1.1 Upper Limb Embryology and Growth

8/12/2019 1.1 Upper Limb Embryology and Growth

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Upper limb embryology and growth

MANOJ RAMACHANDRANConsultant Paediatric and Young Adult Orthopaedic Surgeon,

Barts Health NHS Trust, London, England


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Objectives

Embryonic limb development

Patterning and differentiation

Limb growth


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Embryonic limb

development


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Molecular level

Cell produces secreted protein

Protein binds to specific receptor

Intracellular changes transcription factors to nucleus

Bind to DNA expression of new target genes


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Examples


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OriginsNotochord/neural tube (3rdweek)

Paraxial mesoderm segmentsinto blocks (somites)

Somites differentiate into:

Sclerotome(vertebrae and ribs)Dermomyotome(dermis and

primordial muscle cells)

Mesoderm forms loosely woventissue, mesenchyme(embryonic connective tissue),which has bone-formingcapacity (4thweek)


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Limb bud 4thweek

Limb bud = mesenchymal core ofmesoderm covered by a layer ofectoderm (with leading bulge)

Bulging ectoderm = apicalectodermal ridge

Core mainly lateral plate mesoderm


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Limb bud 4thweek

Migratory cells invade limb bud:

Myoblasts, angioblasts andmotor axons from somaticmesoderm

Sensory nerves, pigmentcells and Schwann cells fromneural crest

Migratory cells do not invade

growing apex


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How does it start?Early embryo homeobox (HOX) genes

set up segmental body plan alongcranial-caudal axis

Upper limb fields established 4thweek,triggering expression of TBX5, WNT

and FGF that initiate limb budoutgrowth

Tetramelia = failure of limb induction associated with WNT3 and FGF10

mutationsNick Vujicic, Life Without Limbs


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Arms vslegs

Cells similar and behave identically

T-box (Tbx) genes expressed early anddifferentiate fore from hind limb

Tbx-4 hind limb-specificTbx-5 fore limb-specific

Ptx1 gene controls Tbx4 (hind limb) change wing into leg by changing Ptx1

expression


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Molecular control via FGF

FGF

Implant FGF (microcarrier beads) =

supernumerary limbs

FGF-10 knockout = limbless


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Molecular control via FGFMesodermal FGF10, in conjunction

with ectodermal radical fringe (R-FNG), induces ectodermal thickening

to form AER

AER maintains FGF10 expression in

underlying mesoderm (via WNTs)

FGF10 also sustains proliferation of PZ

Reciprocal loop of ectodermal and

mesodermal FGF/WNT proteinsmaintains proximaldistal outgrowth


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Control of limb bud

Within the progresszone, the fate of

mesodermal cells isdetermined by these

multiple signalingcenters

Proximodistal AER FGF-2, -4 and -8

Anteroposterior ZPA Shh

Dorsoventral Dorsal ectoderm Wnt-7a/LMX1B(dorsal)

Ventral ectoderm En-1(ventral)


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ZPAZPA controls radial-ulnar axis from the

posterior (ulnar) limb mesoderm

ZPA expands limb width and posteriorizes

(ulnarizes) limb through secreted sonic

hedgehog (SHH)

AER and ZPA closely linked by reciprocal

feedback loop that maintains SHH

expression at the distal posterior (ulnar)

border of the AER during progressive

outgrowth


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ZPA

ZPA grafts to anterior induces duplicatedmirror digits

ZPA

stimulates mesenchymal cell proliferation

induces changes in AER

acts via SHH to produce BMP-2 and -7

which pattern digits


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Dorsal-ventral axisWNT7a expressed in dorsal limb ectoderm

WNT7a activates expression of LMX1b,

which determines dorsal limb bud identity

Lack of LMX1b = nail-patella syndrome

Ventral ectoderm expresses en-1, which

has inhibitory effect on WNT7a,

restricting it to dorsal ectoderm

Lack of en-1 = relatively more WNT7a

leads to congenital palmar nail syndrome


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The three axes

AER/FGF =transverse arrest

FGF = radialarrest

SHH = ulnararrest

LMX1B = dorarrest


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Growth of limb bud


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Patterning and

differentiation


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Formation of digits

Programmed cell death (apoptosis)plays a role in sculpting the limb

Regions that are sculpted by cell

death:

Interdigital zoneInterior necrotic zone

(separates radius and ulna)

Anterior & posterior necrotic

zones (shape the end of the

limb)


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BMPsSignals for apoptosis are BMP2,

BMP4 and BMP7

Expressed in the interdigital

mesenchyme

Paddle to individual digits

Blocking BMP signaling prevents

interdigital apoptosis

NOGGIN

Absence of apoptosis - syndactyly


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M l diff ti ti


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Muscle differentiation

Migration of myoblasts from somites

Dorsal and ventral muscle masses

Tendons from limb bud mesenchymeinteract with myotubes

N diff ti ti


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Nerve differentiation

Motor axon from the spinal cordinnervate dorsal and ventral musclemasses from fifth week

Sensory axons use motor axons forguidance

Dermatomal patterning

V l diff ti ti


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Vascular differentiation

Fine capillary network branches fromaortic intersegmental arteries -> largecentral artery (primary axial artery)

Drain into marginal sinus under AER

accumulates blood and drains the limbvia peripheral veins

Primary axial artery becomes brachialartery in arm and common

interosseous artery in forearm


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Limb growth

H d b f ?


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How do bones form?

Primary ossification centre develops inmid-diaphysis (chondrocyte

hypertrophy and vascular invasion)

Endochondral ossification at physes

Secondary centres near end of fetaldevelopment

S b t li b th


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Subsequent limb growth

Longitudinal

Physeal endochondral ossification

CircumferentialZone of Ranvier (osteoblasts,

fibroblasts and chondrocytes)

Periosteal appositional ossification

Th h i


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The physis

Cells

Matrix

Vascular invasion and

programmed cell death

Indian hedgehog protein main

mitogen in proliferative layer

Oxygen

Upper limb gro th


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Upper limb growth

First 5 years are characterized by rapid growthfollowed by plateau till puberty

Proportions amongst various bones established

by 5 years of age

Ulna is 80% of the length of the humerus,

whereas the humerus represents 70% of the

length of the femur

Proximal humeral physis contributes to 80% ofhumeral length


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Objectives

Embryonic limb development

Patterning and differentiation

Limb growth


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Thank you!

MANOJ RAMACHANDRANConsultant Paediatric and Young Adult Orthopaedic Surgeon,

Barts Health NHS Trust, London, England

Documents

1.1 Upper Limb Embryology and Growth