Objectives 1) Review endochrondral vs. membranous ossification 2) Understand the role of paraxial mesoderm in limb development 3) Describe formation, growth, and organization of limb buds - Organogenesis: formation of organ rudiments and organ systems!!1) Review endochrondral vs. membranous ossification - Development of bone and muscle occurs during the 4th week of gestation after est. tube w/in a tube body plan!- Bone formation:!- Endochrondral= cartilage model forms 1st and is replaced w/bone!- Incl.: axial, appendicular, cranial base (except part of the clavicle) - Intermembranous= bone forms directly from mesenchymal cells w/o cartilage template!- Incl: majority of bones of face and skull = dermal or membrane bones 2) Understand the role of paraxial mesoderm in limb development - Muscular system: develops from the mesodermal layer!- 3 muscle types: !- smooth: differentiates from splanchnic mesoderm surrounding gut and from ectoderm (anything rel. to viscera)!- cardiac: derived from visceral splanchnic mesoderm surrounding heart tube!- skeletal: derived from paraxial mesoderm !- Derivatives of mesodermal germ layer!- Surrounding the notochord there are developing tissues: our focus = paraxial mesoderm which is immediately lateral to the notochord!- Paraxial mesoderm!- Begins to org. into segments (somitomeres) by 3rd week!- Head region: gives rise to striated muscles of the face, jaw, and throat!- Trunk region: organizes into somites --> give rise to axial skeleton!- Mesodermal cells of somite arrange around a lumen!- Trunk: somites appear w/specified periodocity, from cranial to caudal and increase in # thru the weeks!- After formation, somites reorg. into:!- Mesenchymal sclerotome (ventral)!- Epithelial dermomyotome (dorsal) !- Sclerotome - Develops into the vertebrae and ribs!- Ventral cells surround notochord and form beginning of vertebral body!- Dorsal cells surround neural tube and form beginning of vert. arch and spine!- Lateral cells form vert. transverse processes and ribs!- Each sclerotome resegments and splits in half!- Caudal segment fuses w/cranial seg. of sclero below it!- Each of the 2 segments contributes to a vertebra!- Most cranial 4: fuse to become occipital bone!- 8 cervical somites (or sclero?) develop --> form 7 cervical vertebrae (1st cerv. vert is "lost" to the occipital bone)!- Segmental spinal nerves exit between each vertebra - where the sclerotomes split --> fibrous intervertebral discs develop!- original core: notochordal origina, will die and leave nucleus pulposus!- annulus fibrosus develops from sclerotomal cells!- Paraxial mesoderm (bones)!- lateral condensations (costal processes) develop in asso. w/vertebral arches!- thoracic region: tips lengthen to form ribs - ribs dev. as cartilage precursors until they ossify later !- sternum develops from a pair of longitudinal mesenchymal condensations, sternal bars (at tips of ribs coming around)!- Dermomyotome (from somites) divides into:!- Dermatome= will form dermis of skin - Myotome!- Differentiates into:!- Epimere (dorsal): will form deep epaxial muscles of back (true back m.), inneravated by dorsal primary rami - Hypomere (ventral): will form hypaxial muscles of lateral and ventral body wall in thorax and abdomen, strap neck muscles, and limb muscles. Innervated by ventral primary rami! - Lateral Somitic Frontier!- well-defined border between each somite and parietal layer of lateral plate mesoderm (LPM) = barrier between paraxial and lateral plate mesoderm !- ventrolateral cells migrate across frontier THE PIONEERS! into parietal layer of LPM and join it during folding --> abaxial muscle domain - muscle precursor cells that don't move will stay dermatome cells to become dermomyotome and form primaxial muscle domain - separates 2 mesodermal domains in embryo!- primaxial: region around neural tube, onlysomite-derived cells (paraxial mesoderm), --> m. of back, shoulder girdle, intercostal m.s!- abaxial: parietal layer of LPM w/somite cells that migrated --> m. of body wall (incl. limb musculature)!- Paraxial mesoderm REVIEW - # of somites increases thru 4th (end=26-29 pairs) and 5th week (end= 42-44 pairs)!- each somite forms its own!- sclerotome (cartilage + bone)!- myotome (segmental muscle components)!- dermatome (segmental skin components)!- Each myotome and dermatome have its own segmental nerve component!- this is all in the trunk 3) Describe formation, growth, and organization of limb buds - Limb development!- Begins by end of 4th week: limb buds become visible as outpocketings from ventrolateral body wall!- Each limb bud consists of:!- Outer ectodermal cap!- Inner mesodermal core (from parietal layer of lateral plate mesoderm)!- Upper limbs develop a little before lower limbs!- Finishes by 8th wk.!- Each limb grows and organizes aroudn 3 axes: proximal-distal, anterior-posterior (thumb to pinky), and dorsal-ventral!- Bone shape and type and regulated by HOX genes (homeobox)!- Proximal-distal growth!- Ectoderm of bud induced by below somatic mesoderm to form apical ectodermal ridge (AER)-->limb continues to grow along prox-distal axis!- Prolif and growth higher on dorsal side-->ventral curvature!- Two models: Progress zone model and Early specification model!- If you remove the AER, the growth of the limb is arrested at the point and rest of limb won't develop! - Anterior-posterior axis!- Regulated by zone of polarizing activity (ZPA)= cluster of cells at posterior border of limb near body wall, the cluster of cells indicates which side is posterior!- Dorsal-ventral axis!- Determined by signals from ectoderm!- Hands and feet!- 6 week old embryos --terminal part of limb bud becomes flattened (like a mitten) --> hand and footplates!- fingers and toes formed when programmed cell death in AER separates each digit!- Clinical correlation: if mesenchyme between digits isn't removed by programmed cell death, 2 or more fingers/toes may be fused = syndactyly - Development of internal portion of limbs!- Mesenchyme in buds begins to condense and differentiate into chrondrocytes!- By 6th week, hyaline cartilage model appear!- Endochondral ossification begins by end of embryonic period (will start when limb is completely developed)!- Limb Rotation - in 7th week, limbs rotate about 90 degrees in diff. directions!- Upper rotates laterally!- Lower rotates medially !- this explains why the anterior and posterior divisions of the lumbosacral plexus seems to be inverted!- Limb Muscles!- limb
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