016 Limb development Flashcards
when does upper limb development start?
week 4 (upper limbs)
when does lower limb development start?
week 6
on the human arm skeleton, what is proximal and distal?
- humerus is proximal
- phalanges is distal
on the human arm skeleton, what is anterior and posterior?
- thumb is anterior
- little finger is posterior
on the hand, what is dorsal and ventral?
- dorsal = back of hand
- ventral = palm
name 4 different limb defects
- Grieg syndrome
- Apert syndrome
- Holt-Oram syndrome
- thalidomide defects
describe what Grieg syndrome is and what it is caused by (genetic)
- polydactyly (extra digits) and syndactyly (digits fused)
- heterozygous mutation in GLI3 gene (part of sonic the hedgehog signaling pathway)
- also causes neuro and heart defects
describe what Apert syndrome is and what it is caused by (genetic)
- syndactyly (digits fused)
- heterozygous mutation in FGDR2 gene
- also causes cranium sutures to fuse early
describe what Holt-Oram syndrome is and what it is caused by (genetic)
- phocomelia = long bones reduced or absent
- only arms, not legs
- heterozygous mutations in the TBX5 gene (transcription factor, a T-box gene = DNA binding domains)
- also causes heart defects
what is thalidomide?
- drug given to pregnant women in the 1950s-60s to control morning sickness
- caused serious congenital limb defects
what limb defect does thalidomide cause?
- causes congenital phocomelia of arms or legs or both
- also causes heart defects, deformed ears and eyes and alimentary canals, urinary tracts, blindness and deafness
why are most babies born with limb defects (they don’t die in the uterus)?
- fetus’ dont need limbs to survive in the uterus (compared to if it had a gut defect)
- so most live till birth
what is the first part of limb development and when?
- limb buds form (day 24 for arms, day 28 for legs)
what is the structure of limb buds and what do they become?
- epidermal jacket/outer layer (skin) enclosing lateral plate mesoderm
- lateral plate mesoderm becomes : bones, tendons, ligaments, dermis, muscle sheath, blood vessels
- apical ectodermal ridge (AER) forms at distal end of limb bud
what is the AER/apical ectodermal ridge?
- ridge of columnar cells at the distal end of limb bud
what is the genetic difference between the forelimb and hindlimb?
- forelimb = Tbx5 transcription factor expressed only
- hindlimb = Tbx4 transcription factor expressed only
where is Tbx5 expressed, and what does a mutation cause?
- forelimb
- Holt-Oram syndrome = short long bone in arm
where is Tbx4 expressed and what does a mutation cause?
- hindlimb
- congenital clubfoot, small patella syndrome
how/where does the limb bud mesenchyme proliferate?
- along the proximal-distal axis
- the humerus/ femur first, phalanges last
- digit 4 first then3,2 then 1 and 5 together
describe the process of endochondral ossification in fetus’
- mesenchyme condenses to form the limb bone precursors
- these cells form chondrocytes that produce cartilage
- centre of the bone becomes vascularised first and osteoblasts are formed, which replace the cartilage with mineralised bone
- primary ossification sites all grow at the same time in the centre of all the long bones in the fetus
- secondary ossification centres form around birth
what does skeletal muscle develop from?
- paraxial mesoderm = somites
- they migrate from the myotome to the limb bud in about the 5th week of development forming dorsal and ventral masses
how are the myotome cells attracted to the limb bud?
- HGF, hepatocyte growth factor, secreted by the limb bud mesenchyme
what happens if there is a mutation in the HGF or HGF receptor gene?
- no muscle
describe how innervation of the limb develops
- motor and sensory neurons form on ventral side of spinal cord coz of SHH
- limb muscle doesn’t attract axons but regulates the final branching pattern (don’t know what attracts them)
describe how neuromuscular junctions develop in fetus
- multiple motor neurons will innervate a single myotube
- at birth only the most active synapse is retained and others are destroyed (only 1 per myotubule)
how do we know that the AER (apical ectodermal ridge) is needed for limb development?
- experiment with chick embryo
- remove AER early on = truncated limb, only humerus and smaller, no forearm or hand
- remove AER late = humerus and forearm, no hand
= shows proximal-distal growth
how does FGF stimulate limb growth?
- FGF beads in AER diffuse into mesenchyme, which stimulates proliferation and limb bud growth
what happens if you implant an FGF bead in the lateral plate mesoderm between the forelimb and hindlimb?
- an extra limb will form between the arm and leg
describe the progress zone of limb growth
- just beneath AER
- Secretion of FGF by the AER maintains a high rate of cell division in the underlying mesenchyme = the progress zone
= high FGF and high mitosis
describe the differentiation zone of limb growth
- more proximal than the progress zone
- further away from AER
= low FGF, low mitosis - cells divide more slowly and differentiate
what decides the proximal and distal fates of the limb patterning?
- FGF from AER determines distal fates (may act as a morphogen gradient)
- retinoic acid from the flank - derivative of vit A - determines proximal fates (e.g. humerus)
what is the zone of polarising activity (ZPA)
- area of mesenchyme that contains signals instructing limb bud to form along the anterior-posterior axis
- this region expresses SHH
describe how limbs have anterior-posterior patterning in limb development
- the zone of polarising activity (ZPA) in the mesenchyme of the limb bud
- expresses Shh genes which determine anterior-posterior patterning (ie, thumb anterior, pinky posterior)
- if you take out a graft of ZPA and implant it on the opposite side, you will get duplicate digits and mirror image symmetry
describe the Anterior-Posterior morphogen gradient in limb/digit development
- ZPA was believed to be the source of a morphogen, which specifies different digits at different concentrations
- highest concentration = most posterior digits (pinky)
- lowest concentration = most anterior digits (thumb)
- ZPA grafts cause counter-gradient and duplicates digits
what happens if you activate SHH expression in anterior and posterior parts of the limb bud?
- mirror-image duplication of the digits identical to that of a ZPA graft
what happens if you have increased Shh?
- ## preaxial-polydactyly ( extra digits, thumb)
what happens if you have decreased Shh?
- shortened limbs with very few / no digits
- acheiropodia (rare) - mostly due to inbreeding in brazil, isolated tribes
what is the relationship between FGF, Shh, Gremlin and Bmp?
- FGF (AER) induces Shh expression (ZPA)
- Shh induces Gremlin expression (in distal mesenchyme)
- Gremlin inhibits Bmp signaling
- Bmp signaling reduces FGF expression
- loop, positive feedback = continuous growth
how is limb growth stopped?
- as the limb grows , the Shh domain remains the same size, while cells that previously expressed Shh cannot express Gremlin
- thus a barrier of unresponsive cells, which increase in size with limb bud growth, separated the Shh domain from the Gremlin domain
- = reduced Gremlin = increased Bmp = increased reduction of FGF = reduced Shh in ZPA = growth stops
describe apoptosis of the digits in limb development
- initially, digits are all connected (webbed fingers)
- apoptosis occurs to remove/kill interdigital cells
- apoptosis is controlled by Bmp signaling molecules which are only active in interdigital region
- Noggin and Gremlin both inhibit Bmp and is found in the digits (no apoptosis)
what is syndactyly and how is it caused?
- where 2 or more digits are fused together
- most common congenital abnormality
- caused by defective apoptosis of interdigital region
