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
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