limb bud formation
forelimb bud forms earlier, hindlimb bud forms later
limb bud
contains precursors of skeletal elements and tendon sheaths, muscles and vessels migrate in later, so all limb and limb girdle bones lateral plate mesoderm
limb elongation
proximo-distal differentiation is mediated by the apical ectodermal ridge
normal elongation
stylopod, zeugopod, autopod
late ablation
stylopod, zeugopod
early ablation
stylopod
limb orientation
crania-caudal orientation generated by the zone of polarising activity which lies caudally
polydactyly
common cause is ectopic medial expression of ZPA-related genes
apoptotic digit separation
paddle has to form digits, forms clefts early pn in the paddle - cells start to apoptose and form digits
syndactyl
when digits don’t separate
bone-endochondral ossification
mechanism of bone formation in most trunk and limbs - mesenchymal condensation in portion of limb
- blastemal condensation
large, tightly-packed mesenchymal cells
- chondrification
cells differentiate into chondrocyte - the cartilage model
- hypertrophy
chondrocytes at the centre stop dividing and swell - interstitial growth
connective tissue around model is compressed into perichondrium
- early mineralisation
central chondroblasts start to mineralise matrix, perichondrial cells become osteoblasts – cortical bone collar – compact bone
- vascular invasion
invades through cortical bone, accompanied by osteoblasts - these osteoblasts lay down primary spongiosa, eventually cancellous bone
- further lengthening of diaphysis
chondrocyte proliferation and hypertrophy, marrow cavity enlarges
- secondary centres in epiphyses
follows same processes: chondrocyte hypertrophy – vascular invasion – ossification
- no marrow cavity in epiphyses
- diaphysis thickens due to traction on the periosteum.
- leaves a think disc of cartilage between epiphyses and diaphysis
- growth plates
cause most pre and post natal growth , one at each end of most long bones and vertebral centra
growth plate structure
- reserve cartilage - hyaline
- proliferation - columns form
- maturation - stop dividing and swell
- hypertrophy and calcification
- cartilage degeneration, osteoblast invasion
- osteogenesis - mineralisation on cartilage spicules the metaphysis
- - shaft lengthens
joint formation
proximally in the limb joints form in the pre-existing gaps between blastemal condensations
in the autopod, joints may act to divide condensations
joint formation mechanism
- a mesenchyme ‘joint plate’ infills the gap
- joint plate capitates and differentiates into capsule and synovial membrane
- synovial cells are worn off the articular surfaces by metal movements in utero
limb muscle formation
limb muscle precursors are not made within the limb bud, but invade it in to form the myotomes
forms from ventral hypomere
