Embryonic and early fetal period development and morphogenesis of human craniovertebral junction.

Several studies have focused on the cartilaginous, articular, and ligamentous development of the craniovertebral joint (CVJ), but there are no unifying criteria regarding the origin and morphogenetic timetable of the structures that make up the CVJ. In our study, serial sections of 53 human embryonic (n = 27) and fetal (n = 26) specimens from O'Rahilly stages 17-23 and 9-13 weeks, respectively, have been analyzed. Our results demonstrate that the chondrification of the pars basioccipitalis and exoccipitalis becomes observable at stage 19, and all future bones in the CVJ are in their cartilaginous form except for the future odontoid process. In addition, two chondrification centers appear for the body of the axis. From stage 21, the apical, alar, and transverse atlantal ligaments begin to acquire a ligamentous structure and the odontoid process initiates its chondrogenic phase. Stage 22 witnesses the first signs of the articular cavities of the atlanto-occipital joint, and by stage 23 all joints have cavities except for the transverse-odontoid joint, which will wait until week 9. In week 10, the ossification of the basilar part of the occipital bone begins, followed by the rest of the structures except for the odontoid process, which will start at week 13, thus completing the osteogenesis of all bones in the CVJ. The results of this study could help in establishing the anatomical basis of the normally functioning CVJ and for detecting its related pathologies, abnormalities, and malformations.

Hypoplastic occipital condyle and third occipital condyle: review of their dysembryology.

Disruption or embryologic derailment of the normal bony architecture of the craniovertebral junction (CVJ) may result in symptoms. As studies of the embryology and pathology of hypoplasia of the occipital condyles and third occipital condyles are lacking in the literature, the present review was performed. Standard search engines were accessed and queried for publications regarding hypoplastic occipital condyles and third occipital condyles. The literature supports the notion that occipital condyle hypoplasia and a third occipital condyle are due to malformation or persistence of the proatlas, respectively. The Pax-1 gene is most likely involved in this process. Clinically, condylar hypoplasia may narrow the foramen magnum and lead to lateral medullary compression. Additionally, this maldevelopment can result in transient vertebral artery compression secondary to posterior subluxation of the occiput. Third occipital condyles have been associated with cervical canal stenosis, hypoplasia of the dens, transverse ligament laxity, and atlanto-axial instability causing acute and chronic spinal cord compression. Treatment goals are focused on craniovertebral stability. A better understanding of the embryology and pathology related to CVJ anomalies is useful to the clinician treating patients presenting with these entities.

Development and growth of the craniocervical junction with special reference to topographical relationship between the occipital basion, the anterior arch of atlas, and the odontoid process of axis: A study using human fetuses.

The embryonic occipital bone and odontoid process of the axis are attached and connected by the notochord, but become separated in later development and growth. With special attention to the process of separation, we examined sagittal sections of the craniocervical junction in 18 human fetuses at 8-16 weeks and 22 fetuses at 31-37 weeks. At 8-9 weeks, the anterior arch of atlas was always seen overriding the occipital basal part. The odontoid process was close to the occipital with or without a transient joint cavity until 16 weeks. Near term, the top of the odontoid process was usually higher than the anterior arch, but the former was sometimes (7 of 22) at a level almost equal to or lower than the latter. The apical ligament was evident in a few specimens (5 of 22). A distance between the occipital basion and odontoid process was sometimes less than 1.5 mm (8 of 22) or less than half the thickness of the arch (10 of 22). A transient joint cavity between the basion and odontoid process was often (10 of 22). In three fetuses near term, the atlanto-occipital joint cavity was continuous with the median atlanto-axial joint cavity, and the anterior arch was overriding the occipital basal part. Therefore, rather than stage or age, individual differences were evident in the topographical relationship between the three bony elements at the craniocervical junction. An understanding of the embryology and normal development will aid in the correct interpretation of radiologic images of the pediatric cervical spine.

Arcuate Foramen: Anatomy, Embryology, Nomenclature, Pathology, and Surgical Considerations.

BACKGROUND: The arcuate foramen is an anatomic variant that is thought to arise from ossification of the posterior atlanto-occipital membrane. Owing to potential entrapment of the vertebral artery segment that traverses the foramen, vertebrobasilar ischemia may occur, and the person may experience vertigo, headache, or neck pain. METHODS: We reviewed the literature regarding anatomy (both human and comparative), embryology, nomenclature, pathology, and surgery of the arcuate foramen. RESULTS: Surgically, the presence of an arcuate foramen is important when placing screws into lateral masses of the atlas. In these cases, the screws can damage the V3 segment of the vertebral artery and/or the suboccipital nerve. CONCLUSIONS: It is important to review the current literature on the arcuate foramen to further understand its morphology and clinical relevance.

Bilateral type 1 proatlantal arteries with absence of vertebral arteries.

The persistent proatlantal artery is a well-described communication between the carotid and vertebrobasilar system. However, persistence of bilateral proatlantal arteries is exceptionally rare. Although usually noted as an incidental finding, the presence of a proatlantal artery, particularly when bilateral, may result in unusual symptoms or may have implications for therapy. We report a case of bilateral proatlantal arteries, describe their embryology, and consider potential clinical implications of this finding.

Craniovertebral anomalies.

We have tried to clarify this confusing area by demonstrating the common relationships of these abnormalities. The development of the craniovertebral junction was present in order to understand the formation of the anomalies discussed. The radiologic lines and measurements that have been described are actually to measure the degree of compromise of the functional size of the foramen magnum. This mechanical compromise, either from direct neural compression and/or from a secondary vascular impairment (arterial or venous), leads to the signs and symptoms of cervicomedullary compression.

[Structure and histo-topography of meniscoid structures of the atlanto-occipital and atlanto-axial joints]. / Stroenie i gistotopografiia meninskoidnykh struktur atlantozatylochnogo i atlantoosevykh sustavov.

Anatomical preparation, Pirogov's saw-cuts and serial histotopographic sections, made across the atlantooccipital and atlantoaxial joints in the sagittal, frontal and horizontal planes in 35 corpses have been used in reconstructing and studying the meniscoid structures in the area of the joints mentioned. The meniscoids are composed of fine fibrillar connective tissue with a large amount of fat and vessels. The meniscoid has a triangle form, its base consisting of collagenous fibers, gradually transfers into the articular capsule, and its apex is directed into the articular cavity. The meniscoid, surrounding the tooth of the axial vertebra, at the sides passes into the medial parts of the atlantoocipital and lateral atlantoaxial joints, forming a single meniscoid complex of the joints studied. A tight connection is stated between the meniscoids and the perineural formations of the I and II cervical nerves and with the vertebral vessels.

Prenatal development of the composite occipito-atlanto-axial synovial joint cavity in the dog.

In the dog, the synovial cavities of the atlanto-occipital and atlanto-axial joints communicated to form a single large composite joint cavity. The prenatal development of this composite occipito-atlanto-axial joint cavity was studied by examining 26 serially sectioned dog embryos and fetuses that ranged in size from 19 to 68 mm crown-rump length, and were between 30 and 42 days of gestational age. In the composite occipito-atlanto-axial joint, trilaminar interzones developed at 19-22 mm (30-31 days), joint cavities opened at 27-32 mm (33-34 days), and the atlanto-occipital and atlanto-axial cavities first communicated at 48 mm (37 days).

The human vertebral column at the end of the embryonic period proper. 2. The occipitocervical region.

The present investigation of the cervical region of the vertebral column at eight post-ovulatory weeks is the first such study based on precise reconstructions of staged embryos. At the end of the embryonic period proper, a typical vertebra is a U-shaped piece of cartilage characterized by spina bifida occulta. The notochord ascends through the centra and leaves the dens to enter the basal plate of the skull. The median column of the axis comprises three parts (designated X, Y, Z) which persist well into the fetal period. They are related to the first, second and third cervical nerves, respectively. Part X may project into the foramen magnum and form an occipito-axial joint. Part Z appears to be the centrum of the axis. The articular columns of the cervical vertebrae are twofold, as in the adult: an anterior (atlanto-occipital and atlanto-axial) and a posterior (from the lower aspect of the axis downwards). Alar and transverse ligaments are present. Cavitation is not found in the embryonic period in either the atlanto-occipital or zygapophysial joints, and is generally not present in the median atlanto-axial joint either. Most of the transverse processes exhibit anterior and posterior tubercles. An 'intertubercular lamella' may or may not be present, i.e. the foramina transversaria are being formed around the vertebral artery. The spinal ganglia are generally partly in the vertebral canal and partly on the neural arches, medial to the articular processes. During the fetal period, the articular processes shift to a coronal position and this alteration appears to be associated with a corresponding change in the location of the spinal ganglia.

Occipitocervical segmentation in staged human embryos.

Serial sections of 108 human embryos from stage 11 to stage 23 were investigated, and 33 reconstructions were prepared. The existence of 4 occipital somites was confirmed. The important developmental distinction between axial (central) and lateral components obtains in the occipital as well as in the vertebral region. The lateral occipital components begin to show dense areas as the cervical region is approached. The lateral occipital and vertebral components arise in registration with the initial sclerotomes. In both the occipital and the vertebral region the related nerves and intersegmental arteries traverse the loose areas of the sclerotomes. The axial occipital region is not segmented, whereas the cervical components develop from perinotochordal loose areas. Three complete centra (known as XYZ) develop in the atlanto-axial region, although they are related to only 2 1/2 sclerotomes and only 2 neural arches. The height of the XYZ complex equals that of 3 centra elsewhere, and not 2 1/2, as previously maintained. The experimental findings in the occipitocervical region of the chick embryo show both similarities to, as well as differences from, the data for the human embryo. A scheme showing the early development of the entire vertebral column is included.