DEVELOPMENT OF SKELETAL SYSTEM
BONE DEVELOPMENT
After Fertilization- ~25yrs
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Regulated by hormones & nutrition
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Sex hormones
Thyroxine
• Stimulates Bone growth
Parathyroid
• Stimulates osteoclast & osteoblast activity
Calcitonin
• Inhibits osteoclast
2 main categories
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Osteogenesis
Maintenance & replacement in adults
Bone growth
OSSIFICATION
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Growth hormone
Formation of bone
Replacement of existing tissue with bone
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Fibrous CT or cartilage, mesenchymal tissue
Calcification
– Deposit of calcium salts w/in tissue
– Any tissue
OSSIFICATION
OSSIFICATION OF BONE
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Ossification (or osteogenesis) is the process of laying
down new bone material by cells called osteoblasts. It is
synonymous with bone tissue formation.
Two types of ossification
 Intramembranous Ossification
 Endochondral Ossification
INTRAMEMBRANOUS OSSIFICATION
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Intramembranous ossification mainly occurs during formation of
the flat bones of the skull but also the mandible, maxilla, and
clavicles; the bone is formed from connective tissue.
The steps in intramembranous ossification are:
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Development of ossification center
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Calcification
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Formation of trabeculae
Development of periosteum
INTRAMEMBRANOUS OSSIFICATION
INTRAMEMBRANOUS OSSIFICATION
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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.
INTRAMEMBRANOUS OSSIFICATION
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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
INTRAMEMBRANOUS OSSIFICATION
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REGULATION
– Fetus- hormonal cues
– Mechanical stress
– Connective tissue injury
– Bone Injury
– Maintenance
INTRAMEMBRANOUS OSSIFICATION
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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.
ENDOCHONDRAL OSSIFICATION
Secondary center of ossification:
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Appears in each end (epiphysis) of long bones.
The cartilage between the primary and secondary ossification centers is
called the epiphyseal plate.
Growth continues until the individual is about 21 years old or until the
cartilage in the plate is replaced by bone.
The point of union of the primary and secondary ossification centers is
called the epiphyseal line.
STAGES IN ENDOCHONDRAL OSSIFICATION
ENDOCHONDRAL OSSIFICATION
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Endochondral ossification, on the other hand, occurs in long bones and
most of the rest of the bones in the body; it involves an initial hyaline
cartilage that continues to grow.
The steps in endochondral ossification are:
Development of cartilage
Growth of cartilage
Development of the primary ossification center
Development of the secondary ossification center
Formation of articular cartilage and epiphyseal plate.
ENDOCHONDRIAL OSSIFICATION
DEVELOPMENTAL
CLSSIFICATION
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Long, short, and irregular bones develop by endochondral
ossification, where cartilage is replaced by bone.
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Flat bones develop by intramembranous ossification,
where bone develops within sheets of connective tissue.
GERM LAYERS
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A germ layer, occasionally referred to as a germinal epithelium, is a
group of cells, formed during animal embryogenesis. Germ layers are
particularly pronounced in the vertebrates.