Development of the back

Segmentation of Paraxial Mesenchyme

Epithelial epiblast cells that ingress through the lateral aspect of the primitive node and the rostral primitive streak (see Fig. 10.3 ) become committed to a somitic lineage as they undergo an epithelial to mesenchymal transition. The position of ingression of the epiblast informs the specific destination of the cells. Those ingressing through the lateral portion of Hensen's node form the medial halves of the somites, whereas those ingressing through the primitive streak, approximately 200 μm caudal to the node, form the lateral halves of the somites. The two somite halves do not appear to intermingle. After passing through the streak the mesoblastic cells retain contact with epiblast and hypoblast basal laminae both during migration and after reaching the lateral sides of the notochord. Here the cell population is referred to as paraxial mesenchyme. The unsegmented mesenchymal population (presomitic mesenchyme), also termed segmental plate, subsequently undergoes a mesenchymal to epithelial transition to form somites in a process called somitogenesis. The first pair of somites in human embryos develops at stage 9 (about 26 postfertilization days; see Fig. 23.3 ) and 38–39 pairs of somites are present at stage 14 ( ).

Somitogenesis is an autonomous process. Somites will form from cultured presomitic mesenchyme with or without the presence of neural tube tissue or primitive node tissue. Transcriptome profiling of human paraxial mesenchyme, presomitic and mature somite cells, derived from stage 13–14 human embryos, has identified some of the regulatory pathways in the human and highlighted specific differences from equivalent mouse embryo somitic stages ( ). Myogenic, osteogenic and chondrogenic cells have been grown in 3D culture from both human embryonic stem cells (hESCs) and human induced pluripotential stem cells (hiPSCs), cultured according to specific human profiling information for somitogenesis ( , ).

Bilateral segmentation of the paraxial presomitic mesoblastic populations, which divide into discrete epithelial spheres, occurs as a sequential process along the craniocaudal axis. In avian embryos, a pair of somites is formed every 90 minutes until the full number is obtained. The molecular pathway for this synchronous segmentation has been termed the segmentation clock and has been identified as a conserved process in vertebrates from fish to mammals. It is based on the rhythmic production of messenger RNAs (mRNAs) for the transcription of genes related to Notch, a large transmembrane receptor, and a number of other factors, including members of the Wnt and fibroblast growth factor (FGF) signalling pathways.

Intrinsically coordinated pulses of mRNA expression appear as a wave within the presomitic mesoblast as each somite forms. As new cells enter the paraxial mesenchyme caudally, they begin phases of upregulation of the cycling genes, followed by downregulation of these genes. During each cycle, the most cranial presomitic mesoblast will segment and undergo mesenchymal to epithelial transformation to form the next somite. Experimental evidence from chick embryos shows that newly formed paraxial mesoblast cells undergo 12 such cycles before they finally form a somite, the period from ingression through the primitive streak to segmentation into a somite taking approximately 18 hours ( ). The somite number and the rate of somite formation varies between vertebrate species, taking 2 hours in mouse and 5 hours in human embryos ( ). Those vertebrates with elongated bodies and many somites form somites more rapidly relative to their developmental rate than those with shorter bodies, a finding that supports the concept that somite number is controlled, in part, by species-specific cyclical properties of the presomitic mesoblast ( , ).

The final determination of somitic boundary formation has not yet been fully elucidated but seems to require a periodic repression of the Notch pathway genes. The caudal presomitic mesoblastic cells are thought to be maintained in an immature state by their production of FGF8, and become competent for segmentation when FGF8 levels drop below a certain threshold. For an overview of vertebrate segmentation and its clinical implications, see and .

Somite Development

Five main stages can be identified in somitogenesis and somite development ( Figs 18.1 – 18.2 ). A local portion of the most cranial paraxial mesoblast undergoes compaction. The compacted cells undergo a mesenchymal to epithelial transformation, resulting in an epithelial sphere of cells that surrounds free somitocoele cells. This stage marks segmentation of the paraxial mesenchyme and somite formation. Shortly after the somite boundaries have been defined, the mesenchymal sclerotome is formed by a region-specific epithelial to mesenchymal transition of the ventral and ventromedial walls of the somite. As the embryo enlarges, the sclerotomal populations on each side become contiguous with the notochord and the neural tube. The rest of the dorsal lateral somitic epithelium remains as the epithelial plate of the somite, also termed the dermomyotome, a proliferative epithelium that will give rise to (nearly) all of the striated muscles of the body. Formation of the epaxial myotome begins. Segmentation of the paraxial mesoblast, mesenchymal to epithelial transformation to form epithelial somites and the resultant somite developmental processes (epithelial to mesenchymal transformation to form the sclerotome) all occur in a craniocaudal progression caudal to the otic vesicle from stage 9.

Cells of the spherical epithelial somite are polarized. Their apical region contains Golgi apparatus, actin and α-actinin close to the luminal surface, while their basal surface, corresponding to the outer circumference of the somite, rests on a basal lamina that contains collagen, laminin and fibronectin. Basal, filopodia-like processes pass through this basal lamina to contact the basal laminae of the neural tube medially, notochord ventromedially and surface ectoderm dorsolaterally. Dorsoventral patterning of the somites is dependent on the activity of Sonic Hedgehog (SHH) from the notochord/neural tube floor plate, Wnt1 and Wnt3a from the neural tube roof plate, and Wnt6 from the overlying ectoderm. A role of somitic filopodial processes in signal transduction is likely for Wnt signalling from the surface ectoderm ( ).

The diverse sclerotomal cell populations have been studied mostly in the chick embryo, where the fates of the cells have been tracked ( ) ( Figs 18.3 – 18.6 ). The main mass of the sclerotome is termed the central sclerotome, the portion close to the notochord is termed the ventral sclerotome, and the portions adjacent to the dorsomedial and ventrolateral dermomyotomal margins are termed the dorsal sclerotome and lateral sclerotome, respectively. The central sclerotome remains close to the dermomyotome and will give rise to the pedicles and ventral parts of the neural arches, and the proximal ribs. The ventral sclerotomal cells that laterally abut the notochord, proliferate and fuse with their contralateral counterparts to form an axial cell population within the extracellular matrix of the perinotochordal space, now termed the perinotochordal sheath. The dorsal sclerotomal cells develop relatively late, invade the space between the surface ectoderm and growing neural tube, and form the dorsal part of the neural arches. The lateral sclerotomal cells give rise to distal parts of the ribs ( ). Generally, there is a dorsolateral expansion of the whole sclerotome rather than the medial migration of a population of sclerotomal cells ( ), as can be seen in Figs 18.3 – 18.5 . Sclerotomal cells also give rise to the meninges surrounding the spinal cord, local tendons and ligaments, and endothelial cells of blood vessels ( ). The somitocoele cells, which remain mesenchymal throughout somite formation, give rise to the vertebral joints, intervertebral discs and the proximal ribs ( ).

The dermomyotome is a proliferative rectangular epithelial cell sheet that releases cells from its four borders ( Fig. 18.7 ). Proliferation at the dorsomedial edge produces muscle precursor cells that elongate along the cranial to caudal extent of the dermomyotome underneath its ventral (apical) surface as they move laterally. With a slight delay, cells similarly elongate from the cranial and caudal edges of the dermomyotome. The cells that are produced from these three edges are termed the epaxial myotome and will give rise to skeletal muscle dorsal to the vertebrae, i.e. the epaxial (back) musculature. Cells produced by the ventrolateral edge of the dermomyotome are termed the hypaxial myotome. Cells from occipital and cervical somites migrate ventrally, giving rise to the intrinsic muscles of the tongue and respiratory diaphragm respectively. Cells arising adjacent to the limb buds migrate laterally to enter them ( Ch. 19 ). At interlimb levels, the ventrolateral edge of the epithelial dermomyotome extends into the body wall and gives rise to the intercostal and abdominal muscles. Dermomyotomal cells that do not form myotomal muscle fibres contribute to the dermis and subcutis of the back ( ).

It was once thought that the somite gave rise to segmental portions of the dermis of the skin, as well as bone and muscle. However, it is now clear that the somitic contribution to the skin from the epithelial plate is limited to de-epithelialization of dermomyotomal cells over the epaxial muscles alone, a much smaller distribution than the segmental portion of skin usually implied by the term dermatome. The concept that an embryological dermatome, derived from the somite, produces all of the dermis of the skin is no longer tenable.

The regularity of somite formation provides criteria for staging embryos. The staging scheme proposed by noted that morphogenetic events occur in successive somites in a craniocaudal progression. The somite most recently formed from the unsegmented paraxial mesenchyme was designated as stage I, the next most recent as stage II, and so on. When the embryo has formed an additional somite, the stages of the previously formed somites increase by one Roman numeral. According to this scheme, in the chick embryo, compaction occurs at stage 0, epithelialization at stage I, formation of mesenchymal sclerotome cells from stage V, and myotome formation from stage VI. Early migration of the ventrolateral lip of the dermomyotome and production of myotome cells are still occurring at stage X.

Development of Sclerotomes

Sclerotomal cell populations form from the ventral half of the epithelial somite. An intrasegmental boundary (fissure or cleft, sometimes termed von Ebner's fissure), initially filled with extracellular matrix and a few cells, appears within the sclerotome and divides it into loosely packed cranial and densely packed caudal halves. The dermomyotome covers both half-sclerotomes dorsally. The bilateral sclerotomal cell populations of each segment migrate towards the notochord and surround it to form the perinotochordal sheath. They undergo a matrix-mediated interaction with the notochord, differentiating chondrogenetically to form the cartilaginous precursor of the vertebral centrum. The perinotochordal sheath starts to express type II collagen, and this is believed to mark a chondrogenic fate in those mesenchyme cells that contact it. Each vertebra is formed by the combination of much of the caudal half of one bilateral pair of sclerotomes with much of the cranial half of the next caudal pair of sclerotomes, their fusion around the notochord producing the blastemal centrum of the vertebra ( Figs 18.8 – 18.10 ). The mesenchyme adjoining the intrasegmental sclerotomic fissure now increases greatly in density to form a well-defined perichordal disc that intervenes between the centra of two adjacent vertebrae and is the future anulus fibrosus of the intervertebral symphysis (‘disc’) (see below).

The basic pattern of a typical vertebra is initiated by this recombination of caudal and cranial sclerotome halves (resegmentation) (see Fig. 18.9 ; Fig. 18.11 ), followed by differential growth and sculpturing of the sclerotomal mesenchyme that encases the notochord and neural tube. The centrum encloses the notochord and lies ventral to the neural tube. Sclerotomal differentiation starts in human embryos at about stage 10, when 10 somite pairs are present. The loosely packed cranial and densely packed caudal halves of the sclerotome can be seen at stage 15, with the neural processes extending dorsolaterally to the spinal ganglia ( ). The neural arch consists of paired bilateral pedicles (ventrolaterally) and laminae (dorsolaterally) that coalesce in the midline dorsal to the neural tube to form the spinous process. This is seen initially in the mid-thoracic region by postfertilization week 9. On each side, three further processes project cranially, caudally and laterally from the junction of the pedicle and laminae. The cranial and caudal projections are the blastemal articular processes (zygapophyses) that become contiguous with reciprocal processes of adjacent vertebrae; their junctional zones mark the future zygapophysial or facet joints. The lateral projections are the true vertebral transverse processes. Bilateral costal processes (ribs) grow anterolaterally from the ventral part of the pedicles (i.e. near the centrum) from the neighbouring perichordal disc, and, at most thoracic levels, with accessions from the next adjacent caudal pedicles. The costal processes expand to meet the tips of the transverse processes. Transverse processes are first seen in lower cervical and thoracic vertebrae during stage 18 (see Fig. 18.6 ). The definitive vertebral body is compound, formed from a median centrum (derived from the cells of the perinotochordal sheath), and bilaterally from the expanded pedicle ends (derived from the migrating sclerotomal populations) (see Fig. 18.11 ). These portions of the vertebral body fuse at the neurocentral synchondroses.

The segmental nature of the vertebrae is promoted by the notochord and neural floor plate, which induce the ventral elements of the vertebrae and repress dorsal structures such as the spinous processes. Experimental excision of the notochord in early chick embryos results in fusion of the centra and formation of a cartilaginous plate ventral to the neural tube. Dorsal segmentation is influenced by the spinal ganglia: experimental removal of the ganglia results in fusion of the neural arches and the formation of a uniform cartilaginous plate dorsal to the neural tube. Vertebrae are specified as to region very early in development. If a group of thoracic somites is transplanted to the cervical region, ribs will still develop. In marked contrast, the sclerotome is restricted and the myotome will produce muscle characteristic of its new location.

Development of vertebrae

The initial movements of sclerotomal cells round the neural tube and the expression of type II collagen signal the blastemal stage of vertebral development (see Fig. 18.11 ). In human embryos, vertebral bodies consist of loose mesenchyme until stage 17, and intervertebral discs are composed of dense mesenchyme continuous with the neural processes. Both mesenchymal populations show cartilage differentiation from stage 18. Each centrum chondrifies from one cartilage anlage. Each half of a neural arch is chondrified from a centre, starting in its base and extending dorsally into the laminae and ventrally into the pedicles, to meet, expand and blend with the centrum. There are 33 or 34 cartilaginous vertebrae by stage 23, but the spinous processes have not yet developed and so the overall appearance is of total spina bifida occulta. Fusion of the spines starts in the thoracic region at postfertilization week 9 and is seen in the sacral region by postfertilization week 15. For details of neural arch closure at all vertebral levels see . The transverse and articular processes are chondrified in continuity with the neural arches. Intervening zones of mesenchyme that do not become cartilage mark the sites of the facet joints and the complex of costovertebral joints, within which synovial cavities later appear.

A typical vertebra is ossified from three primary centres, one in each half-vertebral arch and one in the centrum (see Fig. 46.34 ). Centres in arches appear at the roots of the transverse processes and ossification spreads backwards into laminae and spines, forwards into pedicles and posterolateral parts of the body, laterally into transverse processes, and upwards and downwards into articular processes. Conventionally, centres in vertebral arches are said to appear first in upper cervical vertebrae and then in successively more distal vertebrae, reaching lower lumbar levels by about postmenstrual week 12. However, a radiographic study of unsexed human fetuses found a pattern that differed from such a simple craniocaudal sequence. Centres first appeared in the lower cervical/upper thoracic region, quickly followed by others in the upper cervical region. After a short interval, a third group appeared in the lower thoracolumbar region and remaining centres then appeared, spreading regularly and rapidly in craniocaudal directions ( ).

The major part of the vertebral body, the centrum, ossifies from a primary centre dorsal to the notochord. Centra are occasionally ossified from bilateral centres that may fail to unite. Suppression of one of these produces a cuneiform vertebra (hemivertebra), one cause of lateral spinal curvature (scoliosis). At birth and during the early postnatal years the centrum is connected to each half-neural arch by a synchondrosis or neurocentral joint. In thoracic vertebrae, costal facets on the bodies are posterior to neurocentral joints.

During the first year, the arches unite first in the lumbar region and then throughout the thoracic and cervical regions. In typical upper cervical vertebrae, centra unite with arches during year 3, but union is not complete in lower lumbar vertebrae until year 6. The upper and lower surfaces of the bodies and apices of the transverse and spinous processes are cartilaginous until puberty, at which time five secondary centres appear: one in the apex of each transverse and spinous process and two anular epiphyses (ring apophyses) for the circumferential parts of the upper and lower surfaces of the vertebral body. Costal articular facets are extensions of these anular epiphyses and they fuse with the rest of the bone at about 25 years. There are two secondary centres in bifid cervical spinous processes. Exceptions to this pattern of ossification are described in the appropriate subsections in Chapter 46 .

Occipitocervical junction

In humans, the occipitocervical (craniovertebral, spinomedullary) junction between the head and neck is placed at the boundary between the fourth and fifth somites ( , , ). At stage 12 it is identified by the presence of hypoglossal nerve rootlets ( Fig. 18.12A ) and at stages 14–15 it lies between the hypoglossal rootlets and the first spinal ganglion ( Fig. 18.12B –C ). In avian embryos, where all embryonic stages may be obtained experimentally, the occipitocervical boundary occurs within the fifth somite ( ). The onset of avian sclerotome formation in occipital and upper cervical somites is initiated simultaneously in multiple somites and occurs later than at thoracolumbar levels, in contrast to the serial cranial to caudal progression of somite maturation in the trunk ( ).

The segmental pattern present in the development of the somites can be seen rostrally in the developing skull base. Occipital somites are rostral to the first cervical nerve (see Fig 17.7 ). The first somite, defined by its position caudal to the vagal neural crest or ganglion, can be seen between stages 9–12. Mesenchymal condensations equivalent to the centra of occipital somites 2, 3 and 4 become apparent and form the basiocciput. Occipital sclerotomes 3 and 4 are the most distinct at stage 14, by which time the first three sclerotomes have fused. The hypoglossal rootlets pass through the less dense portion of occipital sclerotome 4, accompanied by the hypoglossal artery. Occipital sclerotome 4 forms an incomplete centrum axially and exoccipital elements laterally. Exoccipital elements are regarded as corresponding to neural arches and they form the rim of the foramen magnum. The occipital condyles develop from the cranial part of sclerotome 5, derived from the first cervical somite. The centra of occipital segments 1–4 are fused by stage 15 in the human embryo and form the basiocciput. The dorsocranial boundary of the foramen magnum is formed by stage 23 ( ).

In the occipitocervical junctional region, the centra formed from sclerotomes 5, 6 and 7 have a different fate from those more caudally placed, whereas the lateral portions of these sclerotomes generally develop similarly to those of lower ones. In a study of occipitocervical segmentation in human embryos, Müller and O'Rahilly designated the three complete rostral centra that develop in the atlanto-axial region X, Y and Z ( Fig. 18.13 ; , ). They noted that the height of the XYZ complex is equal to that of three centra elsewhere. X is on the level of sclerotome 5, and Y and Z are in line with sclerotome 6 and with the less dense portion of sclerotome 7. During stage 17 a temporary intervertebral disc appears peripherally between Y and Z. It begins to disappear in stage 21, although remnants may be found in the adult. No disc develops between X and Y. The origin of the anterior arch of the atlas is unclear: it is evident at stages 21–23 at the level of X or, sometimes, between X and Y. The posterior arch of the atlas arises from the dense area of sclerotome 5 at the level of X. The XYZ complex belongs to the axis, which means that the atlas does not incorporate a part of the central column ( ). The posterior arch of the axis arises from the dense area of sclerotome 6 and is at the level of Y and Z, particularly Z. XYZ correspond to the three parts of the median column of the axis, where X represents the tip of the dens, Y represents the base of the dens, and Z represents the centrum of the axis. The latter differs from other cervical vertebrae in that it is thicker and square-shaped. A review of the development and associated anomalies of this region is given in .

The bony elements of the occipitocervical junction may be thought of as a central pillar (basiocciput and dens of the axis) surrounded by two rings (the rim of the foramen magnum and occipital condyles superiorly, and the anterior and posterior arches of the atlas inferiorly). Bony anomalies involving this embryologically complex region may produce altered, unstable anatomical relationships that result in the compression of underlying neural and/or vascular structures and may also compromise cerebrospinal fluid dynamics ( , , ). The development of the cervical spine, particularly of the upper cervical vertebrae, is closely related to the development of the basiocciput and exocciput, and anomalous development will affect both regions. For example, abnormal prolapse of the dens into the foramen magnum, a condition termed basilar invagination, has been attributed variously to hypoplasia of the basiocciput, occipital condyles or atlas; deficiencies in the bony arches of the atlas with spreading of the lateral masses; or atlanto-occipital assimilation ( ). Malfusions of the caudal portion of occipital sclerotome 4 and the cranial portion of cervical sclerotome 1 may produce defects of the occipital condyles. The pro-atlas, a transient bony structure derived from the fourth occipital sclerotome, usually fuses with the three upper occipital sclerotomes to form the occipital bone and the dorsal part of the foramen magnum. Very rarely, remnants of the pro-atlas persist into adult life: several malformations and anomalies of the most caudal of the occipital sclerotomes have been attributed to failures of pro-atlas segmentation ( , ).

Chiari malformation type I (CM-I) is defined radiographically as a displacement of the cerebellar tonsils of 5 mm or more below the foramen magnum in young adults. Classic CM-I is thought to be a congenital anomaly, in which the normally developing hindbrain is compressed within a hypoplastic posterior cranial fossa, forcing the cerebellar tonsils down through the foramen magnum. Patients with classic CM-I have a characteristically shallow posterior cranial fossa, particularly below Twining's line (a line joining the tuberculum sellae and internal occipital protuberance). Basioccipital shortness means that the foramen magnum is constricted transversely; the suggested pathogenesis is premature stenosis of the basi-exoccipital and exosupra-occipital synchondroses ( ). Tonsillar herniation is aetiologically heterogeneous and there are many causes that do not appear to be related to skull-base hypoplasia; CM-I can occur in association with disorders that appear to be unrelated to skull-base hypoplasia and may be acquired ( ).

Most defects of the atlas do not contribute to abnormal occipitocervical anomalies and are not associated with basilar invagination. Abnormalities of the axis are usually concerned with fusion of the dens with the centrum of the second cervical sclerotomes. Using the classification of the three complete centra that develop in the atlanto-axial region as X, Y and Z ( , ), failure of fusion of X with the YZ complex produces an ossiculum terminale, a dissociated apical odontoid epiphysis. Failure of fusion of the XY complex with Z at the dentocentral synchondrosis, or maintenance of the transitory intervertebral disc at this point, produces an os odontoideum, thought to be induced by excessive movement at the time of ossification of the dens ( ). Hypoplasia and aplasia of the X and Y centra, and aplasia of the Z centrum, will all lead to a reduced size of the dens: there are widely differing views about whether this will lead to atlanto-axial instability. For further details of the development of the human craniovertebral joints and associated ligaments, see .

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