SN7 SHORT NOTE 5M

Describe the development of the vertebral column and the limbs.

Anatomy · 1st Professional — Pre-Clinical

Resegmentation is the whole point — it allows the myotome, which remains segmental, to span the joint between two vertebrae and so to move the column
Without it the muscles would run from the middle of one vertebra to the middle of the same one, and no movement would be possible

Three primary centres — one for the centrum and one for each half of the neural arch; appear in the fetal period
The halves of the arch fuse in the first 3 to 5 years, and with the centrum by about 6 years at the neurocentral joint
Five secondary centres at puberty — tip of the spine, each transverse process, and two annular epiphyses on the upper and lower surfaces of the body; fuse by about 25 years

Limb buds appear in week 4 — upper on day 24, lower on day 26; the upper limb is always slightly ahead
Apical ectodermal ridge (AER) — a thickening of ectoderm at the tip that controls proximodistal growth
Zone of polarising activity (ZPA) — controls the anteroposterior (thumb to little finger) axis
Digits are formed by apoptosis of the tissue between the digital rays, in weeks 6 to 8
The limbs rotate in week 7 — the upper limb laterally 90° and the lower limb medially 90°

Anomaly	Nature	Note
Amelia	Complete absence of a limb	—
Meromelia / phocomelia	Partial absence; the hand or foot attached close to the trunk	Classically caused by thalidomide, weeks 4–7
Syndactyly	Fused digits — failure of apoptosis between the rays	The commonest limb anomaly
Polydactyly	Extra digits	Often familial; seen in Patau syndrome
Congenital talipes equinovarus	Club foot	Associated with oligohydramnios and neural tube defects
Amniotic band syndrome	Constriction rings or intrauterine amputation	Mechanical, not genetic

The critical period for the limbs is weeks 4 to 7, which is why thalidomide taken over a few days produced such specific defects depending on exactly when it was taken
Spina bifida results from failure of the vertebral arches to fuse — the same halves whose ossification is described above
Hemivertebra — failure of one half of a centrum, causing congenital scoliosis
Block vertebra — failure of segmentation, with fusion of adjacent vertebrae, as in Klippel–Feil syndrome
Sacralisation and lumbarisation — transitional vertebrae from shifted boundaries of resegmentation; a common cause of confusion in identifying levels on radiographs, and of low back pain
Developmental dysplasia of the hip is screened for at birth by the Ortolani and Barlow tests; late diagnosis leads to a permanent limp

All questions in Systemic Embryology