Development of the Limbs

formation of the limb bud

• proliferation of the somatic lateral plate mesoderm gives rise to the limb buds (wk.4)

levels of the limbs along the body axis are determined by which genes

hox genes

what does the lateral plate mesoderm form

• skeletal elements

• connective tissues

• vasculature of the limbs

upper limb bud

• appear around wk.4

• small bulges on lateral body wall→ C5 to T1

• end of wk 4, lower limb buds appear→ L1 to S1

formation of somites

• paraxial mesoderm forms somites

• around 42-44 pairs by the end of 4th week

what do somites give rise to

• sclerotome→ axial skeleton

• dermomyotome→ dermis and skeletal muscle

• pre-myogenic cells→ migrate from dermatomyotome to limb bud

muscle masses

• muscle precursor cells originating from somites migrate into limb

• cells divide into 2 groups:

  • dorsal muscle mass

  • ventral muscle mass

what does dorsal muscle mass give rise to

• extensors

• supinators

• abductor

what does ventral muscle mass give rise to

• flexors

• pronators

• adductors

innervation of the limbs

• spinal nerves divide into dorsal and ventral rami

  • dorsal rami→ muscles of the back

  • ventral rami→ muscles of thoracic and abdo. wall and limbs

• in limbs, nerves divide again into ventral and dorsal divisions

  • ventral→ ventral mass (flexors)

  • dorsal→ dorsal mass (extensors)

proximodistal growth

• upper and lower limb buds protrude laterally from the trunk

proximodistal growth

• limb bud is encased in ectoderm→ skin

• mesenchymal core of limb bud secretes fibroblast growth factor 10 (FGF10)

• FGF10 induces thickening of overlying ectoderm along the tip of the limb bud→ apical ectodermal ridge (AER) ((pictures))

what is the AER

• signalling centre

• controls growth and elongation of the limb

progress zone

• region below AER

• contains rapidly dividing undifferentiated mesenchymal cells providing material for growing limb

• AER releases FGF signals→ stimulate cells in the progress zone to keep dividing and prevent them from differentiating too early

• new cells added to growing limb from progress zone→ limbs lengthen

Amelia

• complete absence of limb

• very early failure of FGF signalling/AER function

• PZ doesn’t develop

meromelia

• partial absence of limb

• later or partial loss of FGF signalling/ AER function

• arrest of outgrowth

• vascular abnormalities may also be responsible

failure of segments of AER

• induction and maintenance of AER essential for limb outgrowth

• mutation can result in failure of middle of AER to form properly

• can result in split (cleft) hand/foot

differential growth

• dorsal aspect of limb grows faster than ventral which causes limb bud to curve around body wall

limb rotation

• Upper limb undergoes lateral rotation

• lower limb undergoes medial rotation

dermatomes

• area of skin supplied by a single spinal nerve

• regions organised in segmental pattern along body

• limb rotation alters segmental arrangement of dermatomes in trunk

digits

• distal limb bud flattens to form hand and foot plates in wk. 6

• apoptosis in AER splits into 5 segments (A)

• continued PD outgrowth in 5 segments under AER to form and lengthen digit (B)

• apoptosis between digits separates them (C)

syndactyly

• fused digits

• failure of normal apoptosis between developing digits

craniocaudal patterning

• explains why thumb is lateral and finger is medial

• determined by region of mesenchyme in caudal part of limb bud→ zone of polarising activity (ZPA)

ZPA

• ZPA in caudal part of bud secretes SHH (sonic hedgehog)

• high conc. SHH in caudal= caudal structures e.g. little finger

• SHH diffuses in cranial direction

• ring, middle and index develop under progressively lower levels of SHH

• low conc. cranially induces cranial structures e.g. thumb

sonic the hedgehog

• SHH signalling from ZPA signals formation of caudal elements

  • loss of ZPA therefore results in loss of caudal elements

  • upregulation of ZPA signals results in additional caudal elements

  • duplication of ZPA results in duplication of caudal elements

dorso-ventral patterning

• determine which side of the limb will be the back and the front

dorsal side in dorso-ventral patterning

• signalled by Wnt7a proteins from skin over limb

• Wnt7a activates LMX1B gene in underlying tissue which specifies dorsal indentity

ventral side e.g. palm or sole

• involves BMP signals

• prevent dorsal signals from working

nail patella syndrome

• defect in LMX1B gene

• rare condition affecting nails, bones, kidneys, eyes

causes of limb abnormalities

• genetics

• drugs

• environmental toxins/poisons

• maternal illness

• idiopathic

phocomelia

• digits develop prematurely

• proximal elements of limb absent

• can be due to genetic factors or teratogens

  • associated with thalidomide use

amniotic band syndrome

• constriction of limbs by fibrous amniotic bands

• leads to congenital amputations or other malformations