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Gastrulation Embryology of the spine and spinal cord Andrea Rossi, MD Neuroradiology Unit Istituto Giannina Gaslini Hospital Genoa, Italy [email protected] LEARNING OBJECTIVES: LEARNING OBJECTIVES: 1) To understand the basics of spinal 1) To understand the basics of spinal cord development cord development 2) To understand the general rules of the 2) To understand the general rules of the development of the spine development of the spine 3) To understand the peculiar variations 3) To understand the peculiar variations to the normal spine plan that occur at to the normal spine plan that occur at the CVJ the CVJ Summary of week 1 Week 2-3 GASTRULATION "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Lewis Wolpert (1986) Gastrulation Conversion of the embryonic disk from a bilaminar to a trilaminar arrangement and establishment of the notochord The three primary germ layers are established The basic body plan is established, including the physical construction of the rudimentary primary body axes As a result of the movements of gastrulation, cells are brought into new positions, allowing them to interact with cells that were initially not near them. This paves the way for inductive interactions, which are the hallmark of neurulation and organogenesis Day 16 H E Day 15 Dorsal view of a 0.4 mm embryo BILAMINAR DISK CRANIAL Epiblast faces the amniotic sac node Hypoblast Primitive pit (primitive endoderm) faces the yolk sac Primitive streak CAUDAL Prospective notochordal cells Dias Dias During gastrulation, epiblastic cells start migrating toward the Cells migrating through the node to both sides of primitive streak, pass through it to the interface of epiblast the midline form a rod-like rudiment which initially is and hypoblast, canalized notochordal process and then migrate laterally along the interface to form: Subsequently, prospective notochordal cells form a plate of 1) the endoderm; and 2) the mesoderm tissue which rolls up to become the definitive notochord Canalization Notochordal process & canal The notochord is required for the development of the neuroectoderm (neural plate) Intercalation Neurenteric canal Existence in humans is debated Excalation Notochord Carnegie stage 7 Carnegie stage 8 Primary Week 3-4 neurulation PRIMARY The zipper-like NEURULATION model The fusion of the neural folds Disjunction involves first the neural ectoderm, then the surface ectoderm Anterior neuropore 30 days Somite 5 Posterior neuropore 31 days Fusion of the posterior neuropore: End of primary neurulation 31 d Tortori Donati P et al, Neuroradiology 2000 Interface between primary and secondary neurulation (Nievelstein, 1993) Primary neurulation: brain and Primary uppermost neural tube 9/10 of spinal cord Somite 32 (S3) Secondary neurulation: Secondary neural tip of conus medullaris tube Courtesy of and filum terminale Dr RAJ Nievelstein Utrecht, the Netherlands Week 4-6 SECONDARY NEURULATION Secondary neurulation Secondary neurulation Medullary cord Secondary neural tube Catala, Paris (caudal eminence or tail bud) canalization Secondary neurulation Only the primordia for R Nievelstein, Utrecht the terminal ventricle and filum terminale and, possibly, part of the conus medullaris corresponding to the coccygeal metameres of Primary neural the spinal cord are Secondary tube formed during neural tube secondary neurulation Nievelstein R et al, Teratology 48:21-31, 1993 Day 50: canalization Retrogressive differentiation 1) The presomitic stage LEARNING OBJECTIVES: Notochord 1) To understand the basics of spinal cord development 2) To understand the general rules of the development of the spine 3) To understand the peculiar variations to the normal spine plan that occur at the CVJ 2) Somitogenesis 9 10 10 11 Somites: 4 occipital 8 cervical 12 thoracic 5 lumbar 5 sacral 3-6 coccygeal Dormans JP, Kusumi K: Klippel-Feil syndrome. Clin Orthop 2004;424:187 3) Differentiation of the somitic mesenchyme 4) Resegmentation of the sclerotome Neugliederung Remak R, 1855) Robert Remak (26 July 1815 29 August 1865) was a Polish/German embryologist, physiologist, and neurologist, born in Posen, Prussia Dr. Remak obtained his medical degree from Friedrich Wilhelm University in Berlin in 1838 specializing in neurology He is best known for reducing Karl Ernst von Baer's four germ layers to three: the ectoderm, mesoderm, Wnt and endoderm Shh (future vertebra) 4) Resegmentation of the sclerotome 4) Resegmentation of the sclerotome Neugliederung Remak R, 1855) SOMITES SCLEROTOMES Segmental artery in neural Segmental artery foramen Intersomitic boundary with segmental vein Basivertebral vein Dense upper half Loose lower half Loose upper half Axial sclerotome: Lateral sclerotome: Dense lower half Vertebral body Neural arch Intervertebral boundary Posterior vertebral zone elements (Annulus fibrosusLateral of IV disk) Axial Lateral (neural arch, pedicle, etc) Upper (dense) Upper (dense) Vertebral body Lower (loose) Lower (loose) Does not form bone; Lateral Axial Lateral promotes emergence of nerve roots (IV foramen) LEARNING OBJECTIVES: 1) To understand the basics of spinal cord development 2) To understand the general rules of the development of the spine 3) To understand the peculiar variations to the normal spine plan that occur at the CVJ The CVJ is the product of the 4 occipital somites and the first 3 cervical somites The occipital somites are peculiar! No intersomitic boundary zone Segmentation of the first 3 occipital axial sclerotomes does not occur Rostral part of basiocciput (clivus) Proatlas Hypochordal bow Clival tubercle Derivatives of the proatlas: The clival tubercle Attachment of the fibrous pharyngeal raphe Caudal part of basiocciput and basion Exocciput (occipital condyles and opisthion) Pang & Thompson Clival tubercle Apical dens condylus tertius The atlas does not derive from the proatlas! General rule: 1 sclerotome 1 vertebral body Special rule: Proatlas 2 vertebral half- bodies basion & apical dens severance line Hypochordal bow Junction between head and spine follows an C1 anterior arch intersomitic, not an intersclerotomic, boundary Derivatives of the C1 sclerotome: Basal dens Anterior arch of atlas Lateral masses and posterior arch of atlas The basal dens is of the atlas The whole atlantal ring derives from C1 sclerotome The axis (C2): body + basal dens + apical dens Derivatives of the C2 sclerotome: 2.5 vertebral bodies Body of axis Lateral and posterior elements of C2 The axis: body + basal dens + apical dens No IV boundary zones are formed Apicodental synchondrosis Subdental synchondrosis.
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