L1 - Bone Flashcards
Describe how bones are organs comprised on many tissues
Nerves innervate the periosteum and bone
Blood vessels run in canals within bone
Cartilage covers articular surface
Connected to muscles via tendons and to other bones via ligaments
Houses bone marrow cavities containing blood forming cells and fat cells
Describe bone as a CT
Strong and fracture resistant
Comprised of cells surrounding or embedded in a mineralized EXM
Impregnated with inorganic material but it is a living and dynamic tissue
Formed from mesenchymal SCs
Well vascularized
What is the function of bone (as organs)?
Provides a structural framework for the body (support), acts as levers for skeletal muscles (movement), protection of underlying organs, reservoir for minerals including 99% of the body’s Ca and blood cell formation
What are osteoprogenitors?
Mesenchymal stem cells that produce osteoblasts
Found in periosteum and endosteum
What do osteoblasts from from and where are they found?
Mesenchymal stem cells and are only found on the bone surface
What is the function of osteoblasts?
Produce bone matrix/osteoid
Initiate and control mineralization of osteoid
What is the organic component of bone matrix?
Type I collagen and proteoglycans
What is the inorganic component of bone matrix?
Ca phosphate and hydroxyapatite crystals (mineral salts) lying between fibers
What are the different fates for an osteoblast?
Become an osteocyte embedded in bone
Become a bone lining cell (flat resting cell on the surface)
Or die by apoptosis
What are osteocytes?
Mature osteoblasts trapped in bone matrix
Located in lacuna
Have spider like long processes that extend in canaliculi
What is the purpose of the processes found on osteocytes?
Allow them to communicate with other cells by gap junctions
Receive nutrient flow via canaliculi
What are the functions of osteocyte cells?
Communicate with osteoblasts to increase deposition of bone matrix
Maintain bone health (mechanosensory cells and control activity of osteoblasts as well as osteoclasts)
Maintain bone matrix
What are osteoclasts differentiated from?
Fusion of monocytes
Describe osteoclasts
Multinucleated cell located on bone surfaces
Create Howship’s lacunae
Important endocrine target for regulating Ca levels
What is the function of osteoclasts?
Remove bone tissue (bone resorption) by degrading the matrix by secreting acids and proteases
Exhibit a ruffled border which resorbs bone through H+ and lysosomal proteins
What is an epiphysis?
Knobby regions at ends of bone
Compact bone is superficial with trabecular bone deep to the surface
Contains epiphyseal plate
What is a diaphysis?
Elongated shaft of long bone
What is a metaphysis?
Between the diaphysis and epiphysis of long bone
Adjacent to growth plate
Consists of spongy bone
What is a the epiphyseal line?
Within the metaphysis
Hyaline cartilage within children and adults
What is a medullary (marrow) cavity?
Cylinders also space in the diaphysis containing bone marrow
What is periosteum?
Dense irregular CT covering the external surface of bone
Does not cover articular cartilage
Neurovascular
Anchored by perforating fibers embedded in the bone matrix
Contains osteoblasts and osteoblast precursors
What is endosteum?
CT lining of all trabeculae and marrow cavity
Contains osteoprogenitor cells, reticular fibers of bone marrow and CT fibers
What is an osteon?
A cylindrical layered structure comprising of mature compact bone
Runs parallel to the diaphysis
Consists of concentric lamellae (bone matrix surrounding a central canal)
Describe an osteon
Has a central canal containing BVs and nerves providing osteocytes with a source of nutrition and O2
Osteocytes are arranged in concentric layers (concentric lamellae) surrounding the vessels
What are lacunae?
Where osteocytes reside within an osteon
What are canaliculi?
Channels for osteocyte processes and reach to the central blood vessel of the osteon
Interconnected by gap junctions with those of other osteocytes
What is the cement line?
Outer boundary of the osteon
What are circumferential lamellae?
Line entire inner and outer circumference of the diaphysis
What are interstitial lamellae?
Remnants of previous concentric lamellae
What are the two major types of bone?
Cortical (compact)
Trabecular (spongy or cancellous)
Describe cortical (compact) bone
Forms outer walls of bone
Primary structures: lamellar (layers) either as circumferential arrangement (osteon) or partial rings (interstitial lamellae)
Describe trabecular (cancellous or spongy) bone
Forms a network of bony plates or rods
No osteons
Proximal and distal ends of long bones
Can be woven or lamellar bone arrangement
What are the two types of bone arrangement?
Woven or lamellar
What is woven bone?
Immature or primary bone
Quickly laid down bone made of irregular and random arrangements of cells and loose collagen with lower mineral content and higher cell density
Usually first bone formed in response to fracture repair
Where is woven bone found?
In alveolar sockets and where tendons insert into bone
Describe lamellar bone
Mature
Laid down in highly organized layers with higher mineral content
Replaces woven bone during remodeling and in maintaining compact bone
What are the two types of ossification that can occur?
Intramembranous and endochondral
Describe intramembranous ossification
Osteoblasts differentiate directly from mesenchyme and begin secreting osteoid
No cartilage involvement
Which bones form via intramembranous ossification?
Flat bones of the skull (calvaria), most facial bones and mandible, central part of the clavicle
What is endochondral ossification?
Develops initially on a hyaline cartilage model
All bones except some bones of the skull and central clavicles
Complete in early childhood
Responsible for lengthening bones
Describe the process of endochondral ossification
- Bone collar forms around the diaphysis of the hyaline cartilage model
- Cartilage in the center of the diaphysis calcifies and then develops cavities
- The periosteal bud invades the internal cavities and spongy bone begins to form
- The diaphysis elongates and a medullary cavity forms as ossification continues
- Secondary ossification centers appear in the epiphysis
- The epiphyses ossify
What occurs in both intramembranous and endochondral bone formation?
Woven bone is formed first and then replaced by lamellar bone
What is appositional growth?
Only way to law down bone
Osteoblasts lay down a layer of bone matrix on top of another layer
Initial bone that is formed is woven which then matures into lamellar
Describe bone growth in diameter
Growth in circumference of long bones occurs via osteoblasts in the periosteum
Begins with formation of the bone collar on the cartilaginous diaphysis
Accompanied by enlargement of the marrow cavity by activity of osteoclasts in the endosteum
What is the purpose of bone remodeling?
Repair of micro damage
Ca metabolism: 500mg of Ca may enter or leave the adult skeleton each day
What is coupled activity?
Resorption by osteoclasts is followed by formation by osteoblasts
Resorption must be linked to formation in order to maintain bone
Describe remodeling in trabecular bone
Replaced every 3-4 years
Osteoclasts remove bone matrix and then signal osteoblasts to come in and deposit fresh new bone
Describe remodeling of compact bone
Replaced every 10 years
Osteoclasts tunnel through old bone forming a remodeling tunnel and signal for BVs to follow
Along with BVs osteoblast precursors (osteoprogenitors) flow
Osteoblasts begin depositing concentric rings of lamellar bone at the back end of the tunnel until the tunnel is filled back in leaving a central BV
Thus a new osteon is formed
Describe bone fracture repair
Torn blood vessels within fracture release blood -> clots and produces fracture hematoma
Hematoma removed by macrophages —> replaced by fibrocartilage like mass, soft callus
If the periosteum is torn, periosteum re-established continuity over soft callus
Procallus invaded by blood vessels and osteoblasts
Fibrocartilage is gradually replaced by woven bone —> forms hard callus
Woven bone is remodeled as compact and cancellous bone
Vasculature is re-established
What is the epiphyseal growth plate?
Hyaline cartilage found between the primary and secondary ossification centers
Damage to this plate during growth will disrupt lengthening of bone
What is the zone of proliferation?
Chondrocytes within this plate undergo mitosis and increase cell numbers forming stacks of chondrocytes
What is the zone of hypertrophy?
Chondrocytes deposit ECM spreading them apart from one another while they also increase in size (interstitial growth)
These cells produce factors that initiate vascular invasion of the cartilage which pushes the epiphysis away from the diaphysis thereby lengthening the bone
What is the zone of calcification?
Cartilage cells then calcify the surrounding matrix and die
What is the zone of ossification?
Invading blood vessels bring in osteoprogenitor cells that differentiate into osteoblasts and cover hardened cartilage with woven bone matrix
What is the correct order of zones at the epiphyseal plate?
- Zone of proliferation
- Hypertrophic zone
- Calcification zone
- Ossification zone
What is achondroplasia?
Autosomal dominant mutation in FGFR3 which is an important mediator of chondroblast proliferation
Most common form of dwarfism
Shortening of long bones, small mid face (defects in cranial base) and altered spinal curvature
What is osteoporosis?
Characterized by low bone mass due to resorption occurring faster than bone deposition
Occurs most often in women after menopause primarily due to estrogen loss but also in the elderly
Strong genetic component
What is estrogen replacement therapy for prevention of osteoporosis after menopause controversial?
Due to associated increased risks of heart attack, strokes and cancer
What is vitamin D required for?
Ca absorption
Deficiency leads to osteomalacia and rickets
What is osteomalacia?
Occurs in adults
Bones are inadequately mineralized and bones are soft
What is rickets?
Occurs in children - analogous to osteomalacia but is more severe
Weakened long bones, abnormal head and rib cage
What is an osteoclast’s fate?
Eventual cell death by apoptosis
What do active osteoblasts become?
Cuboidal mononuclear cells
