Limb Development Overview the appendicular system forms between the 4-8 weeks of gestation limb bud development appears to be under the control of fibroblast growth factors (FGF) enlargement of the limb bud is due to the interaction between the apical ectodermal ridge (AER) and the mesodermal cells in the progress zone. first identifiable by transvaginal ultrasound at 8 weeks Steps of limb development notochord expresses Shh Shh regulates limb bud formation limb bud is combination of lateral plate mesoderm and somatic mesoderm growing outwards into ectoderm (called apical ectodermal ridge) limb bud formed at embryonic stage 12 (26 days after fertilization) mesenchyme condenses into preskeletal blastemal at core of limb bud chondrification occurs where mesenchyme differentiates into chondrocytes All upper limb bones are endochondral except distal parts of distal phalanges (membranous) From proximal (humerus, 36 days after fertilization) to distal (distal phalanges, 50 days) Factors required for chondrification login to view 11 more bullets Formation of joints requires repression of chondrogenesis at sites of future joints proteins involved – WNT4, WNT14, growth and differentiation factor 5 (also known as cartilage-derived morphogenetic protein 1) shoulder interzone appears at 36 days, hand interzones appear at 47 days Finger separation digital rays are evident within hand paddle at stage 17 (41 days) interdigital mesenchyme cells undergo programmed cell death (stage 19 to 22)( days 47-54) transcription factor Msx2 is expressed in interdigital mesenchyme, regulates BMP4-mediated programmed cell death pathway transcription factor Hox-7 also expressed in interdigital zones Limb patterning Proximodistal first signaling center to appear is AER login to view 2 more bullets removal /defect in AER results in proximal limb truncation login to view 2 more bullets FGFs are the signaling molecule login to view 4 more bullets Anteroposterior second signaling center to appear is ZPA (zone of polarizing activity), along posterior limb bud login to view 2 more bullets signaling molecule is Shh compound (dose dependent) login to view 12 more bullets Dorsoventral axis third signaling center is non-AER limb ectoderm /Wnt signalling center (progress zone, PZ) dorsal limb ectoderm expresses WNT7a login to view 2 more bullets ventral ectoderm expresses en-1 (engrailed-1 protein, antagonistic to WNT7a) login to view 2 more bullets Key Genes Sonic Hedgehog (Shh) genes secreted by ZPA involved with HOX gene expression anterior-posterior (radioulnar) growth login to view 2 more bullets concentration gradient dictates formation of digits login to view 2 more bullets activates Gremlin login to view 1 more bullet Hox genes anterior-posterior (radioulnar) patterning login to view 1 more bullet regulate somatization of the axial skeleton, essentially patterning digit formation Wnt genes (Wnt7a) expressed in dorsal (non-AER) ectoderm (Wnt signalling center) dorsal-ventral growth Mutations removal of AER truncated limb duplication of ZPA mirror-image duplication of the limb Key Genes/Regions Gene/Region Expresses Regulates Apical ectodermal ridge (AER) FGF8 is dominant (also FGF 4, 9, 17, which are redundant) Proximal to distal growth and interdigital necrosis Zone of polarizing activity (ZPA) SHH Anterior-posterior (radio-ulnar) growth Non-AER limb ectoderm (dorsal) Dorsal ectoderm expresses WNT7a, that activates Lmx1b (regulates dorsal patterning) Dorso-ventral growth Non-AER limb ectoderm (ventral) Ventral ectoderm expresses en-1, antagonistic to WNT7a (regulates ventral patterning) Dorso-ventral growth Spine and Spinal Cord Development Somites the spinal column originates from pairs of mesodermal structures known as somites somites develop in a cranial to caudal direction on either side of the notochord and neural tube this process is dependent on the presence of the paraxis gene somite layers sclerotome login to view 1 more bullet myotome login to view 1 more bullet dermatome login to view 1 more bullet Dorso-vental patterning dorso-vental patterning of the neural tube determined by counteracting activities of Sonic Hedgehog (Shh) login to view 1 more bullet canonical Wnt/β-catenin login to view 1 more bullet Metameric shift phenomenon the phenomenon of how the spinal nerves, which originally ran in the center of the sclerotome, exit between the two vertebral bodies at each level. Progression neural crest forms PNS, pia mater, spinal ganglia, sympathetic trunk neural tube forms spinal cord notochord forms anterior vertebral bodies and nucleus pulposus Ossification centers vertebrae have 3 primary ossification centers centrum (anterior vertebral body) neural arch (posterior elements, pedicles, small portion of anterior vertebra) costal element (anterior part of lateral mass, transverse process, or rib) Intervertebral disc nucleus pulposus forms from notochord annulus fibrosus forms from sclerotome