· somites differentiate into
· sclerotome
· dermatomyotome
· cells from sclerotome migrate
· ventromedially to form vertebral bodies
· dorsally to form neural arch
· ventrolaterally to form costal processes
· ventromedial migration of cells from sclerotomes
· towards notochord
· sclerotomes lie in paired segmental masses alongside notochord
· each sclerotome differentiates into
· loosely packed cells in cranial half
· densely packed cells in caudal half
· fissure forms between cellular halves
· sclerotomic fissure of von Ebner
· cells near fissure migrate to notochord and encase it
· from proximal caudal part
· forms perichordal ring
· is precursor of intervertebral disc
· denser caudal half unites with less dense cranial half of suceeding sclerotome to form substance of vertebra
· called sclerotomal resegmentation
· when resegmentation complete, notochord surrounded by series of vertebral centra derived from cells from adjacent somites
Notochord
· notochord gradually regresses
· between vertebral bodies, notochordal cells undergo mucoid change to form nucleus pulposis
· perichordal ring becomes annulus fibrosis
· dorsal migration of cells from sclerotomes
· from denser half (now cranial) of sclerotome around neural tube
· form vertebral arch and pedicles
· ventrolateral migration of cells from sclerotomes
· lie between myotomes opposite intervertebral disc
· form costal processes
· 1st 4 somites called occipital somites
· next 7 somites called cervical somites
· dense band of tissue ventrally called hypochordal bow
· 4 occipital somites fuse to form basiocciput
· caudal portion of 4th forms
· arch of atlas
· tip of dens
· alar and apical ligaments
· 1st and 2nd cervical somites fuse to form C2
· odontoid process, centrum and arch
· hypochordal bow contributes to anterior arch of atlas
· centres of chondrification appear during 6th embryonic week
· starts at cervicothoracic junction
· two centres appear on each side of notochord
· fuse to make chondrification centre
· one centre forms in each half of vertebral arch
· fuse dorsally to form dorsal arch
· begin to form spinous process
· two other centres appear at junctions of centrum and neural arches
· form transverse processes
· cartilagenous model of vertebrum forms as solid unit
· begins in 10th week
· three primary centres
· single centre in body (centrum)
· one in each half of vertebral arch
· ossification occurs as in long bones
· hypertrophy of cells
· calcification of matrix
· penetration by blood vessels
· starts at thoracolumbar junction
· progresses craniad and caudad
· fuse during foetal life
1. anterior centre to posterior centre
· on each side anterior to pedicle
· at neurocentral synchondrosis
2. posterior to posterior centre
· in midline at posterior synchondrosis
· anterior primary ossification centre associated with spherical physis
· postnatally, physis enlarges toward sides of centrum and intervertebral discs
· does not participate in growth
· growth centre expands to demarcate parallel growth plates located superiorly and inferiorly
· analagous to tubular bones
Vertebral development
· body consists of centrum and adjacent part of neural arch
· posterior centres make variable contribution to postero-lateral part vertebral body
· significant in cervical spine
· less and less in thoracic and lumbar spine
· ossification of centrum leaves 2 cartilage plates
· at superior and inferior margins of vertebral body
· form physes
· produces increase in height of vertebral body
· by typical enchondral ossification
· develop during in adolescence
Ring apophysis
· linear foci of ossification occur within end-plate epiphyses
· between age 11 and 14
· form radiologic ring
· analogue of secondary ossification centre in tubular bones
· extends to neurocentral synchondrosis
· does not involve entire circumference of vertebral body (only centrum)
· does not contribute to growth of vertebral body
· growth occurs at contiguous physis
· fusion of end plate ossification centre with primary ossification centre begins at age 14-15
Processes
· three centres on tips of
· spinous process (1)
· transverse processes (2)
Costal processes
· contribute to different parts of vertebrae in different regions
· thoracic - rib
· cervical - anterior half of arch for vertebral artery
· lumbar - transverse processes
· sacral - part of alae
Thoracic ribs
· primary centre of ossification appears at 9th week near future angle of rib
· ossify towards sternal and vertebral ends
· secondary ossification centres develops in vertebral end during adolescence
· sternal end remains cartilaginous
· contribute to different parts of vertebral body
· cervical - posterior half of arch for vertebral body
· thoracic - transverse process
· lumbar - mamillary body and accessory tubercle
· sacral - alae
· diametric growth occurs in two ways
· through perichondrium which surrounds centrum
· through diametric growth of physis and plate epiphysis via membranous ossification
· vertical growth occurs due to endochondral ossification at growth plates
· enlargement of canal can occur as long as the three synchondroses are open
· endochondral ossification occurs at each
· allows units to grow away from each other and enlarge canal
· canal reaches adult size at age 3-6 years when synchondroses close
· two posterior primary ossification centres
· form prenatally
· expand posteriorly to form neural arch
· meet posteriorly at posterior synchondrosis
· also expand anterolaterally
· single anterior secondary ossification centre
· forms between 9 and 12 months of age
· remnant of hypochordal bow
· expands towards each posterior ossification centre
· forms two neurocentral synchondroses
· these fuse by age 4
· canal at maximum (adult) size at this stage
· five primary and two secondary ossification centres
· odontoid develops 2 primary ossification centres
· usually coalesce by age 3 months
· centres separated from primary ossification centre of centrum by dentocentral synchondrosis
· odontoid forms cartilagenous epiphysis at each end
· then forms secondary ossification centre in each
· centrum has superior and inferior epiphyses and subsequent ossification centres
· superior dens epiphysis = chondrum terminale
· inferior dens epiphysis + superior centrum epiphysis = dentocentral synchondrosis
· longitudinal growth occurs in three regions
· chondrum terminale
· dentocentral synchondrosis
· inferiorly through end plate physis
· secondary ossification centre of chondrum terminale called ossiculum terminale at age 8-10
· fuses with rest of dens at age 10-13