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Embryology of the spine and

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

"It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." (1986) Gastrulation

Conversion of the embryonic disk from a bilaminar to a trilaminar arrangement and establishment of the

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 and

Day 16 H E Day 15 Dorsal view of a 0.4 mm BILAMINAR DISK

CRANIAL faces the amniotic sac

node (primitive ) faces the yolk sac

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 which rolls up to become the definitive notochord Canalization Notochordal process & canal The notochord is required for the development of the ()

Intercalation Neurenteric canal Existence in 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 , then the 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: and Primary uppermost 9/10 of spinal cord 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)

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 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 's four germ layers to three: the ectoderm, mesoderm, Wnt and endoderm

Shh

(future ) 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 ; 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