Like a hole in the head: Development, evolutionary implications and diseases of the cranial foramina

Imaging in osteopetrosis

Imaging investigations are critical in the management of children with suspected and confirmed osteopetrosis. In severe cases, imaging can provide rapid confirmation of the diagnosis, whilst in milder cases, imaging findings may be the first or only indicators of the disease. Imaging can also identify major complications, including fractures and neurological compromise. We review the pathophysiological basis for the imaging findings in osteopetrosis, focusing on the impact of loss of various osteoclast functions leading to elevated bone density, hyperostosis, modelling abnormalities and bone fragility. We give an overview of the specific imaging findings, both skeletal and neuroradiological, in the spectrum of osteopetrotic disorders, including in the related entities of pyknodysostosis and dysosteosclerosis. We also explore potential radiological differential diagnoses.

Variations and Classification of Bony Septations of the Jugular Foramen: An Anatomic and Histologic Study With Application to Imaging and Surgery of the Skull Base

INTRODUCTION

Approaches to the jugular foramen can be challenging. Adding to this difficulty is the potential for a bony septation to occur in the foramen thus subdividing its contents. Although such bony septations in the jugular foramen are known, the anatomic details of these structures have not been studied well. Therefore, the present anatomic study was performed.

METHODS

One hundred adult human skulls (200 sides) underwent evaluation for the presence or absence of a bony septation within the jugular foramen. The source, morphology, and size were all documented. A classification scheme was developed to better describe the varied types of morphology of the jugular foramen bony septa. Select bony septations were submitted to histologic analysis using hematoxylin and eosin and Masson's trichrome.

RESULTS

Bony bridges were found on 8.5% of sides. These were bilateral in 3% of skulls. Eight (47%) were incomplete (type I). Nine sides (53%) were found to have completely articulated bony bridges (type II) and these had articulation between the bony processes as either touching (type IIa), joint-like (type IIb), or completely fused (type IIc) morphologies. Multiple septa were observed on 2.5% of sides and these were classified as type III septa. Histologically, both incomplete and complete septa were found to be composed of normal bone tissue.

CONCLUSIONS

Our study found significant variations regarding the bony septa of the jugular foramen. Knowledge of this anatomy can be useful for preoperative planning and intraoperative navigation with approaches to the skull base that target pathology of the jugular foramen.

Remodeling of the hyomandibular skeleton and facial nerve positioning during embryonic and postembryonic development of teleost fish

The vertebrate skeleton changes its shape during development through the activities of chondrocytes, osteoblasts and osteoclasts. Although much is known about the mechanisms for differentiation in these cells, it is less understood how they behave in a region-specific manner to acquire unique bone shapes. To address this question, we investigated the development of the hyomandibular (Hm) system in zebrafish. The Hm originates as cartilage carrying a single foramen (the Hm foramen), through which the facial (VII) nerve passes. We reveal that Schwann cells, which myelinate the VII nerve, regulate rearrangement of the chondrocytes to enlarge the Hm foramen. The Hm cartilage then becomes ossified in the perichondrium, where the marrow chondrocytes are replaced by adipocytes. Then, the bone matrix along the VII nerve is resorbed by osteoclasts, generating a gateway to the bone marrow. Subsequent movement of the VII nerve into the marrow, followed by deposition of new bone matrix, isolates the nerve from the jaw muscle insertion. Genetic ablation of osteoblasts and osteoclasts reveals specific roles of these cells during remodeling processes. Interestingly, the VII nerve relocation does not occur in medaka; instead, bone deposition distinct from those in zebrafish separates the VII nerve from the muscle insertion. Our results define novel mechanisms for skeletal remodeling, by which the bone shapes in a region- and species-specific manner.

Osteoclast activity sculpts craniofacial form to permit sensorineural patterning in the zebrafish skull

Efforts to understand the morphogenesis of complex craniofacial structures have largely focused on the role of chondrocytes and osteoblasts. Along with these bone-creating cells, bone-resorbing osteoclasts are critical in homeostasis of adult skeletal structures, but there is currently limited information on their role in the complex morphogenetic events of craniofacial development. Fundamental aspects of skull formation and general skeletal development are conserved from zebrafish to mammals. Using a cathepsinK reporter, we documented osteoclast location in the developing zebrafish skull over several weeks, from 5.18 mm to 9.6 mm standard length (approximately 15 to 34 days post fertilization). While broad distribution of osteoclasts is consistent across individuals, they are sparse and the exact locations vary among fish and across developmental time points. Interestingly, we observed osteoclasts concentrating at areas associated with neuromasts and their associated nerves, in particular the hyomandibular foramina and around the supraorbital lateral line. These are areas of active remodeling. In contrast, other areas of rapid bone growth, such as the osteogenic fronts of the frontal and parietal bones, show no particular concentration of osteoclasts, suggesting that they play a special role in shaping bone near neuromasts and nerves. In csf1ra mutants lacking functional osteoclasts, the morphology of the cranial bone was disrupted in both areas. The hyomandibular foramen is present in the initial cartilage template, but after the initiation of ossification, the diameter of the canal is significantly smaller in the absence of osteoclasts. The diameter of the supraorbital lateral line canals was also reduced in the mutants, as was the number of pores associated with neuromasts, which allow for the passage of associated nerves through the bone. Our findings define important and previously unappreciated roles for osteoclast activity in shaping craniofacial skeletal structures with a particular role in bone modeling around peripheral cranial nerves, providing a scaffold for wiring the sensioneural system during craniofacial development. This has important implications for the formation of the evolutionarily diverse lateral line system, as well understanding the mechanism of neurologic sequelae of congenital osteoclast dysfunction in human craniofacial development.

The Need for Head Space: Brachycephaly and Cerebrospinal Fluid Disorders

Brachycephalic dogs remain popular, despite the knowledge that this head conformation is associated with health problems, including airway compromise, ocular disorders, neurological disease, and other co-morbidities. There is increasing evidence that brachycephaly disrupts cerebrospinal fluid movement and absorption, predisposing ventriculomegaly, hydrocephalus, quadrigeminal cistern expansion, Chiari-like malformation, and syringomyelia. In this review, we focus on cerebrospinal fluid physiology and how this is impacted by brachycephaly, airorhynchy, and associated craniosynostosis.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape