The accessory neural arch: development, morphology, and systematic distribution

The accessory neural arch is an oddly distributed character present in several non-acanthomorph teleostean taxa. Its homology was often implied but never satisfyingly tested. In this study, we attended this pending problem. We analyzed the morphology, development, and systematic distribution of the accessory neural arch in teleosts. Using a comprehensive taxon sampling of cleared and stained specimens, we evaluated if the accessory neural arch fulfils existing homology criteria. We then combined these data with recent genetic phylogenies and ancestral character state estimation to reconstruct the evolutionary history of the accessory neural arch. While its gross morphology and development fit homology criteria, results from ancestral character state estimations suggest multiple independent evolutions within teleosts. Although the accessory neural arch cannot be homologous between several teleostean taxa, the concept of parallelism may explain the presence of such a similar character in a variety of non-acanthomorph teleostean taxa.

A morphological characterization of the lumbar neural arch in females and males with degenerative spondylolisthesis

Background Although Degenerative Spondylolisthesis (DS) is a common osseous dysfunction, very few studies have examined the bony morphology of lumbar the neural arch in the population afflicted with DS. Therefore, this study aimed to characterize the neural arch (NA) morphology along the entire lumbar spine in individuals with degenerative spondylolisthesis (DS) and compare them to healthy controls. Methods One hundred CTs from a database of 500 lumbar CTs of spondylolisthesis were selected. We excluded vertebral fractures, non-L4-L5 slips, previous surgeries, vertebral spondyloarthropathies, and scoliosis. Scans were divided into a study group of 50 individuals with single-level DS (grades 1–2) at L4–5 (25 males and 25 females), and an age-sex matched control group of 50 individuals. Linear and angular measurements from all lumbar segments included: vertebral canals, intervertebral foramens, pedicles, and articular facets. Results Compared with the controls, all individuals with DS had greater pedicle dimensions in the lower lumbar segments (∆ = 1 mm–2.14 mm) and shorter intervertebral foramens in all the lumbar segments (∆range:1.85 mm–3.94 mm). In DS females, the lower lumbar facets were mostly wider (∆ = 1.73–2.86 mm) and more sagittally-oriented (∆10°) than the controls. Greater prevalence of grade-3 facet arthrosis was found only in the DS population (DS = 40–90%,controls = 16.7–66.7%). In DS males, degenerated facets were observed along the entire lumbar spine (L1-S1), whereas, in DS females, the facets were observed mainly in the lower lumbar segments (L4-S1). Individuals with DS have shorter intervertebral foramens and greater pedicle dimensions compared with controls. Conclusions Females with DS have wider articular facets, more sagittally-oriented facets, and excessively degenerated facets than the controls. This unique NA shape may further clarify DS’s pathophysiology and explain its greater prevalence in females compared to males.

Fifth lumbar vertebrae solitary osteochondroma arising from the neural arch, a case report

Background Osteochondromas are a benign outgrowth of bone and cartilage and one of the most common bone tumors that usually occur in long bones, with only 1–4% being located in the spine, more frequently in the cervical and upper dorsal region, and rarely the lumbar spine. Here, we report a case of lumbar spine (L5) osteochondroma arising from the neural arch. Case presentation A 30-year-old man presenting with a solid painless mass at the lower lumbar region. No neurological symptoms. Radiological examinations revealed an exophytic lesion in the fifth lumbar articular process, and the spinous process appears to be a solitary osteochondroma. Lumbar spine magnetic resonance imaging showed a bony lesion covered by a 2-mm-thick cartilaginous cap. Surgical en bloc resection of the mass was performed, and the histopathological examination confirmed the diagnosis of osteochondroma. No evidence of recurrence at the end of 4-year follow-up. Conclusion Osteochondromas are benign tumors rarely present in the spine; diagnosis can be made by the typical appearance of the cartilaginous cap over the mass in the magnetic resonance imaging. Surgical excision is the best management option.

Regionalization, constraints, and the ancestral ossification patterns in the vertebral column of amniotes

The development of the vertebral column has been studied extensively in modern amniotes, yet many aspects of its evolutionary history remain enigmatic. Here we expand the existing data on four major vertebral developmental patterns in amniotes based on exceptionally well-preserved specimens of the early Permian mesosaurid reptile Stereosternum: (i) centrum ossification, (ii) neural arch ossification, (iii) neural arch fusion, and (iv) neurocentral fusion. We retrace the evolutionary history of each pattern and reconstruct the ancestral condition in amniotes. Despite 300 million years of evolutionary history, vertebral development patterns show a surprisingly stability in amniotes since their common ancestor. We propose that this conservatism may be linked to constraints posed by underlying developmental processes across amniotes. However, we also point out that mammals and birds differ more strongly from the ancestral condition than other clades, which might be linked to a stronger regionalization of the column in these two clades.

Quantitative assessment of the vertebral pneumaticity in an anhanguerid pterosaur using micro-CT scanning

Research on the postcranial skeletal pneumaticity in pterosaurs is common in the literature, but most studies present only qualitative assessments. When quantitative, they are done on isolated bones. Here, we estimate the Air Space Proportion (ASP) obtained from micro-CT scans of the sequence from the sixth cervical to the fourth dorsal vertebra of an anhanguerine pterosaur to understand how pneumaticity is distributed in these bones. Pneumatisation of the vertebrae varied between 68 and 72% of their total volume. The neural arch showed higher ASP in all vertebrae. Anhanguerine vertebral ASP was generally higher than in sauropod vertebrae but lower than in most extant birds. The ASP observed here is lower than that calculated for the appendicular skeleton of other anhanguerian pterosaurs, indicating the potential existence of variation between axial and appendicular pneumatisation. The results point to a pattern in the distribution of the air space, which shows an increase in the area occupied by the trabecular bone in the craniocaudal direction of the vertebral series and, in each vertebra, an increase of the thickness of the trabeculae in the zygapophyses. This indicates that the distribution of pneumatic diverticula in anhanguerine vertebrae may not be associated with stochastic patterns.

Diversity and Evolution of Mineralized Skeletal Tissues in Chondrichthyans

The diversity of skeletal tissues in extant vertebrates includes mineralized and unmineralized structures made of bone, cartilage, or tissues of intermediate nature. This variability, together with the diverse nature of skeletal tissues in fossil species question the origin of skeletonization in early vertebrates. In particular, the study of skeletal tissues in cartilaginous fishes is currently mostly restrained to tessellated cartilage, a derived form of mineralized cartilage that evolved at the origin of this group. In this work, we describe the architectural and histological diversity of neural arch mineralization in cartilaginous fishes. The observed variations in the architecture include tessellated cartilage, with or without more massive sites of mineralization, and continuously mineralized neural arches devoid of tesserae. The histology of these various architectures always includes globular mineralization that takes place in the cartilaginous matrix. In many instances, the mineralized structures also include a fibrous component that seems to emerge from the perichondrium and they may display intermediate features, ranging from partly cartilaginous to mostly fibrous matrix, similar to fibrocartilage. Among these perichondrial mineralized tissues is also found, in few species, a lamellar arrangement of the mineralized extracellular matrix. The evolution of the mineralized tissues in cartilaginous fishes is discussed in light of current knowledge of their phylogenetic relationships.

New mid-cervical vertebral morphotype of Spinosauridae from the Kem Kem Group of Morocco

An unusual mid-cervical vertebra belonging to a large spinosaurid from the Cenomanian Kem Kem Group of Morocco is described. It is compared to the characteristic morphology of each reconstructed cervical position in Spinosaurus aegyptiacus, based on a recent composite reconstruction that incorporates most previously referred material from this unit. Rather than conforming to any of the previously identified cervical positions in its morphology, the specimen displays a unique combination of mid-cervical characters, with the relatively compact centrum suggesting a position as C4, and the form of the neural arch laminae suggesting a position as C5 or C6. Furthermore, it displays two characters that are previously unknown in spinosaurid mid-cervicals from the Kem Kem Group: a rounded hypapophyseal tuberosity that is not continuous with a ventral keel, and a moderately developed, dorsally oriented epipophysis that does not overhang the postzygapophysis posteriorly. The diagnostic value of positionally variable cervical vertebral characters in spinosaurid systematics is discussed. Although limited, the new data could lend support to the controversial hypothesis that two spinosaurid taxa are represented in the Kem Kem Group.