CC BY
0
i.m. krasnov et al., 2024
vertebrogenesis: what the discoveries of the 21st century added into the classical understanding of the embryogenesis of the spine in general and of the craniovertebral zone in particular. scientific review
I.M. Krasnov1, M.A. Mushkin2, A.Yu. Mushkin2,3
1St. Petersburg State University, St. Petersburg, Russia 2Pavlov First St. Petersburg State Medical University, St. Petersburg, Russia 3St. Petersburg Research Institute of Phthisiopulmonology, St. Petersburg, Russia
Classical concepts of embryogenesis of the spine, supplemented by modern data on the role of extracellular matrix factors, specific cell adhesion molecules, signaling molecules, and Hox and Pax genes are presented. They allow us to get closer to understanding the molecular genetic cascades possibly regulating the development of the axial skeleton. Particular attention is paid to the data on the influence of these factors on the morphogenesis of the craniovertebral zone and its defects, primarily associated with segmentation disorders. Key Words: vertebrogenesis, embryogenesis of the spine, craniovertebral zone.
Please cite this paper as: Krasnov IM, Mushkin MA, Mushkin AYu. Vertebrogenesis: what the discoveries of the 21st century added into the classical understanding of the embryogenesis of the spine in general and of the craniovertebral zone in particular. Scientific review. Russian Journal of Spine Surgery (Kh-irurgiya Pozvonochnika). 2024;21(2):81—89. In Russian. DOI: http://dx.doi.org/10.14531/ss2024.281-89.
Understanding the mechanisms of spinal embryogenesis is required to explain not only the diverse anatomical variations of its development, but also possible abnormal processes associated with it [1-4]. Among all the parts of the human skeleton, the craniovertebral junction (CVJ) has the most complicated range of functions that provide head movements, protection of medulla oblongata and cranial cervical spinal cord. Such unique functionality requires specific development of the structures that form the CVJ - the occipital bone, the C1 and C2 vertebrae and ligaments related to them.
The objective is to provide a present-day idea of embryonic development of the spine and possible ways of its improvement in experimental vertebro-logy, and possibly - in practical one.
The Present-day Idea of Embryogenesis of the Spine
NB! Preparation of this review didn't include the task of describing all the stag-
es of embryogenesis; therefore, we will focus on stages that are of crucial significance for vertebrogenesis.
Normal embryonic development of the spine includes 6 sequential, however, overlapping phases.
1. Phase of gastrulation andformation of somites and notochord. During the first two weeks after fertilization, a human embryo goes through several stages of cell division and restructuring and forms a one-layer embryoblast that divides into epi- and hypoblast, and then transforms into a three-layer embryo. Knowing the early phases of vertebrogenesis includes a fundamental fact that the embryonic cells acquire spatial ventral-dorsal and craniocaudal polarity; its markers are the occurrence of the primitive streak, Hensen's node and the elongating noto-chord surrounded on both sides by the somitic mesoderm that is the base for the subsequent development of axial skeleton [1-3, 5-9].
2. Condensation of somitic mesoderm, formation of somites. The first somites
81
(condensates of mesodermal cells) develop at the beginning of the third week of embryogenesis in the future cervical area. There are up to five cranial somites by the start of the closure of the developing neural tube; the subsequent ones are growing in the rostral-caudal direction and are accompanied by a simultaneous wave of neurulation. It is thought that the axial differentiation of vertebrae is determined by the various expression of different homeotic genes (Hox genes, homeobox genes) at the appropriate level (the so-called "Hox profile") and by the transcription factors controlled by them (please see below). As a result, in the week 4 of pregnancy, the paraxial mesoderm adjacent to the neural tube and segments of the notochord develops in 42 somites: 4 occipital, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 8-10 coccygeal [3, 10-15].
3. Development of sclerotome and der-momyotome. Differentiation of somite cells regulated by the notochord and the ventral plate of neural tube floor results
in the formation of its segments - medial (sclerotome that gives rise to the axial skeleton), external (dermatome that forms the dermis and subcutaneous tissues of the back), and internal (myotome that determines the development of myocytes of the dorsal muscles of the trunk). These segments express different molecular markers: sclerotome - Pax-1 and Pax-9, dermomyotome - Pax-3 and Pax-7, as well as Myo-D and others.
The events of embryogenesis phases 1-3 are provided in Fig. 1.
The subsequent development of somites is usually divided into 3 subsequent phases [3, 12,16].
4. Development of membranous somite and resegmentation. At week 5 of pregnancy, all somites, with the exception of several cranial ones, undergo a reorganization of their rostral-caudal borders, i.e. resegmentation. Upon that, each sclerotome divides into cranial and caudal halves expressing different cellular and molecular markers. Dense caudal part of the sclerotome combines with the less dense cranial part of the underlying one; in this manner a vertebral body is formed that does not correspond to the initial segmentation (this process is called a "metameric shift"). A cleft between new vertebrae filled with mesenchyme (von Ebner's fissure) is transformed into an intervertebral disc, with a single formation, nucleus pulposus, that originates entirely from the notochord in its center. After the metameric shift, spinal nerves that initially emerged from the neural tube are located between the precartilagi-nous vertebral bodies; they get through the center of sclerotome and innervate segmental myotomes. Segmental arteries are developed near the center of each precartilaginous vertebral body.
The CVJ develops from the fourth occipital sclerotome and the first and second cervical sclerotomes. The fourth occipital sclerotome, or so-called transitional sclerotome of the proatlas, originates from the fusion of the caudal half of the fourth somite and the rostral half of the fifth somite (in Russian literature, "proatlas" usually refers to the bone between the occipital bone and the C1
vertebra). A break line is developed at the segmentation border that is the final cell-level separation of the skull and spine. Thus, the condyles and condylar processes, the foramen magnum (FM) ring, part of the C2 dens, as well as the apical, cruciate, and alar ligaments raise from the fourth occipital sclero-tome (somites 4 and 5). The first cervical sclerotome (somites 5 and 6) gives rise to all components of the atlas and most part of the C2 dens; the second cervical sclerotome (somites 6 and 7)
gives rise to the other parts of the second cervical vertebra. As a result, seven cervical vertebrae develop from eight cervical somites (Fig. 2) [3, 12, 16-19].
During resegmentation, somites 4-7 give raise to the proatlas (PA), the first (C1) and second (C2) cervical sclero-tomes, and the intervertebral rim zones (IRZ1_3) that, in turn, form the following anatomical structures:
- PA - clivus of the occipital bone (C), its anterior tubercle (AT), basion (B), occipital condyles (OC), edges of the FM
82
i.m. krasnov et al vertebrogenesis: what the discoveries of the 21st century added into the classical understanding of the embryogenesis
(EFM), opisthion (O), and the apex of the C2 odontoid process (AOP);
- C1 - basal segment of the C2 odontoid process (BSOP), as well as the anterior arch of the C1 atlas (AAA), its lateral masses (LMs), superior and inferior articular surfaces (SAS and IAS), and the posterior arch (PA).
- C2 - the epistropheus body (AB), its superior and inferior surfaces (SS and IS);
- IRZj - the superior synchondrosis of the odontoid process (SSOP);
- IRZ2 - the inferior synchondrosis of the odontoid process (ISOP);
- IRZ3 - the first intervertebral disc (ID).
5. Chondrification phase. The development of chondrification centers starts in the week 6 of embryogenesis in the cervicothoracic region and spreads cranially and caudally; it is induced by the substances secreted by the noto-chord and neural tube. As a rule, three paired chondrification centers develop in each vertebra: ventral (vertebral body) ones surround the notochord ventral to the neural tube; dorsal ones form the posterior arches and spi-nous processes; the third (intermediate) ones develop between the dorsal and ventral pairs giving raise to the transverse processes and costal arches. Ventral centers develop earlier than the dorsal ones, and the intervertebral disc ring is originated from cells that gather around the notochord, while its physaliferous cells develop in the centrally located gelatinous nucleus. The anterior and posterior longitudinal ligaments (ALL and PLL) are developed at the same stage from the mesenchymal cells surrounding the cartilaginous vertebrae [3, 16].
6. Ossification phase. Vertebral ossification starts in the week 8-9 of embryogenesis and continues after birth. Ossification of the vertebral body centers occurs slightly earlier than the ossification of the dorsal arch; it starts in the thoracolumbar region (T10-L1), and quickly spreads cranially to T2 and cau-dally to L4; more cranial and caudal vertebrae are involved later. Ossification of the dorsal arches from C1 to L1, in turn, starts simultaneously and continues in the craniocaudal direction. All ossifica-
tion centers are visible by 14 weeks of pregnancy [3, 16, 20].
Cartilaginous endplates are differentiated cranially and caudally to the ventral ossification centers; on their periphery, between the intervertebral disc and the growing ossification center of the vertebra, there is an ellipsoid ring apophysis with relatively stronger lateral and ventral parts. Secondary ossification centers in apophyses and apexes of the spinous and transverse processes develop only by 11-14 life years; they merge by 15-16 years. Complete fusion of the primary and secondary ossification centers occurs in late adolescence.
In summary, the fundamental periods of vertebrogenesis can be represented as follows (Table 1).
Contribution of the 21st Century Discoveries to the Understanding of Congenital and Hereditary Vertebral Abnormalities
Progress in embryology and molecular genetics has helped to come closer to understanding the mechanisms of encoding genetic programs and their dependence on microenvironmental factors. Information on biochemical gradients, morphogens, mechanical forces, cell migration, and cell adhesion is the base for a general concept of the embryogenesis of congenital spine abnormalities including the craniovertebral junction zone [12, 19].
Molecular basis of morphogenesis. Histo- and organogenesis is based on decreased cell pluripotency and differentiation of cell lines; these processes provide positional information and biochemical specificity of cells starting from their first divisions [21]. Upon that, morphogenesis is determined by a limited set of division processes, as well as changes in shape, composition, migration, and growth of cells. The extracellular matrix (ECM) is a highly organized structure that includes collagen, proteoglycans and other macromolecules; it is considered to have an effect on these processes.
According to the idea of the migration system, at the differentiation stage, interaction with it results in the cells acquir-
ing morphofunctional polarity, ability to diffuse into the corresponding areas of the embryo and to form specific associations with the help of cell adhesion molecules [22-25].
In addition to recognizing ECM signals, migrating cells also communicate via long-distance signaling molecules (morphogens) that regulate differential gene expression and control cell specification in tissues due a concentration gradient. Morphogen molecules are released from a local but active source, aggregate together and/or with other molecules, and move through the extracellular medium due to limited diffusion. Gradient type is determined by the release rate, the diffusibility of the morphogen, and the kinetics of its clearance in target tissues. By interacting with heparan sulfate proteoglycans (HSPG) and other extracellular proteins, morphogens bind to the cell surface having an effect on their transport and reception. Linear transfer of morphogen can have an effect on intracellular signaling pathways resulting in stepwise activation of the transcription (reading) effect; the latter is involved in the target gene response to the concentration and duration of morphogen exposure. Thus, target genes are regulated not only by the morpho-gen signaling pathway, but also by preexisting factors and cross-talk. Feedback mechanisms equilibrate fluctuations in morphogen generation, have an effect on the interpretation of signals, and scale up the development of the pattern mediated by them [26-28].
Key vertebrogenesis-regulating genes. It is considered that genes related to the homeobox (Hox) and paired box (Pax) have a critical role in controlling the development of the craniocervical junction [29].
Hox genes are involved in the development of a molecular coordinate system that determines the basic plan of embryo's structure. Works on the functions of proteins encoded by Hox genes revealed that they were involved in cell differentiation and organ development, as well as participated in hematopoie-sis, development of neural circuits, and tissue regeneration even in adulthood
83
1st 2nd
3rd
II
Complex of sclerotomes involved in the formation of >fie occipital bone
II
/
4th
5th
6th 7th
.................
Somites
Table 1 Deterministic periods of prenatal and postnatal vertebrogenesis
Prenatal phases of vertebrogenesis
Somitization of paraxial mesoderm Week 4
Formation of the membranous somite and resegmentation Week 5
The occurrence of chondrification centers Week 6
The beginning of vertebrae ossification (the occurrence of ossification nuclei) Weeks 8-9
Visualization of all spinal ossification centers Week 14
Postnatal phases of vertebrogenesis
The occurrence of the ossification nucleus of C2 dens 3 years
Fusion of the odontoid process and the body of the C2 vertebra, complete ossification of the dens 4-6 years
Fusion of the ossification nucleus of the dens and the odontoid process of the C2 vertebra 6 years
The occurrence of ossification foci of the vertebral endplates 11 — 14 years
Fusion of ossification foci of the vertebral endplates 15 years
Fusion of primary and secondary ossification centers of the spine 15—16 years
Completion of postnatal formation of the bone components of the cervical spine, synostosis of the apophyses of the vertebral bodies 14—17 years
84
i.m. krasnov et al vertebrogenesis: what the discoveries of the 21st century added into the classical understanding of the embryogenesis
[30-34]. Hox proteins are associated with a wide range of abnormalities that develop in connection with mutations or improper regulation [35, 36], including vertebral transformations: for example, excessive expression of Hox-7 in mice results in posterior translocation of the cervical vertebrae followed by the development of an aberrant bone, the proatlas, from the last occipital somites, instead of the occipital bone. The true atlas is instead developed with a complete vertebral body [3].
All Pax genes in vertebrates include a highly conserved DNA sequence called the "paired box". Encoded Pax proteins, often referred to as master regulators, are tissue-specific transcription factors that control the development, structuring, and function of organ tissues during cellular differentiation by maintaining cell identity. Despite the well-known function of Pax proteins in the development of abnormalities, the role of their isoforms in the embryogenesis of cra-niovertebral deformities is further studied [37-39].
The key to understanding the anatomy of vertebral malformations may be in the regulation of segmentation mechanisms: while Hox genes determine the rostral-caudal identity of cells within somites after primary segmentation, Pax-1 expression is of the first importance in resegmentation because of its effect on cell differentiation into tissues [40]. Out of the nine known Pax genes, all, except Pax-1 and Pax-9, are involved in the development of the central nervous system. The exceptions, especially Pax-1, control boundary formation between tissues (cell populations): it is thought to encode a transcription factor due to regulating molecules expressed by different populations of surface cells [41, 42] that are required for the resegmentation of cell adhesion molecules (such as NCAM (cytotactin)) and intercellular communication (such as connexins) [41, 43-46]. At the same time, the effect of Pax-1 on the intervertebral boundary zone (IBZ), with the mesenchyme of the future intervertebral disc, helps in the zonation (segregation) of sclerotomes.
Pax-1 expression is detected in pre-differentiated somites very early: SHH (Sonic Hedgehog) protein-mediated signals from the chord and ventral plate of the neural tube floor induce somite dividing into sclerotome and dermomy-otome; this coincides with intense Pax-1 expression of ventral part of sclerotome, suggesting that Pax-1 also plays a mediating role in the dorsoventral specification of somites [47, 48]. After somitic differentiation, Pax-1 expression is found within both lateral and axial sclerotomes where its fluctuations coincide with crucial resegmentation events. For example, during condensation of the sclerotome into the loose and dense halves, Pax-1 expression is weak within the loosely cellular prevertebrae but intense within the IBZ [41, 48]. Later, during chondrification and development of a homogeneous vertebral body, Pax-1 is repressed, however, persists in high levels at the interbody zones, where the centers are separated until intervertebral disc development is underway. Pax-1 level is also enhanced during condensation of the lateral sclerotome to form the neural arch [48-51].
On the contrary, normal fusion of certain adjacent sclerotomes takes place only when Pax-1 expression is turned off. At the CVJ of chick embryos, Pax-1 repression is timed exactly when the occipital sclerotomes fuse to form the basioccipital. The fusion of the two C2 odontoid process primordia with its body also coincides with the down-regulation of the Pax-1 gene, while ectopic Pax-1 expression disrupts normal assemblage of the dens axis and base of the occipital bone [48, 51]. Pax-1 is also highly expressed within the junctional zone between the proatlas and the first cervical sclerotome. It may thus also play a role in the separation of the head from the trunk. Murine Pax-1 mutants are associated with multiple fusion of vertebral bodies and fusion of the odontoid process with the anterior atlantal arch, reminiscent of the human Klippel - Feil syndrome. [51, 52].
Hyper- and hyposegmentation defects in humans are likely to be explained by over- and under-expression of Pax-1 during certain periods of vertebral devel-
opment. Under the control of the Pax and Hox genes, the proatlas hypocenter forms the anterior tubercle of the clivus, while its center gives raise to the dens apex and apical ligament. The neural arch is divided into a ventral-rostral segment that forms the occipital condyles, the anterior margin of the FM, alar and cruciate ligaments, and a dorsal-caudal segment that forms the posterior arch and lateral masses of the atlas [12, 39]. In early development, a dense band of connective tissue forms an arch that is ventral to each vertebral segment. The proatlas-related hypochordal arch usually regresses, and the one related with the first cervical somite contributes to the development of the C1 anterior arch [12, 53, 54].
The mesoderm and ectoderm are involved in the development of the posterior cranial fossa and the growth of skull parts located anterior to the chord
[55], while the parts located caudal to the dorsum sellae are of vertebral, including chordal (chorda dorsalis), origin
[56]. The clivus has typical endochon-dral osteogenesis, that is, the initial development of cartilage with its subsequent reabsorption and ossification [57]. During development, the notochord emerges from the skull base onto the outer surface of the future clivus, where irregular processes are grown [56]. In the os basisphenoidale, the chord reaches the caudal pituitary gland where the skull base starts to develop [58]. The brain is surrounded by a thick mesenchymal mass that covers the chord and gives raise to the clivus and dorsum sellae [59]. These mechanisms fully explain the tendency for tumor growth in this area, such as chordomas and chondromas/ chondrosarcomas.
The Pax-1 gene is also involved in the development of such a defect as the median (third) occipital condyle (the so-called condylus tertius), a residual occipital vertebra located anterior to the FM and sometimes fused with the dens or atlas; it limits movements in the CVJ [53]. In turn, atlantoaxial instability is associated with the hypoplasia of occipital con-dyles and condylus tertius; it is accompanied not only by the retaining proatlas,
85
but also by the odontoid process hypoplasia, stenosis of the FM and cervical spinal canal, compression of the medulla oblongata and vertebral artery, and weak transverse ligaments [54].
Groups of molecules involved in vertebrogenesis. Progress in molecular genetics over the past 20 years allowed coming closer to understanding the molecular genetic signaling cascades that are of fundamental importance in the development of the axial skeleton. Williams et al. [60] made it possible to identified groups of genes and proteins involved in the processes of embryonic vertebrogen-esis, as well as typical diseases caused by the impaired activity of the corresponding molecules. Thus, in Klippel-Feil syndrome that is characterized, first of all, by the impaired segmentation of cervical vertebrae, several authors define Meox-1 (Mesenchyme Homeobox-1) as one of the triggering agents of sclerotome differentiation [61] that interacts with Pax-1 and Pax-9 inducing Bapx-1 expression [60, 62, 63]. Activation of the Bapxl-Sox-9 positive feedback mechanism allows sclerotome responding to the signal transmission by bone morphogenetic proteins (BMP) aimed at the differentiation of chondrocytes [64-66]. Thus, Bapx-1 is of critical significance for their formation during the development of vertebral bodies and intervertebral discs [67, 68]. Pang and Thompson call Pax-1 as the resegmentation gene [40]. Among the molecular markers of the abnormal-
ity, the rostral (Mesp-2 and Tbx-18) and caudal (Ripply-1 and -2, Uncx-4.1 and Pax-1/9) markers indicating the sclero-tome polarity are considered critically important [69-71]. Identification of mutations in the GDF6 gene in Klippel-Feil syndrome suggests its important role in vertebral segmentation [72, 73].
Examples of several processes and their regulating agents are provided in Table 2.
Conclusion
Classical experiments on embryogenesis regulation performed at the end of the latest century were improved at the beginning of this one by the identification of types of molecular markers and genes that determine the development of certain anatomical structures and their abnormalities. The craniovertebral zone, having the most complex anatomy in comparison with all sections of the axial skeleton, can become an object for the analysis of the
fundamental mechanisms of abnormal vertebrogenesis and its possible regulation.
New knowledge can not only bring us closer to the intended development of experimental models of congenital abnormalities of the axial skeleton, but also becomes the basis for targeted methods of regulating such conditions as primary abnormality of vertebrae and ribs segmentation, their secondary synostosis (including postoperative), as well as the development of a postoperative bone union. Thus, they may have unexpected applied effects that considering the rapid accumulation of scientific knowledge may become one of the future focus areas in vertebrology.
The study had no sponsors. The authors declare that they have no conflict of interest. The study was approved by the local ethics committees of the institutions. All authors contributed significantly to the research and preparation of the article, read and approved the final version before publication.
86
References
1. Menezes AH, Ryken TC, Brockmeyer DL. Abnormalities of the craniocervical junction. In: McClone DG, ed. Pediatric Neurosurgery: Surgery of the Developing Nervous System. 4th ed. Philadelphia: WB Saunders. 2001:400-422.
2. O'Rahilly R, Meyer DB. The timing and sequence of events in the development of the human vertebral column during the embryonic period proper. Anat Embryol (Berl). 1979;157:167-176. DOI: 10.1007/BF00305157.
3. Brockmeyer DL. Advanced Pediatric Craniocervical Surgery. 1th ed. Thieme, 2006:1-30.
4. Ulrikh EV, Mushkin AYu, Gubin AV. Vertebral Pathology in Syndromes. Novosibirsk, 2016.
5. Modak SP. [Experimental analysis of the origin of the embryonic endoblast in birds]. Rev Suisse Zool. 1966;73:877-908. In French.
6. Nicolet G. [An autoradiographic study of the presumptive fate of the primitive streak in chick embryos]. J Embryol Exp Morphol 1970;23:70-108. In French.
7. Nicolet G. Avian gastrulation. Adv Morphog 1971;9:231-262. DOI: 10.1016/ B978-0-12-028609-6.50019-8.
8. Rosenquist GC. A radioautographic study of labeled grafts in the chick blas-to-derm: development from primitive-streak stages to stage 12. Contrib Embryol.1966;38(262):71-110.
9. Vakaet L. Some new data concerning the formation of the definitive endoblast in the chick embryo. J Embryol Exp Morphol 1962;10:38-57.
10. Bagnall KM, Sanders EJ. Higgins SJ, Leam H. The effects of somite removal on vertebral formation in the chick. Anat Embryol (Berl). 1988;178:183-190. DOI: 10.1007/ BF02463652.
11. Flint OP, Ede DA, Wilby OK, Proctor J. Control of somite number in normal and amputated mutant mouse embryos: an experimental and a theoretical analysis. J Embryol Exp Morphol. 1978;45:189-202.
12. Menezes AH. Craniocervical developmental anatomy and its implications. Childs Nerv Syst. 2008;24:1109-1122. DOI: 10.1007/s00381-008-0600-1.
13. Muller F, O'Rahilly R. Somitic-vertebral correlation and vertebral levels in the human embryo. Am J Anat 1986;177:3-19. DOI: 10.1002/aja.1001770103.
14. Parke WW. Development of the spine. In: Herkowitz HN, Garfin SR, Balderston RA, Eismont FJ, Bell GR, Wiesel SW, eds. Rothman-Simeone the Spine. 4th ed. Philadelphia: WB Saunders. 1999:3-27.
15. Tam PP. The control of somitogenesis in mouse embryos. J Embryol Exp Morphol. 1981;65Suppl:103-128.
16. Menezes AH. Embryology, development, and classification of disorders of the cra-niovertebral junction. In: Bambakidis NC, Dickman CA, Spetzler RF, Sonntag VKH, eds. Surgery of the Craniovertebral Junction. Thieme Medical Publishers, 2013:3-12.
17. Choi KS, Cohn MJ, Harfe BD. Identification of nucleus pulposus precursor cells and notochordal remnants in the mouse: implications for disk degeneration and chordoma formation. Dev Dyn. 2008;237:3953-3958. DOI: 10.1002/dvdy.21805.
18. Slipka J, Tonar Z. Outlines of Embryology. 2nd ed. Charles University in Prague, Karo-linum Press, 2019.
19. Flynn JM, Weinstein S, eds. Lovell and Winter's Pediatric Orthopaedics. 8th ed. LWW, 2020.
20. Садофьева В.И. Нормальная рентгеноанатомия костно-суставной системы детей. Л., 1990. [Sadofieva VI. Normal X-ray Anatomy of the Osteoarticular System in Children. Leningrad, 1990].
21. Wolpert L. Positional information and pattern formation. Philos Trans R Soc Lond B Biol Sci. 1981;295:441-450. DOI: 10.1098/rstb.1981.0152.
22. Loganathan R, Rongish BJ, Smith CM, Filla MB, Czirok A, Benazeraf B, Little CD. Extracellular matrix motion and early morphogenesis. Development. 2016;143:2056-2065. DOI: 10.1242/dev.127886.
23. Saiyin W, Li L, Zhang H, Lu Y, Qin C. Inactivation of FAM20B causes cell fate changes in annulus fibrosus of mouse intervertebral disc and disc defects via the alterations of TGF- and MAPK signaling pathways. Biochim Biophys Acta Mol Basis Dis. 2019;1865:165555. DOI: 10.1016/j.bbadis.2019.165555.
24. Shi DL. Planar cell polarity regulators in asymmetric organogenesis during development and disease. J Genet Genomics. 2023;50:63-76. DOI: 10.1016/j.jgg.2022.06.007.
25. Simonova OB, Burdina NV. [Morphogenetic movement of cells in embryogenesis of Drosophila melanogaster: mechanism and genetic control]. Ontogenez. 2009;40(5):355-372. In Russian.
26. Ashe HL, Briscoe J. The interpretation of morphogen gradients. Development. 2006;133:385-394. DOI: 10.1242/dev.02238.
27. Rogers KW, Schier AF. Morphogen gradients: from generation to interpretation. Annu Rev Cell Dev Biol. 2011;27:377-407. DOI: 10.1146/annurev-cellbio-092910-154148.
28. Barresi MJF, Gilbert SF. Developmental Biology. Oxford University Press, 2020. 758 p.
29. Zeoli T, Iwanaga J, Bui CJ, Dumont AS, Tubbs RS. Duplication of the odontoid process with other congenital defects of the craniocervical junction: case report and review of the literature. Anat Cell Biol. 2020;53:522-526. DOI: 10.5115/acb.20.189.
30. Collins EM, Thompson A. HOX genes in normal, engineered and malignant hema-topoiesis. Int J Dev Biol. 2018;62:847-856. DOI: 10.1387/ijdb.180206at.
31. Gofflot F, Lizen B. Emerging roles for HOX proteins in synaptogenesis. Int J Dev Biol. 2018;62:807-818. DOI: 10.1387/ijdb.180299fg.
32. Philippidou P, Dasen JS. Hox genes: choreographers in neural development, architects of circuit organization. Neuron. 2013;80:12-34. DOI: 10.1016/j. neuron.2013.09.020.
33. Roux M, Zaffran S, Zappavigna V, Convay SJ. Hox genes in cardiovascular development and diseases. J Dev Biol. 2016;4:14. DOI: 10.3390/jdb4020014.
34. Schiedlmeier B, Santos AC, Ribeiro A, Moncaut N, Lesinski D, Auer H, Kornacker K, Ostertag W, Baum C, Mallo M, Klump H. HOXB4's road map to stem cell expansion. Proc Natl Acad Sci USA. 2007;104:16952-16957. DOI: 10.1073/ pnas.0703082104.
35. Shah N, Sukumar S. The Hox genes and their roles in oncogenesis. Nat Rev Cancer. 2010;10:361-371. DOI: 10.1038/nrc2826.
36. Tischfield MA, Bosley TM, Salih MA, Alorainy IA, Sener EC, Nester MJ, Oystreck DT, Chan WM, Andrews C, Erickson RP, Engle EC. Homozygous HOXA1 mutations disrupt human brainstem, inner ear, cardiovascular and cognitive development. Nat Genet. 2005;37:1035-7. DOI: 10.1038/ng1636.
37. Dahl E, Koseki H, Balling R. Pax genes and organogenesis. Bioessays. 1997;19: 755-765. DOI: 10.1002/bies.950190905.
38. Deutsch U, Dressler GR, Gruss P. Pax 1, a member of a paired box homologous murine gene family, is expressed in segmented structures during development. Cell. 1988;53:617-625. DOI: 10.1016/0092-8674(88)90577-6.
39. Menezes AH, Fenoy KA. Remnants of occipital vertebrae: proatlas segmentation abnormalities. Neurosurgery. 2009;64:945-954. DOI: 10.1227/01. NEU.0000345737.56767.B8.
40. Pang D, Thompson DN. Embryology and bony malformations of the craniovertebral junction. Childs Nerv Syst. 2011;27:523-564. DOI: 10.1007/s00381-010-1358-9.
41. Smith CA, Tuan RS. Human PAX gene expression and development of the vertebral column. Clin Orthop Relat Res. 1994;(302):241-250.
42. Wallin J, Mizutani Y, Imai K, Miyashita N, Moriwaki K, Taniguchi M, Koseki H, Balling R. A new Pax gene, Pax-9, maps to mouse chromosome 12. Mamm Genome. 1993;4:354-358. DOI: 10.1007/BF00360584.
87
43. Pedrosa AE, de Azevedo GBL, Cardoso JV, Guimar es JAM, Defino HLA, Perini JA. Polymorphisms in paired box 1 gene were associated with susceptibility of adolescent idiopathic scoliosis: A case-control study. J Craniovertebr Junction Spine. 2022;13:318-324. DOI: 10.4103/jcvjs.jcvjs_54_22.
44. Jones FS, Chalepakis G, Gruss P, Edelman GM. Activation of the cytotactin promoter by the homeobox-containing gene Evx-1. Proc Natl Acad Sci USA. 1992;89: 2091-2095. DOI: 10.1073/pnas.89.6.2091.
45. Jones FS, Prediger EA, Bittner DA, De Robertis EM, Edelman GM. Cell adhesion molecules as targets for Hox genes: neural cell adhesion molecule promoter activity is modulated by cotransfection with Hox-2.5 and -2.4. Proc Natl Acad Sci USA. 1992;89:2086-2090. DOI: 10.1073/pnas.89.6.2086.
46. Musil LS, Goodenough DA. Gap junctional intercellular communication and the regulation of connexin expression and function. Curr Opin Cell Biol. 1990;2:875-880. DOI: 10.1016/0955-0674(90)90086-t.
47. Koseki H, Wallin J, Wilting J, Mizutani Y, Kispert A, Ebensperger C, Herrmann BG, Christ B, Balling R. A role for Pax-1 as a mediator of notochordal signals during the dorsoventral specification of vertebrae. Development. 1993;119: 649-660. DOI: 10.1242/dev.119.3.649.
48. Wilting J, Ebensperger C, M ller TS, Koseki H, Wallin J, Christ B. Pax-1 in the development of the cervico-occipital transitional zone. Anat Embryol (Berl). 1995;192:221-227. DOI: 10.1007/BF00184746.
49. Dietrich S, Gruss P. Undulated phenotypes suggest a role of Pax-1 for the development of vertebral and extravertebral structures. Dev Biol. 1995;167:529-548. DOI: 10.1006/dbio.1995.1047.
50. Dietrich S, Schubert FR, Gruss P. Altered Pax gene expression in murine notochord mutants: the notochord is required to initiate and maintain ventral identity in the somite. Mech Dev. 1993;44:189-207. DOI: 10.1016/0925-4773(93)90067-8.
51. Wallin J, Wilting J, Koseki H, Fritsch R, Christ B, Balling R. The role of Pax-1 in axial skeleton development. Development. 1994;120:1109-1121. DOI: 10.1242/ dev.120.5.1109.
52. Stapleton P, Weith A, Urb nek P, Kozmik Z, Busslinger M. Chromosomal localization of seven PAX genes and cloning of a novel family member, PAX-9. Nat Genet. 1993;3:292-298. DOI: 10.1038/ng0493-292.
53. Rao PV. Median (third) occipital condyle. Clin Anat. 2002;15:148-151. DOI: 10.1002/ca.1111.
54. Tubbs RS, Lingo PR, Mortazavi MM, Cohen-Gadol AA. Hypoplastic occipital condyle and third occipital condyle: review of their dysembryology. Clin Anat. 2013;26:928-932. DOI: 10.1002/ca.22205.
55. Di leva A, Bruner E, Haider T, Rodella LF, Lee JM, Cusimano MD, Tschabitscher M. Skull base embryology: a multidisciplinary review. Childs Nerv Syst. 2014;30:991-1000. DOI: 10.1007/s00381-014-2411-x.
56. Hofmann E, Prescher A. The clivus: anatomy, normal variants and imaging pathology. Clin Neuroradiol. 2012;22:123-139. DOI: 10.1007/s00062-011-0083-4.
57. Sajisevi M, Hoang JK, Eapen R, Jang DW. Nasopharyngeal masses arising from embryologic remnants of the clivus: a case series. J Neurol Surg Rep. 2015;76:e253-257. DOI: 10.1055/s-0035-1564603.
58. Kawase T, Shiobara R, Ohira T, Toya S. Developmental patterns and characteristic symptoms of petroclival meningiomas. Neurol Med Chir (Tokyo). 1996;36:1-6. DOI: 10.2176/nmc.36.1.
59. Rai R, Iwanaga J, Shokouhi G, Loukas M, Mortazavi MM, Oskouian RJ, Tubbs RS. A comprehensive review of the clivus: anatomy, embryology, variants, pathology, and surgical approaches. Childs Nerv Syst. 2018;34:1451-1458. DOI: 10.1007/s00381-018-3875-x.
60. Williams S, Alkhatib B, Serra R. Development of the axial skeleton and intervertebral disc. Curr Top Dev Biol. 2019;133:49-90. DOI: 10.1016/bs.ctdb.2018.11.018.
88
61. Zhao J, Li S, Trilok S, Tanaka M, Jokubaitis-Jameson V, Wang B, Niwa H, Nakayama N. Small molecule-directed specification of sclerotome-like chondropro-genitors and induction of a somitic chondrogenesis program from embryonic stem cells. Development. 2014;141:3848-3858. DOI: 10.1242/dev.105981.
62. Rodrigo I, Bovolenta P, Mankoo BS, Imai K. Meox homeodomain proteins are required for Bapx1 expression in the sclerotome and activate its transcription by direct binding to its promoter. Mol Cell Biol. 2004;24:2757-2766. DOI: 10.1128/ MCB.24.7.2757-2766.2004.
63. Rodrigo I, Hill RE, Balling R, M nsterberg A, Imai K. Pax1 and Pax9 activate Bapx1 to induce chondrogenic differentiation in the sclerotome. Development. 2003;130:473-482. DOI: 10.1242/dev.00240.
64. Kempf H, Ionescu A, Udager AM, Lassar AB. Prochondrogenic signals induce a competence for Runx2 to activate hypertrophic chondrocyte gene expression. Dev Dyn. 2007;236:1954-1962. DOI: 10.1002/dvdy.21205.
65. Tribioli C, Lufkin T. The murine Bapx1 homeobox gene plays a critical role in embryonic development of the axial skeleton and spleen. Development. 1999;126:5699-5711. DOI: 10.1242/dev.126.24.5699.
66. Yamashita S, Andoh M, Ueno-Kudoh H, Sato T, Miyaki S, Asahara H. Sox9 directly promotes Bapx1 gene expression to repress Runx2 in chondrocytes. Exp Cell Res. 2009;315:2231-2240. DOI: 10.1016/j.yexcr.2009.03.008.
67. Akazawa H, Komuro I, Sugitani Y, Yazaki Y, Nagai R, Noda T. Targeted disruption of the homeobox transcription factor Bapx1 results in lethal skeletal dys-plasia with asplenia and gastroduodenal malformation. Genes Cells. 2000;5:499-513. DOI: 10.1046/j.1365-2443.2000.00339.x.
68. Herbrand H, Pabst O, Hill R, Arnold HH. Transcription factors Nkx3.1 and Nkx3.2 (Bapx1) play an overlapping role in sclerotomal development of the mouse. Mech Dev. 2002;117:217-224. DOI: 10.1016/s0925-4773(02)00207-1.
69. Kawamura A, Koshida S, Takada S. Activator-to-repressor conversion of T-box transcription factors by the Ripply family of Groucho/TLE-associated mediators. Mol Cell Biol. 2008;28:3236-3244. DOI: 10.1128/MCB.01754-07.
70. Leitges M, Neidhardt L, Haenig B, Herrmann BG, Kispert A. The paired homeobox gene Uncx4.1 specifies pedicles, transverse processes and proximal ribs of the vertebral column. Development. 2000;127:2259-2267. DOI: 10.1242/dev.127.11.2259.
71. Morimoto M, Sasaki N, Oginuma M, Kiso M, Igarashi K, Aizaki K, Kanno J, Saga Y. The negative regulation of Mesp2 by mouse Ripply2 is required to establish the rostro-caudal patterning within a somite. Development. 2007;134:1561-1569. DOI: 10.1242/dev.000836.
72. Tassabehji M, Fang ZM, Hilton EN, McGaughran J, Zhao Z, de Bock CE, Howard E, Malass M, Donnai D, Diwan A, Manson FD, Murrell D, Clarke RA. Mutations in GDF6 are associated with vertebral segmentation defects in Klippel-Feil syndrome. Hum Mutat. 2008;29:1017-1027. DOI: 10.1002/humu.20741.
73. Wei A, Shen B, Williams LA, Bhargav D, Gulati T, Fang Z, Pathmanandavel S, Diwan AD. Expression of growth differentiation factor 6 in the human developing fetal spine retreats from vertebral ossifying regions and is restricted to cartilaginous tissues. J Orthop Res. 2016;34:279-289. DOI: 10.1002/jor.22983.
Address correspondence to:
Krasnov Ilya Mikhailovich,
St. Petersburg State University,
7-9 Universitetskaya emb., St. Petersburg, 199034, Russia,
st102310@student.spbu.ru
Received 27.03.2024
Review completed 17.04.2024
Passed for printing 26.04.2024
i.m. krasnov et al vertebrogenesis: what the discoveries of the 21st century added into the classical understanding of the embryogenesis
Ilya Mikhailovich Krasnov, student, St. Petersburg State University, 7-9 Universitetskaya emb., St. Petersburg, 199034, Russia, ORCID: 0009-0002-0901-6466, stl02310@student.spbu. ru;
Mikhail Aleksandrovich Mushkin, MD, PhD, orthopedic traumatologist, Department of traumatology and orthopedics, Pavlov First Saint Petersburg State Medical University, 6-8L'va Tolstogostr., St. Petersburg, 194064, Russia, ORCID: 0000-0001-8520-9425, mikhail_mushkin@mail.ru;
Aleksandr Yuryevich Mushkin, DMSc, Prof., Chief Researcher, Head of Department of Vertebrology, Traumatology and Orthopaedics, Head of the Scientific and Clinical Laboratory for Pathology of Locomotor System in Children, St. Petersburg Research Institute of Phthisiopulmonology, 2-4 Ligovsky prospekt, St. Petersburg 191036, Russia; Professor, Pavlov First Saint Petersburg State Medical University, 6-8 L'va Tolstogo str., St. Petersburg, 194064, Russia, ORCID: 0000-0002-1342-3278, aymushkin@mail. ru.
89
