CH 7 Bone formation and remodeling Flashcards
Ossification
Formation of bone a.k.a. osteogenesis
Two types of ossification
1. Intramembranous ossification
2. Endochondral ossification
Intramembranous ossification: stage, types of bones, starting material
-Embryonic stage
-Produces flat bones of skull, clavicle, part of mandible
-Starting material is embryonic mesenchyme
Stages of intramembranous ossification
- Deposition of osteoid tissue
- Calcification of osteoid tissue
- Mesenchyme condensing
- Formation of surface compact bone
Intramembranous ossification:
Stage 1: Deposition of Osteoid tissue
Mesenchyme condenses; mesenchymal cells line blood vessels and become osteoblasts; they secrete pre-bone
Collagenous osteoid tissue
Pre-bone; scaffolding of collagenous fibers
Intramembranous ossification:
Stage 2. Calcification of osteoid tissue
-Calcium phosphate and hydroxyapatite crystallize on collagen fibers
-Osteoblasts trapped in lacuna and become osteocytes
-blood vessels compact to central location
Intramembranous ossification:
Stage 3. Mesenchyme condensing
-Mesenchyme becomes periosteum around edges
-Formation of spongy bone (trabeculae)

Trabeculae
 honeycomb shaped Slender calcification in spongy bone; from blood vessels getting trapped
Intramembranous ossification:
4. Formation of compact bone at surface
Osteoblasts near periosteum deposit layers of compact bone; bone sandwich
Bone sandwich in Stage four of intramembranous ossification
Mesenchyme
Periosteum
Compact bone
Spongy bone
Compact bone
Periosteum
Mesenchyme
Endochondral ossification:
Develops from, bone types, plate?
-Cartilage to bone
-Most bones: long bones, vertebrae, ribs, sternum, pelvic girdle, etc.
-Epiphyseal plate on either side of bone

Epiphyseal plate
Growth zone surrounded by metaphysis
Stages of endochondral ossification
- Hyaline cartilage formation
- Formation of primary ossification center
- Infiltration of blood vessels
- Birth: formation of secondary marrow cavity
- Child: filling of spongy bone
- Late teen/adult: closing of epiphyseal plate
Endochondral ossification stage one: hyaline cartilage formation
-Mesenchyme develops into hyaline cartilage
-Surrounded by Perichondrium (chondrocytes)
Perichondrium
Fibrous outer shell that produces chondrocytes in stage one of endochondral ossification
Endochondral ossification
stage 2: formation of primary ossification center
-Chondrocytes inflate and die= Primary ossification center
-Perichondrium turns into Periosteum starts producing osteoblasts and develops bony collar
Primary ossification center and stage two of endochondral ossification
Lacuna join into one cavity; walls between calcify
Bony collar
Periosteum produces osteoblasts that create bone around highland cartilage
Endochondral ossification
Stage 3: vascularization of bone
-Blood vessels and osteoclasts invade primary ossification center (make Primary marrow cavity)
-Osteoblast deposit layers on bone
-Secondary ossification center as chondrocyte death push to epiphysis
-Metaphysis
Metaphysis
Transitional Cartlidge to bone area; growth zone
Endochondral ossification
Stage 4: formation of secondary marrow cavity (birth)
-Blood vessels infiltrate with osteoclasts and create secondary marrow cavity in epiphysis
-Osteoblasts continue laying out tissue; Primary marrow cavity and larges and creates diaphysis
Endochondral ossification
Stage five: epiphysis fills with spongy bone (child)
-Creation of spongy bone in secondary marrow cavity from osteoblasts
-Cartilage left only at articular joints and Metaphysis surrounding epiphyseal plates
Endochondral ossification
Stage six: closing of epiphyseal plate (late teen/adult)
-All remaining cartilage used up; gap closes into epiphyseal line
-Primary and secondary marrow cavity join together
In bone growth and remodeling, Which directions can bones grow?
Growth: length
Remodeling: width
Bone elongation
Occurs at epiphyseal plate With metaphysis on each side
Zones of metaphysis
1. Zone of reserve cartilage
2. Zone of proliferation
3. Zone of cell hypertrophy
4. Zone of calcification
5. Zone of bone deposition
Zones of metaphysis:
Zone 1, reserve cartilage
Closest to end of bone; has hyaline cartilage and chondrocytes
Zone of metaphysis:
Zone 2, Proliferation
Chondrocytes multiplying and dividing into lacuna
Zones of metaphysis:
Zone 3. Cell Hypertrophy
Chondrocytes get bigger and start to die
Zones of metaphysis
Zone 4. Calcification
As chondrocytes die, calcification of matrix around lacuna where chondrocytes are
Zones of metaphysis
Zone 5. Bone deposition
Full death of chondrocytes, breakdown of lacuna walls, bone put down by osteoblast, forming of trabeculae of spongy bone which Remains for life
Subject to lifelong remodeling; all zones in epiphysis turn into zone five
How does growth occur in adolescents?
Multiplication of chondrocytes in metaphysis zone two and hypertrophy of chondrocytes in zone three push zone one towards the end of the bone= elongation
Epiphyseal line
Epiphyseal plate is closed; no more cartilage to elongate
Appositional growth
Growth of bones in diameter and thickening along outer perimeter
Which kind of ossification provides appositional growth?
Intramembranous ossification
Bone remodeling
Continual process in which old bone is absorbed by osteoclasts and some osteocytes, while osteoblasts deposit new bone
Three R’s for remodeling
- Release minerals to blood
- Reshape bone in response to use/disuse
- Repair microfractures
Wolff’s Law
Architecture of bone is determined by the mechanical stress placed on it
More stress= more compact structuring by osteoblasts
Less stress= less compact from osteoclasts
