Bone Flashcards
Wolff’s Law
- not really a strict law, but..
- laid the foundation for our understanding of bone as a mechanically optimising tissue
- In general: bone is added where strain is high and removed where strain is low (i.e. modelling), resulting in structures matching strain fields of the organ
- The idea is that bone attempts to ensure that it is neither under- nor overstrained
- Bonea adapts to load
Typical Long Bone
(Bone Organs)
- Diaphysis (made of compact bone)
- Epiphyses
- Metaphyses (ends at physis of growth plate)
- Cortex
-Compact, cortical bone
•Medullary cavity
- Marrow
- Trabeculae, cancellous bone
•Periosteum covered (dense connective tissue that wraps around the outside of the bone) -> has blood vessels and nerves (painful when breaks)
- covers everywhere, but the joint surface
- Except articular regions: HAC
•Very vascular
-Nutrient foramen: long bones have one or more through diaphysis (artery or arteriole)
- The middle of diaphysis (medullary cavity) has yellow marrow (fat, adipose tissue)
- In younger animals, and in parts of adult animals (the sternum) has hematopoietic red bone marrow- where blood cells are made
- bone marrow transplant–> want to get out red bone marrow for chemo patients
Cortical Bone
- dense bone
Trabecular Bone
- spongy bone or cancellous
Bones Functions
(Multiple, regional, Functions of Bones)
- Resist tension, compression, bending, torsion, shear
- bones are the fulcrum in a lever arm
- Articulations (provide sites for attachment)
- Joint surfaces
- Muscle action
- Tendon, ligament and joint capsule attachment
- long Fatigue life (deal with load over and over) and high monotonic strength (stength under a single impact)
- Space for other organs (skull, ribs)
- Air filter & transfer surface (nasal turbinates)
- Transmit vibration (malleus, incus, stapes)
Bone Function
(Metabolism)
- Haematopoiesis (make new blood cells)–> occurs in marrow of long bones
- Calcium store (most of calcium in body is in your bones)
Modelling
•Bone deposition and/or resorption with a net change in organ shape
- shape of the bone tissue change
- mechanical stress env’t, more bone where there is stress, less bone where there is less stress
- bone matches the stresses
•Evens out stresses
- Modeling is when bone resorption and bone formation occur on separate surfaces (i.e. formation and resorption are not coupled).
- Response according to stress on the bone
- This improves monotonic strength
- happening all the time–> ex: tennis arm, responds to activity by modelling. Diaphysis will get thicker
- stress field is not even so bone is trying to make it even
Remodeling
*Cells are the same (osteoclasts and osteoblasts)
•Bone resorption (osteoclasts) followed by deposition filling the resorption pit
-osteoclasts dig a hole, osteoblasts ill it back up
•Repairs fatigue damage
Bone remodeling (or bone metabolism) is a lifelong process where mature bone tissue is removed from the skeleton (a process called bone resorption) and new bone tissue is formed (a process called ossification or new bone formation).
-this improves fatigue life
“Rubber” Bone
- place in vinegar
- left with all the bone except the mineral composition
In contrast:
baking a bone rids of the collagen matrix and leaves just the minerals where it can crumble on impact
Bone Matrix
*Composite of
- Type I collagen (35%, tensile E)–> great with tensile strength, why you find it in tendons/ligaments
- Hydroxyapatite mineral (60%, compressive E)–> good at resisting compression
-two together gives composite behavior: need to resist compression and tension
- Water (4%)
- Proteoglycan (1%)
*Osteoid (organic portion of bone matrix, no mineral in it) is as above but water not mineral, what osteoblasts actually make.
-jelly like substance where the water portion is replaced by mineral over time
Osteoblasts
•Secrete unmineralised bone matrix, osteoid
-On bone surface (fills in the hole)
•Derived from mesenchymal stem cell: like a fibroblast (MSC’s are generally found in bone marrow)
-lineage: more like fibroblasts, cartilage, lipid adipocytes
•Bury themselves in osteoid and become osteocytes
Osteocytes
•Adult cell of mature bone
-resorbing and forming bone around them
- Maintain local bone matrix mineralisation
- Sense mechanical load (controversial)
- lie within an osteon
- processes communicate with neighboring osteocytes
- osteon is very vascular (lumen) and cellular
Osteoclasts
•Resorb bone
-“dig holes”
•Derived from _haematopoietic tissue (_red bone marrow): like a macrophage (WBC’s)
–> not a WBC but in that family
**High degree of communication among bone cells
Osteocyte structure
- Cell body in lacuna
- Very many long cell processes in canaliculi
- Narrow (80nm) interstitial space around OC
- Assumed to be bone’s mechanosensor
- Maybe through fluid flow, or tugging of matrix on processes
- Detect microdamage
- Apoptosis
- Resorb mineral
- Osteocytic osteolysis->They destroy bone through a rapid, transient (relative to osteoclasts) mechanism called osteocytic osteolysis
- Ca / P homeostasis -response to drop in calcium or rise in blood calcium. Too little calcium, start resorbing bone to release it
- Signal to OB and OCl
Osteoblast Structure
- sit on surface
- Cuboidal cell
- Secrete osteoid
- Unmineralised gel deposited on bone surface
- Gel accumulates hydroxyapatite – kickstarted by matrix vesicles (outbuddings of osteoblast cell surface, kick start matrix mineralization process)
- Buries itself in osteoid, becoming osteocyte
- Signal to osteoclasts & their precursors
- M-CSF, Rank signalins systems
–> osteoblasts can tell osteoclasts to form
–>Stimulate osteoclast differentiation and resorption
–>Coordinating resorption/formation
–>OB osteoprotegrin blocks Rank ligand binding, inhibits Osteoclast activation
- Form membrane and network with OC processes
- Separates bone extracellular fluid from marrow
- Control of Ca2+ and HPO42- flow
