Skeletal System

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Skeletal System
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Bone development

• Bone growth
• 6 weeks after fertilization- 25yrs
• Regulated by hormones nutrition
• Growth hormone
• Sex hormones
• Thyroxine
• Stimulates Bone growth
• Parathyroid
• Stimulates osteoclast osteoblast activity
• Calcitonin
• Inhibits osteoclast
• 2 main categories
• Osteogenesis
• Maintenance replacement in adults

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Bone growth

• Ossification
• Formation of bone
• Replacement of existing tissue with bone
• Fibrous CT or cartilage, mesenchymal tissue
• Calcification
• Deposit of calcium salts w/in tissue
• Any tissue

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Ossification
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Calcification
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Ossification Types

• Intramembranous ossification
• Aka dermal ossification
• Derived from mesenchyme or fibrous connective
  tissue
• Embryonic bone development, chronic mechanical
  stress, CT injury
• Endochondrial ossification
• Bone replaces existing cartilage framework
• Joints

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Intramembranous Ossification
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Endochondrial Ossification
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Intramembranous Ossification

• Mesenchymal cells (aka stem cells) develop into
  osteoblast
• Embryonic in developing fetus
• Fibrous connective tissue in adult
• Bone cartilage develop in existing embryonic
  connective tissue
• Remnant mesenchymal cells persist in adult tissue

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Intramembranous ossification

• Dermal bones aka membrane bones
• Roofing bones of skull (frontal, parietal)
• Mandible
• Clavicle
• Patella
• Sesamoid bones
• Typically develop deep in dermis
• Membrane bones may develop in other CT under
  chronic mechanical stress
• Ex Heterotropic bones

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Typically Develops in Dermis
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Dermal Bones
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Sesamoid Bones
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Sesamoid Bones
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Intramembranous Ossification

• Regulation
• Fetus- hormonal cues
• Mechanical stress
• Connective tissue injury
• Bone Injury
• Maintenance

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Fetal Skeleton Development
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Chronic Mechanical Stress
Dermal bones develop in CT bone spurs
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Bone Spur
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Heterotopic BoneConnective Tissue Injury
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Injury Sites
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Intramembranous Ossification

• Vascularization Blood supply to tissue
• Differentiation Mesenchymal cells proliferate,
  aggregate differentiate into osteoprogeniter
  cells
• Osteoprogenitor cells will differentiate to
  supply osteoblasts
• Matrix Secretion Clustered osteoblast secrete
  matrix (collagen fibers ground substance)
• Ossification center
• Trapped osteoblasts differentiate into osteocytes
• Calcium salts from blood supply calcify osteoid
• Spicule Formation Spicules form out from
  ossification center
• Spongy Bone Formation Mesenchymal cells
  continually differentiate forming multiple
  ossification sites
• Ossification proliferates in areas of
  vascularization
• Spicules from adjacent ossification sites join to
  form Trabeculae
• Remodeling Conversion of spongy bone to compact
  bone by osteoclasts
• Formation of Haversion Canal System

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Developing Cranial Bones
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Mesenchymal Tissue
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Mesenchymal Tissue Becomes Vascularized
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Mesenchymal Cells Aggregate and Differentiate
into Osteoprogenitor Cells
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Mesenchymal Cells Aggregate Differentiate into
Osteoprogenitor Cells
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Osteoprogenitor Cells Differentiate into
Osteoblast
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Clustered Osteoblasts are Ossification Center
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Ossification Center
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Clustered Osteoblast Secrete Matrix
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Osteoblast Secrete Matrix
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Trapped Osteoblasts Mature to Osteocytes
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Spicules Form out from Ossification Center
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Mesenchymal Cells Continually Differentiate
Forming Multiple Ossification Sites
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Mesenchymal Cells Continually Differentiate
Forming Multiple Ossification Sites
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Spicules of Deposited Bone Form from Multiple
Ossification Centers
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Multiple Ossification Sites Form Bone within
Mesenchymal Tissue
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Spicules from Multiple Ossification Centers Join
to Form Spongy Bone
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Spicules Form Trabeculae of Spongy Bone
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Spicules Form Trabeculae
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Ossification Continues in Areas of Vascularization
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Ossification Continues in Areas of
Vascularization
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Spongy Bone Converted to Compact Bone
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Osteoclast Remodel Spongy Bone to Compact
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Bone Remodeling
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Forms Haversian System
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