11 skeletal and muscleembryo/lectures/exam 3/11 skeletal... · 2006-06-26 · skeletal development...
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Skeletal DevelopmentMultiple Cellular Origins
1 - Paraxial MesodermSomite, SclerotomeAxial Skeleton (e.g. vertebra)
2 - Lateral Plate MesodermAppendicular Skeleton – (e.g. limb)
3 - Neural CrestHead Skeleton
Established as
1 - Hyaline Cartilage – replaced by Endochondrial Ossification
2 – Intramembranous Bone Formation - direct ossification
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Intramembranous Bone
Intramembranous bone = dermal bone (e.g. skull, clavicle)
Mesenchymal condensation, becomes vascularized
Osteoid Tissue (prebone) - cells differentiate into osteoblasts - matrix deposition - Calcium Phosphate
Osteoblast Osteocytes - trapped in matrix
Bone Spicules organized around blood vessels -concentric layers = Haversian system.
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Compact Bone - Osteoblast in periphery lay down layers of compact bone
Spongy bone - beneath bony plates - osteoclasts breaks down bone
Continual bone remodeling via action of osteoblasts and osteoclast
Bone marrow differentiates from mesenchyme in spongy bone
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Endochondrial Bone
Endochondral ossification – Hyaline cartilage template of bone forms
Cartilage - differentiates from mesenchyme cells
Chondroblasts - condenses - become rounded and deposit matrix - collagen fibers or elastic fiber
Three types of cartilage - hyaline (most common), fibrocartilage, elastic cartilage
Perichondrium - outer layer of cells
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Cartilage template of the limb in the Chick wing
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Endochondrial Bone
Primary ossification center - initiation of ossification
Perichondrial cells differentiate into Osteoblasts - deposit matrix as a collar in center of long bone – diaphysis
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Endochondrial BonePerichondrium becomes Periostium
Ossification spreads towards ends of bone
Osteoclasts differentiate and begin to breakdown bone
Chondrocytes die off – center is invaded by vascular system – the bone marrow.
Cells also invade and differentiate into osteoblasts -forming bone spicules that are remodeled by osteoclasts
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Bone Growth
Bone lengthening occurs at diaphyseal-epiphysealjunction - epiphysealcartilage plate (growth plate)
Epiphysis - chondrogenic
Secondary ossification centers in the epiphysis after birth
After growth termination the epiphyseal cartilage plate is replaced with spongy bone
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Skeletal DevelopmentMultiple Cellular Origins
1 - Paraxial MesodermSomite, SclerotomeAxial Skeleton (e.g. vertebra)
2 - Lateral Plate MesodermAppendicular Skeleton – (e.g. limb)
3 - Neural CrestHead Skeleton
Established as
1 - Hyaline Cartilage – replaced by Endochondrial Ossification
2 – Intramembranous Bone Formation - direct ossification
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Skull / Head
Neurocraniumskeleton around the brain
Viscerocraniumskeleton of the face
Both consist of two components: Membranous(Intramembranous ossification)Cartilaginous (Endochondrial ossification)
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NeurocraniumMembranous neurocranium
cranial vault = calvariaflat bones of skull
Sutures - fibrous joints between flat bones
Fontanelles - where several sutures meet
Moldling - bones are soft, sutures are loose – allows for changes during birth
Cartilaginous neurocranium – bones at the base of the skull
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Viscerocranium
Cartilaginous viscerocraniummiddle ear bones - incus, malleus, stapesreichert’s cartilage hyoid bone
Membranous viscerocraniumJaw Bones – maxilla, zygomatic, squamoustemporal bones, mandible
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CraniosynostosisPremature closure of sutures
Abnormal skull shape
Multiple causes:FGF signalingMsx gene function
Apert syndromeCrouzon’s syndrome
Pfeiffer’s syndrome
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FGF Receptor (FGFR) mutations cause craniosynostosis
Autosomal dominant – abnormal dimer function
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Skeletal DevelopmentMultiple Cellular Origins
1 - Paraxial MesodermSomite, SclerotomeAxial Skeleton (e.g. vertebra)
2 - Lateral Plate MesodermAppendicular Skeleton – (e.g. limb)
3 - Neural CrestHead Skeleton
Established as
1 - Hyaline Cartilage – replaced by Endochondrial Ossification
2 – Intramembranous Bone Formation - direct ossification
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Vertebral Column
Three parts to each vertebra - body, vertebral arch, ribs
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Sclerotome cells form a mesenchyme that chondrofiesaround the notochord to form the centrum
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Development of Vertebra
Sclerotome - cells surround notochord on both sidescranial - loosely arranged cellscaudally - densely packed cells
Each vertebra is derived from two sclerotome segments Caudal (dense) cells from a cranial sclerotomeCranial (loose) cells from the next caudal sclerotome
Intervertebral disc between vertebra
Intervertebral disc forms at the interface between loose and dense cells (center of sclerotome)
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The centrum is the primordium of the body
Notochord degenerates in the center of body
Notochord expands in the intervertebral disc region forms the nucleus pulposus = gelatinous disc center
The nucleus pulposus is surrounded by fibrous tissue (concentric) - anulus fibrosus
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Development of Vertebra
Sclerotome cells surround the neural tube - forms the vertebral arch - fuses ventrally with the centrum
Sclerotome cells in the body wall form the costal processes, the ribs
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Primary ossification centers 1 - Surrounding the notochord in the centrum2 - Lateral to the neural tube in the vertebral arch
Secondary ossification centers 1 - anular epiphyses - between body and intervertebral disc)2 - tip of spinous process3 – tips of transverse processes
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Joints: neurocentral joint - centrum / vertebral arch - allows for growth of the spinal cord until 5 years
Costovertebral synchondrosis - vertebral arch / ribs synovial joint
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Cervical
Thoracic
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Coccyx
Sacrum
Lumbar
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Regional characteristics of vertebrae are specified by unique combinatorial expression of Hox genes
Homeotictransformations of vertebrae have been described
Retinoic Acid can cause cranial to caudal segment shifts
Hox Genes
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Ribs / Sternum
Sclerotome cells in the body wall form the costal processes that form the ribs
The Sternum forms from a pair of ventral cartilagenousbands that converge at the ventral midline
Converged sternal bands undergo secondary segmentation – similar to joint formation
Sternal segments later fuse
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Muscle DevelopmentMuscle types – Skeletal, Cardiac, SmoothSmooth muscle : Derived from splanchnic mesoderm
surrounding gut. Cellular elongation without cell fusion
Cardiac muscleDerived - splanchnic
mesoderm Myoblasts adhere
but do not fuseForm intercalated
discs
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Skeletal MuscleHead region skeletal musculature
Derived from head mesenchymeMigration from the cranial somitomeres
Trunk region skeletal musculatureMyoblasts derived from somites Migration - FGF controlledSpindle shaped cells - line up and fuse Multinucleated syncitiumMyofibrils with cross-striations - actin-myosin
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Region-Specific myoblast behavior
Limb Region – myoblastmigration into limb primordia, Differentiation is delayed
Thoracic Region – myotubes form at the somite – then invade the body wall to form the intercostal muscles
Lumbar Region – myoblastmigrate to form the abdominal muscles
Myoblast behavior is controlled by their environment
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Myotome: two partsEpimere Dorsomedial Extensors of Vertebral column Hypomere Ventrolateral limb/body wall
Innervating nerves – Dorsal ramus; Ventral ramus
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Thoracic level – 3 myogenic layers – external intercostal, internal intercostal, transversus abdominis muscles
Ribs maintain segmented musculature, elsewhere fusion large muscle sheets
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Determination of myoblast occurs very earlyKey regulators – Myf-5, Pax3, MyoD