Bone Formation and Joints Flashcards
1
Q
Bone Formation
A
- All bone formed directly (intramembranous) or indirectly (endochondral) from mesenchyme
- Bone matrix laid down as osteoid then subsequently mineralized
- First bone formed is woven bone
- Woven bone modeled or remodeled to form lamellar bone
2
Q
Intramembranous Ossification
General
A
- Bone formed directly from mesenchyme
- Method of bone formation for most flat bones (skull and face) which are generally not weight bearing
- Accounts for bone formation by the periosteum of all bones
- Starts at 8 weeks gestation
3
Q
Intramembranous Ossification
Process
A
- Invasion of blood vessels into mesenchyme.
- Condensation and vascularization of mesenchyme at the ossification center.
- Differentiation of mesenchymal cells into osteoprogenitor cells.
- Osteoprogenitor cells differentiate into osteoblasts.
- Osteoblasts lay down osteoid.
- Osteoid mineralizes forming spicules of woven bone which eventually coalesce into trabeculae.
- Incorported osteoblasts become osteocytes.
- Surrounding mesenchyme continues to proliferate forming osteoprogenitor cells then osteoblasts.
- Spicules continue to grow via appositional growth.
- Osteoclasts introduced and begin to resorb areas of immature woven bone.
- Osteoblasts redeposit lamellar bone.
- Continued shaping of bones via modeling and remodeling.
- Fontenelles made of mesenchymal tissue persist through first years of life.
- Membranes of mesenchyme which surrounded the growing bone become the periosteum and endosteum.
4
Q
Endochondral Ossification
General
A
- Indirect bone formation from mesenchymal cells with cartilage intermediate.
- Occurs in long bones and irregular shaped bones.
- Starts at 12th week gestation
- Allows both interstitial (endochondral) and appositional growth (intramembranous)
5
Q
Endochondral Ossification Process
Primary Ossification Center
A
- Mesenchymal cells differentiate into chondroblasts which deposit a cartilage model of the bone.
- Complex feedback loop induces hypertrophy of chondrocytes at the primary ossification center within the diaphysis.
- Hypertrophic chondrocytes begin producing collagen type X.
- Hypertrophic chondrocytes induce cells of perichondrium to differentiate into osteoblasts.
- Osteoblasts form a ring of bone around the diaphysis called a bony collar.
- CT covering now called a periosteum.
- Chondrocytes die and surrounding cartilage matrix calcifies through alkaline phosphatase activity.
- Osteogenic bud (containing osteoprogenitor cells, hematopoetic cells, and capillaries) invades the center of the model into spaces vacated by dying chondrocytes.
- Osteoprogenitor cells around exterior of blood vessels differentiate into osteoblasts.
- Osteoblasts begin depositing bone inside the cartilage model forming mixed spicules of woven bone and calcified cartilage.
- Zone of ossification spreads proximally and distally in diaphysis as growth continues.
- Diaphysis turned into a hollow cylinder.
- Osteogenic bud persists as nutrient artery.
- Earliest spicules selectively resorbed and replaced with lamellar bone forming medullary cavity.
- Bone reshaped through remodeling to meet needs of the body.
6
Q
Endochondral Ossification Process
Secondary Ossification Centers
A
- Chondrocytes at the epiphyses hypertrophy forming secondary ossification centers.
- Proceeds similarly to primary ossification center starting with chondrocyte hypertrophy then atrophy.
- Eventually epiphysis become completely ossified except for the articular surface of the bone and the epiphyseal plate (growth plate).
7
Q
Epiphyseal Plate
A
Bone continues to increase in length through interstitial growth via endochondral ossification at the growth plate = metaphysis.
- Chondrocytes proliferate and form new cartilage matrix pushing epiphysis and diaphysis apart.
- Bone formed on the diaphyseal side of growth plate replacing dying cartilage.
- Results in the formation of zones within the epiphyseal plate of developing bone:
- Zone of reserve cartilage: area of resting cartilage with minimal mitotic activity
- Zone of proliferation: Rapidly dividing chondrocytes form rows of isogenous cells parallel to the direction of bone growth.
- Zone of hypertrophy: Chondrocytes mature, hypertrophy, and start dying.
- Zone of calcification: Chondrocytes die and cartilage matrix becomes calcified.
-
Zone of ossification: Osteoprogenitor cells invade and differentiate into osteoblasts which deposit bone on surface of calcified cartilage forming mixed spicules.
- Mixed spicules can later be resorbed as marrow cavity elongated or replaced with lamellar bone.
8
Q
Bone Growth
A
- Shaft increases in length by interstitial growth at epiphyseal plate.
- Shaft increases in width through appositional growth from periosteum.
- Bone reshaped:
- As old metaphysis becomes incorporated into diaphysis.
- To customize size and shape of each bone through continued periosteal and endosteal deposition and resorption = modeling
- Epiphyseal plate ossifies at cessation of bone growth post-puberty
9
Q
Bone Modeling
aka
Growth Remodeling
A
- Coordinated asymmetric bone deposition and resorption along periosteum and endosteum surfaces facilitate changes in shape.
- Reshaping of bone occurs in response to stresses placed on it.
- Primarily during during growth and development
- Examples:
- Metaphyseal “waisting” - old metaphysis incorporated into diaphysis
- Diaphyseal enlargement - increase bone diameter
- Diaphyseal drift - alter curvature
- Modeling of skull and facial bones
10
Q
Bone Remodeling
aka
Secondary Remodeling
General
A
- Continual process of bone turnover throughout life
- Two step process of bone resorption follow by bone replacement at a single site.
- Can occur intracortically forming Haversian systems
- Can occur at surface through replacement of lamellar bone at a single site
- Functions:
- Replace damaged or too heavily mineralized bone
- Optimize distribution/organization of bone
- Metabolic needs (calcium regulation)
- # of Haversian systems / mineralization of bone increases with age
- Responsive to local biomechanical stresses & systemic effects of overall activity levels
11
Q
Intracortical Remodeling Process
A
- Cutting cone formed by osteoclasts.
- Osteoblasts follow and fill in space around a centrally growing blood vessel forming closing cone.
- Growth reversal lines (aka cement lines) indicates where resorption stopped and deposition started.
- Concentric rings deposited from outside in gradually narrowing central vascular channel until Haversian system completed.
12
Q
Bone Aging
&
Osteoporosis
A
- Normal secondary remodeling balances resorption with deposition
- Process sensitive to hormonal changes
- Estrogen
- Parathyroid
- Calcitonin
- Can result in greater resorption than deposition causing net bone loss
- Thinner & more porous cortex
- Reduced strength
- Quality of bone decreases with age
- More mineralized bone brittle
- Disorganization of collagen fibers in organic component may reduce compressive or tensile strength
13
Q
Traumatic Fractures
A
Caused by accidental exceeding of normal range of loading to which bone is adapted.
14
Q
Pathological Fractures
A
Caused by normal loading of bone weakened by disease.
- Osteoporotic fractures
- Fractures secondary to removal of bone tumors
15
Q
Simple Fracture
A
- One break in the bone
- Fragments remain aligned
- Skin remains closed
16
Q
Comminuted Fracture
A
- Bone is broken in multiple places
- Skin remains intact
17
Q
Compound Fracture
A
- Skin is broken
- Wound open down to fracture site
- Increased risk for infection
18
Q
Bone Repair Process
A
Phase 1: Inflammation
- Fracture causes rupture of blood vessels in cortex, bone marrow, periosteum, and sometimes in adjacent soft tissues.
- Blood invades fracture sitre forming a hematoma which coagulates within a few hours.
- Influx of cells including fibroblasts, osteoprogenitor cells, and chondroblasts.
- Helps to stabilize the breakage area.
Phase 2: Soft Callus Formation
- Fibrous CT and cartilage gradually convert hematoma into temporary internal callus that ties bone fragments together.
- Blood vessels begin to regrow.
- External callus forms beneath the peristeum.
- Stage lasts 3-4 weeks.
Phase 3: Hard Callus Formation
- Temporary callus replaced by a primary bony callus consisting of woven bone.
- ~ 6 weeks to form and may be noticable for years
- Osteoclasts continue to removal necrotic bone
- Begins 3-4 weeks after injury and continues for 2-3 months until bony union
Phase 4: Remodeling
- Replacement of primary callus by a secondary bone callus made of lamellar bone.
- Woven bone removed by osteoclasts.
- Internal osteon remodeling occurs.
-
Reduction of callus and remodling as needed
- Excess bone removed from exterior of periosteal collar
- Restoration of medullary cavity
- Amount of time needed for complete healing greater than 1 year and dependent on:
- bone involved
- severity of fracture
- apposition of the ends
- stability of fracture ends
- nutritional state and age of individual
20
Q
Sutures
A
-
Sutures
- Joints between bones of the skull
- Classified as squamous or serrated
- Bones articulate by a zone of CT called a sutural ligament or membrane
- Retained unossified mesenchyme
21
Q
Syndesmoses
A
Syndesmoses
- Connected by a sheath of fibrous tissue
- Ex. interosseus membrane between the radius and ulna
22
Q
Gomphoses
A
Gomphoses
- Fibrous articulation
- Only exists between the tooth and alveolus
23
Q
Synchondroses
A
Synchondroses
- Temporary joints found during growth period e.g. growth plates
- Formed of hyaline cartilage
- Replaced once growth ceases by a bony union called a synostosis__
24
Q
Symphyses
A
Symphyses
- Bone surface covered with hyaline cartilage with a layer of fibrocartilage inbetween
- All symphyses located on the mid-line of the body
- Most confined to axial skeleton
- Intervertebral disks
- Manubrio-sternal joint
- Found in the appendicular skeleton only at pubic symphysis
- Most confined to axial skeleton
- Permit limited movement through deformation of fibrocartilage pad
- Usually reinforced by numerous ligaments running across the articulating bones and fibrocartilage
- May fuse with age after 20 y/o forming synotoses
26
Q
Synovial Joint
Components
A
Articular surfaces
Fibrous joint capsule
Synovial membrane
Synovial fluid
May also include :
articular disc or meniscus
intrarticular fat pads or labra
27
Q
Articular Surfaces
A
- Usually covered with hyaline cartilage (articular cartilage)
- Collagen fibers in articular cartilage arranged in arches for optimization of mechanical force distribution
- No nerves or blood vessels
- Nutrition derived from:
- Vascular net in synovial membrane around its periphery
- Synovial fluid
- Blood vessels underlying marrow spaces
28
Q
Fibrous Joint Capsule
A
Forms a cuff around the articular end of the bone forming the joint.
Perforated by articualr vessels and nerves.
29
Q
Synovial Membrane
A
- Lines the non-articular parts of all synovial joints, synovial bursae, and synovial tendon sheaths
- Lines the fibrous joint capsule but not over the articular cartilage.
- Contains two cell types:
- A cells
- Macrophage-like
- Removes debris from joint space
- B cells
- Resemble fibroblasts
- Thought to secrete components of synovial fluid
- A cells
- Underlying CT has rich vascular network
- No basal lamina between lining cells of the membrane and underlying CT
32
Q
Bursae
A
- Fluid filled sacs which tend to surround joints near the areas where tendons insert into bones
- Inflammation or infection in the joint capsule will often spread to the surrounding bursae = bursitis
