80; Bekoff, '81, '82, '92; Bradley and Bekoff, '90, '92; Watson and Bekoff, '90; Cham- bers et al., '95;). However, many of these studies concentrated on the analysis of selected periods of development and did not distinguish between the different phases and patterns of embryo move- ments. Most importantly, no study has been car- ried out under strictly standardized conditions. However, the maintenance of standardized condi- tions is essential for any investigation of normal motor development, because slight variations of influential parameters, such as egg position, tem- perature, or illumination, can substantially influ- ence the observed motility. Although many of these effects are well known, they were seldom consid- ered in quantitative analyses. In the present study we established standard

Background Computer-based three-dimensional (3D) visualizations reconstructed from sectional images represent a valuable tool in biomedical research and medical diagnosis. Particularly with those imaging techniques that provide virtual sections, such as CT, MRI, and CLSM, 3D reconstructions have become routine. Reconstructions from physical sections, such as those used in histological preparations, have not experienced an equivalent breakthrough, due to inherent shortcomings in sectional preparation that impede automated image-processing and reconstruction. The increased use of molecular techniques in morphological research, however, generates an overwhelming amount of 3D molecular information, stored within series of physical sections. This valuable information can be fully appreciated and interpreted only through an adequate method of 3D visualization. Methods and results:In this paper we present a new method for a reliable and largely automated 3D reconstruction from physically sectioned material. The ‘EMAC‘ concept (External Marker-based Automatic Congruencing) successfully approaches the three major obstacles to automated 3D reconstruction from serial physical sections: misalignment, distortion, and staining variation. It utilizes the objectivity of external markers for realignment of the sectional images and for geometric correction of distortion. A self-adapting dynamic thresholding technique compensates for artifactual staining variation and automatically selects the desired object contours. Conclusions Implemented on a low-cost hardware platform, EMAC provides a fast and efficient tool that largely facilitates the use of computer-based 3D visualization for the analysis of complex structural, molecular, and genetic information in morphological research. Due to its conceptual versatility, EMAC can be easily adapted for a broad range of tasks, including all modern molecular-staining techniques, such as immunohistochemistry and in situ hybridization. Anat. Rec. 248:583-602, 1997. © 1997 Wiley-Liss, Inc.

Homeosis, the ectopic formation of a body part, is one of the key phenomena that prompted the identification of the essential selector genes controlling body organization. Shared elements of such homeotic genes exist in all studied animal classes, but homeotic transformations of the same order of magnitude as in insects, such as the duplication of the thorax in Drosophila mutants, have not been described in vertebrates. Here we investigate the capacity of retinoic acid to modify tail regeneration in amphibians. We show that retinoic acid causes the formation of an additional body segment in regenerating tails of Rana temporaria tadpoles. A second pelvic section, including vertebral elements, pelvic girdle elements and limb buds, forms at the mid-tail level. This is the first report of a homeotic duplication of a whole body segment in vertebrate axial regeneration.

Recent advances in developmental genetics of Drosophila have uncovered some of the key molecules involved in the positioning and outgrowth of the leg primordia. Although expression patterns of these molecules have been analyzed in several arthropod species, broad comparisons of mechanisms of limb development among arthropods remain somewhat speculative since no detailed studies of limb development exist for crustaceans, the postulated sister group of insects. As a basis for such comparisons, we analysed limb development in a primitive branchiopod crustacean, Triops longicaudatus. Adults have a series of similar limbs with eight branches or lobes that project from the main shaft. Phalloidin staining of developing limbs buds shows the distal epithelial ridge of the early limb bud exhibits eight folds that extend in a dorsal ventral (D/V) arc across the body. These initial folds subsequently form the eight lobes of the adult limb. This study demonstrates that, in a primitive crustacean, branched limbs do not arise via sequential splitting. Current models of limb development based on Drosophila do not provide a mechanism for establishing eight branches along the D/V axis of a segment. Although the events that position limbs on a body segment appear to be conserved between insects and crustaceans, mechanisms of limb branching may not.

We present a broadly applicable procedure for whole-mount imaging of antibody probes in embryonic tissues at microscopic resolutions based on combining a metal-based immunodetection scheme with x-ray microtomography (microCT). The method is generally accessible, relying on standard enzyme-conjugated secondary antibodies and other readily available reagents, and is demonstrated here with microCT visualizations of acetylated a-tubulin in the chick nervous system and of type II collagen in developing limbs. The tomographic images offer complete three-dimensional representations of molecular patterns obtained with immunostaining methods at the level of organ development, with added possibilities to quantify both spatial distributions and varying densities of gene products in situ. This imaging modality bridges a crucial gap in three-dimensional molecular imaging by combining the histological resolutions of confocal microscopy with a greater specimen size range than optical projection tomography, and thus enables a powerful new approach to long-standing issues of skeletogenic pattern formation in vertebrate limbs. Developmental Dynamics 240:2301–2308, 2011. V C 2011 Wiley-Liss, Inc.

Fibula reduction is a key feature of avian limb evolution. In a combined comparative and experimental approach the present study analyses the trends of fibula reduction in extant birds and their developmental basis. The study of 55 species of birds reveals four different types of tibiotarsus-to-fibula relationships. Extremely small fibulae are associated with two types of limb modification: (1) elongations of the limb primarily affect the tibiotarsus, increasing its length more than that of the fibula; (2) miniaturizations of the limb reduce both tibiotarsus and fibula length, but are reglarly associated with structural reductions of the distal parts of the fibula. True structrual reductions are distinguished from relative size reductions. The specific features of fibula reduction are analyzed through experimental mesenchyme excisions in chick limb buds. The methodical variation of experimental parameters resolves a long-standing controversy about the effects of mesenchyme reductions on the patterns of skeletal formation. Mesenchyme excisions are shown to have unequal effects on the two zeugopod bones, affecting the fibula to a greater degree than the tibiotarsus. Several of the features seen in birds with advanced fibula reductions are paralleled by the effects of mesenchyme reductions. The consequences of this differential susceptibility of the skeletal blastemata are discussed both in terms of pattern formation in limb development and in terms of its bearing on the patterns of evolutionary limb reduction. It is concluded that thresholds of cell number and blastema size in development constrain the patterns of phenotypic variation in avian limbs. © 1992 Wiley-Liss, Inc.

Crocodilians and birds are the extant representatives of a monophyletic taxon known as archosaurs. Their limbs are highly derived in terms of reduction in number of skeletal elements in both the carpus and the tarsus. It is necessary to have a detailed description of crocodilian limb ontogeny to address the evolutionary issue dealing with the origin and organization of the avian limb. In this paper, we present an analysis of the early development of the crocodilian limb skeleton. Contrasting with earlier observations, we redefine the number and composition of carpal, tarsal, and phalangeal elements. This ontogenetic information is then used to introduce a revision of the homologies of the skeletal elements in the crocodilian limb. Some invariances are pointed out in the developmental organization of tetrapod limbs and this evidence serves to readdress several issues concerning the evolution of the avian limb. We present further embryological data in support of the hypothesis that digits 2-3-4 are the components of the wing skeleton in birds. In general, our comparative survey indicates that the elements that appear late in ontogeny are the ones lost in phylogeny. By comparing turtle (primitive) limb development with crocodilian and bird development, we propose a hypothesis in which the derived skeletal patterns found in crocodilians and birds have originated by a heterochronic process of paedomorphosis.

We describe a new methodology for rapid 2D and 3D computer analysis and visualisation of gene expression and gene product pattern in the context of anatomy and tissue architecture. It is based on episcopic imaging of embryos and tissue samples, as they are physically sectioned, thereby producing inherently aligned digital image series and volume data sets, which immediately permit the generation of 3D computer representations. The technique uses resin as embedding medium, eosin for unspecific tissue staining, and colour reactions (beta-galactosidase/Xgal or BCIP/NBT) for specific labelling of gene activity and mRNA pattern. We tested the potential of the method for producing high-resolution volume data sets of adult human and porcine tissue samples and of specifically and unspecifically stained mouse, chick, quail, frog, and zebrafish embryos. The quality of the episcopic images resembles the quality of digital images of true histological sections with respect to resolution and contrast. Specifically labelled structures can be extracted using simple thresholding algorithms. Thus, the method is capable of quickly and precisely detecting molecular signals simultaneously with anatomical details and tissue architecture. It has no tissue restrictions and can be applied for analysis of human tissue samples as well as for analysis of all developmental stages of embryos of a wide variety of biomedically relevant species.

Computer generated three-dimensional stereolithographic models of middle Pleistocene skulls from Petralona and Broken Hill are described and compared. The anterior cranial fossae of these models are also compared with that of another middle Pleistocene skull, Arago 21. Stereolithographic modelling reproduces not only the outer surfaces of skulls, but also features within the substance of the bones, and details of the internal braincase. The skulls of Petralona and, to a somewhat lesser degree, Broken Hill are extremely pneumatized. Previously undescribed features associated with pneumatization are detailed, along with their possible functional significance, polarity, and potential for understanding hominid cranial variation. Petralona and Broken Hill also exhibit a dramatic suite of cerebral features that is probably related to extensive pneumatization of the skull, namely frontal lobes that are tilted and located behind rather than over the orbits, laterally flared temporal lobes, marked occipital projection, and basal location of the cerebellum. Comparison of the anterior cranial fossae of Petralona, Broken Hill, and Arago 21 suggests that external resemblance of skulls may not always correlate with endocranial similarity. We believe that stereolithographic reconstructions have the potential for helping to resolve difficult questions about the origins of Neanderthal and anatomically modern people.

The cavernous sinus is traditionally described as a single anatomical compartment that contains cranial nerves, blood vessels, and connective tissue. A detailed analysis of 45 infant and 4 fetal parasellar regions shows that this view must be modified. The spatial arrangement, the topographic relations, and the expansion of the adipose and connective tissue spaces were analysed and reconstructed 3-dimensionally on a computer. It is shown that 3 different anatomical compartments, which are strictly demarcated by connective tissue, compose the parasellar region of infants. Two represent intracranial continuations of extracranial tissue spaces. The 3rd compartment corresponds to the so-called ‘cavernous sinus’ of the adult. Each of the 3 compartments contains characteristic adipose tissue bodies. Because the cavernous sinus represents only one compartment of the area, we propose to use the term ‘parasellar region’ to designate the entire anatomical region on either side of the sella turcica.

Sympathetic nerves innervate targets in the orbit and the brain. They issue from the superior cervical ganglion and reach the parasellar region via the internal carotid nerve. Information on their further parasellar course and distribution is scant and contradictory. In this study the parasellar sympathetic pathways of 30 human infants and 6 human fetuses were investigated by microdissection and histologically. A common parasellar sympathetic trunk, which reunites all the nerve fibers emanating from the lateral and medial internal carotid plexus, is described as well as its further divisions. It was found that the posterior knee of the infant carotid siphon is free of large sympathetic nerve bundles. In addition a ganglion is described, which is situated in the parasellar adipose body. It is reached by nerve fibers coming from the parasellar sympathetic pathways. Fibers that issue from this ganglion join the periorbita and the orbital muscle of Müller. These anatomical facts are of immediate importance for preventing nerve damage during cavernous sinus surgery. Furthermore, the study improves the anatomical knowledge of the parasellar region and suggests a new concept for the innervation of the orbital muscle.

The development of the vertebrate limb is a hierarchical process of temporal and spatial tissue organization. We begin to understand its cellular and molecular properties, its links to the gene level, and, through the processes of pattern formation, its relationship to phenotypic patterns. In addition, however, a comprehensive theory of limb development must provide a mechanistic basis for evolutionary transformations of limb patterns and for the origin of individualized parts of the limb during vertebrate phylogeny. This paper examines the present concepts about limb development with regard to their capacity to accommodate phylogenetic transformations. Examples from our work on archosaur limb development and evolution are used to illustrate the patterns of transformation and the problems arising from an evolutionary perspective.

The “Archaeopteryx limb” of experimentally treated bird embryos has become a standard quotation in the growing literature on developmental factors in evolution. It has not only been claimed that an early manipulation of the chick limb produces a series of atavistic skeletal features, but the experiment is also frequently interpreted within a genome-centered concept of atavism. The present study provides a morphological, quantitative, and comparative analysis of the skeletal and muscular reactions to the classic barrier insertion experiment of Hampe. The main result of this operation, traditionally seen as a “full length fibula”, is shown to be a relative effect due to tibia shortening, while all the other ancestral skeletal features, which are usually pointed out as being provoked by the elongated fibula, do not appear. The experimentally generated fibula/tibia length ratio and distance, however, mimic the pattern in developing reptilian limbs and are seen to induce secondary effects in the muscular system that are reminiscent of archosaur reptiles. Similar muscle patterns are found as interspecific variations in several bird species. The revised view of the skeletal changes and the additional data on muscular effects allow for a renewed interpretation of the experiment, shifting the emphasis from atavisms to the role of heterochrony, developmental integration, and epigenetic constraint in the evolutionary modification of organismic structures.

The underlying theme of this study is the contribution of developmental mechanisms to the generation of morphological novelty in evolution. The syndesmosis tibiofibularis, an important structural and functional link between the two zeugopod bones of the bird hindlimb, is used as a model for evolutionary novelty. We analyze the structural, developmental and adaptive aspects of its origin in a combined descriptive, experimental, and comparative approach. The ontogeny of the syndesmosis in the chick embryo involves several developmental steps, including the formation of a separate cartilage rudiment that in turn stimulates the formation of an osseous crest on the tibia, which will eventually replace the cartilage element itself. Some of the epigenetic requirements for the formation of the cartilage element and the osseous crest are demonstrated by experimentally increasing the distance between the two zeugopod bones, an operation that results in the absence of both cartilage and crest. Although a syndesmosis tibiofibularis associated with an osseous crest on the tibiotarsus is unique to birds in extant vertebrates, the presence of a distinct crest at the corresponding location in theropod dinosaurs indicates that a syndesmosis also existed in this group of archosaurs. The results of the study suggest that in the case of the syndesmosis tibiofibularis phenotypic evolutionary novelty is based on a caenogenetic feature, i.e. a feature that initially arose in response to changing developmental conditions. In conclusion we propose a model for the stepwise evolutionary modification of the sauropsid hindlimb, integrating adaptive trends and developmental mechanisms that interactively determine the transformations of skeletal limb morphology. The syndesmosis tibiofibularis and the mechanisms of its formation are not only shown to have played a key-role in this process, but its presence in theropod dinosaurs also points towards the origin of birds.

Increasing surgical experience within the cavernous sinus region has generated increased interest in the region’s anatomy. Despite many attempts to understand the morphological complexity of the cavernous sinus, its description and interpretation are still controversial. Two main views oppose each other: classic textbooks of anatomy, but also recent studies (e.g. Harris and Rhoton [11], Bedford [1]), interpret the cavernous sinus as an intradural venous canal, which is more or less trabeculated by fibrous strands. Several other workers of the present century, however, interpret the cavernous sinus as a network of extradural veins (e.g. Taptas [21, 22], Bonnet [3], Parkinson [15,16, 17], a view which is not yet generally accepted. Other controversial problems of the area concern the relationship of the venous pathways with the structures that pass through the cavernous sinus area, like the internal carotid artery and the cranial nerves that run to the orbital cavity.

The paraclinoid area is investigated anatomically for possible microneurosurgical approaches to the C3 segment of the internal carotid artery and to structures in the vicinity of the anterior siphon knee. Removal of the anterior clinoid process reveals a tight connective tissue ring that fixes the internal carotid artery to the surrounding osseous structures at the point of its transdural passage. Transection of this fibrous ring opens a microsurgical pathway to the carotid C3 segment. The artery is surrounded by a loose connective tissue layer that allows blunt preparation along the C3 segment, without compromising the cranial nerves and without damaging venous compartments of the cavernous sinus. This approach provides neurosurgical access to paraclinoidal aneurysms, to partly intracavernous aneurysms, and to carotid-ophthalmic aneurysms, allowing control of the proximal aneurysm neck and of the parent artery itself. In cases of tumors involving the medial sphenoid ridge, the apex of the orbit, or the cavernous sinus, the pericarotid connective tissue can serve as a guide layer for access along the internal carotid artery.

Cranial nerves III, IV, V, and VI are embedded in the deep layer of the lateral wall of the cavernous sinus, and are supplied by arteries which approach the nerves from medially. The arteries arise from two main stems of the internal carotid artery (ICA): the meningohypophyseal trunk (TMH), and the inferior lateral trunk (ILT). The ILT points towards the lateral wall of the cavernous sinus, where it supplies the cranial nerves which converge to the superior orbital fissure. The TMH points dorsally and supplies cranial nerve VI within Dorello’s canal, the Gasserian ganglion, and the dural entrance area around cranial nerve IV. Branches and anastomoses from and to the middle meningeal and ophthalmic artery system, as well as from and to the internal maxillary artery system, may in part replace the arterial supply pattern of the TMH and the ILT. In the sella and clivus area there exist anastomoses with the contralateral intracavernous branches of the ICA. Many of the anastomotic vessels observed in this study can be explained as developmental remnants of ancestral arterial patterns of the skull base.

Background Cephalopods are a highly derived class of molluscs that adapted their body plan to a more active and predatory lifestyle. One intriguing adaptation is the modification of the ventral foot to form a bilaterally symmetric arm crown, which constitutes a true morphological novelty in evolution. In addition, this structure shows many diversifications within the class of cephalopods and therefore offers an interesting opportunity to study the molecular underpinnings of the emergence of phenotypic novelties and their diversification. Here we use the sepiolid Euprymna scolopes as a model to study the formation and differentiation of the decabrachian arm crown, which consists of four pairs of sessile arms and one pair of retractile tentacles. We provide a detailed description of arm crown formation in order to understand the basic morphology and the developmental dynamics of this structure. Results We show that the morphological formation of the cephalopod appendages occurs during distinct phases, including outgrowth, elongation, and tissue differentiation. Early outgrowth is characterized by uniform cell proliferation, while the elongation of the appendages initiates tissue differentiation. The latter progresses in a gradient from proximal to distal, whereas cell proliferation becomes restricted to the distal-most end of the arm. Differences in the formation of arms and tentacles exist, with the tentacles showing an expedite growth rate and higher complexity at younger stages. Conclusion The early outgrowth and differentiation of the E. scolopes arm crown shows similarities to the related, yet derived cephalopod Octopus vulgaris. Parallels in the growth and differentiation of appendages seem to exist throughout the animal kingdom, raising the question of whether these similarities reflect a recruitment of similar molecular patterning pathways. Electronic supplementary material The online version of this article (doi:10.1186/s12983-016-0175-8) contains supplementary material, which is available to authorized users.