Bones Flashcards
Bone functions
Mechanical
-protect tissues/organs; provide framework; form bases of levers
Synthetic
-haemopoiesis
Metabolic
-mineral storage (calcium and phosphorus); fat storage (yellow bone marrow); acid-base homeostasis (absorbs or releases alkaline salts)
Cancellous bone
Network of fine bony columns or plates called trabeculae, spaces are filled by bone marrow.
Aka spongy bone.
Compact bone
Forms the external surfaces of names bones and comprises 80% of body’s skeletal mass.
Aka cortical bone.
Endochondral ossification
Formation of long bones from cartilage template; continued lengthening is by ossification of at epiphyseal plates.
Appositional growth (growth from middle to edges).
Intra-membranous ossification
Formation of bone from clusters of mesenchymal stem cells in the centre of bone.
Interstitial growth (growth in middle).
Appositional growth
Growth from middle to edges
Interstitial growth
Growth in the middle
Epiphyseal plates
Boundary between bone and cartilage
Osteoblasts
Cells that form new bone tissue. They synthesise and secrete bone matrix - collagen and ground substance
They are involved in bone formation and bone remodelling.
Osteoclasts
They break down bone using acid and lysosomes for bone remodelling. They absorb what they remove.
They are bigger, multinucleated and are usually found near a depression of bone.
Osteocytes
Former osteoblasts that have been trapped in bone matrix. They sit in lacunae.
When 2 osteocytes meet they form gap junctions.
They are involved in the regulation of bone remodelling and regulate mineral homeostasis.
They can act like osteoclasts and degrade the bone a little - called osteocytic osteolysis.
Lacunae
The extra-cellular matrix that surrounds a cell (eg osteocytes, chondrocytes).
Periosteum
Membrane outside the bone.
Mesenchymal tissue
Undifferentiated connective tissue in the bone marrow. They can differentiate into cells such as osteoblasts.
Osteoid
Made of mainly collagen 1 as well as hydroxyapatite. Secreted by osteoblasts.
Hydroxyapatite
Part of osteoid laid down by osteoblasts; contains the minerals calcium and phosphate.
Spicules
Bony spurs.
Trabeculae
Form when spicules join up. Rod-like structures making up bone.
Endostium
Where the bone touches the edge of periosteum.
Osteoprogenitor cells
Stem cells in bone that are precursors to more specialised cells, osteoblasts and osteoclasts.
Cement line
Where osteoid meets existing bone.
How is cancellous bone converted to cortical bone?
- Mesenchymal stem cells —> osteoblasts that line recently formed trabecuae.
- They lay down osteoid that is mineralised.
- Osteoblasts become trapped and turn into osteocytes.
- This repeats as concentric lamellae are laid down by osteoblasts.
- Central MSC convert into blood vessels, lymph vessels and nerves.
This occurs from the inside of bone and grows further inwards.
Lamellae
Lamellae laid down at 45 degrees to each other - strengthens.
Volkmann’s canals
The small channels in bone that transmit blood vessels from the periosteum into the bone and that lie perpendicular to and communicate with the Haversian canals.
Arrangement of osteocytes in mature bone
Osteocytes are arranged in concentric lamellae of osteons.
Structure of mature bone
Compact bone:
- Contains Haversian canals (blood, nerves and lymph vessels)
- Concentric lamellae of osteons
Cancellous bone:
- No Haversian or Volkmann’s canals present.
- Teardrop shape due to gravity.
Osteon
Cylindrical vascular tunnels in bone.
Haversian canals
Narrow tubes in bone formed by lamellae. They contain blood vessels, lymph vessels and nerves.
They run parallel to the bone.
The canals and surrounding lamellae are called a Haversian system (or an osteon).
Volkmann’s canals
Any of the small channels in bones that transmit blood vessels from periosteum into bone. They lie perpendicular to and communicate with the Haversian canals.
Bone remodelling
The process of breaking down and reforming bone.
Two steps:
1. Osteoclasts make a wide tunnel in the bone (cutting cone)
2. Osteoblasts make a smaller tunnel of cortical bone (closing cone)
Osteocytic osteolysis
*
Osteocytes act like osteoclasts and the degrade the bone a little. This is increased by PTH (parathyroid hormone).
Osteogenesis imperfecta
Aka brittle bone disease.
Due to mutated collagen fibres that don’t knit together.
Symptoms:
- weakened bones = increased risk of fracture
- short stature
- blue sclera
- hearing loss
- hyper mobility/ flat or arched feet
- poor teeth development
Collagen
Every third a.a is glycine, as it is the only one small enough to fit within the helical structure.
Faulty collagen production leads to osteogenesis imperfecta.
Rickets
Vitamin D deficiency that mainly affects children. Leads to poor calcium mobilisation and ineffective mineralisation.
Symptoms:
- weakened bone development
- soft bones
- shortened height and stature
- pain when walking
- characteristic bowed legs
Osteomalacia
Rickets in adults.
Vitamin D deficiency that leads to lower mineralisation (little to no hydroxyapatite deposits) and increased osteoid.
Increased calcium resorption.
Disease caused by several things; kidney disease, lack of sunlight, some surgeries and certain drugs.
Osteoporosis; 3 types
Primary - Type 1:
Occurs in post menopausal women due to increase in osteoclasts, which is caused by loss of oestrogen.
Primary - Type 2:
Occurs in older men/women due to loss of osteoblast function, caused by loss of both oestrogen and androgen.
Incomplete filling of osteoclast resorption bays.
Secondary:
Result of drug therapy; processes that affect bone remodelling; or metabolic bone diseases.
Achondroplasia
Inherited mutation in FGF3 gene.
FGF promotes collagen formation in cartilage => endochondrial ossification affected
Short stature as long bones cannot lengthen normally.
Types of bone
Long, short, flat, irregular, and sesamoid.
Long bones
Support weight of body and facilitate movement.
Femur, fibula, tibia, humerus, radius, ulnar etc
Short bones
Provide stability and some movement.
Wrist and ankle joint bones etc
Flat bones
Protect internal organs, provide large areas for attachment of muscles.
Skull, thoracic cage, pelvis etc
Irregular bones
Protect internal organs, vertebrae protect spinal cord, provide anchor points for muscle groups.
Vertebrae, sacrum etc
Sesamoid bones
Protect tendons from stress and damage from repeated wear and tear.
Small round bones eg patella
Bone marrow
(Medulla ossea)
Two types - red and yellow
Found inside bones.
Red marrow
Full of developing RBC; rich blood supply; only found in spongy bone.
Function: haemopoiesis.
Yellow marrow
Full of adipocytes; poor blood supply (explains yellow colour).
Function: shock absorber and energy source. Can convert to red marrow and therefore carry out haemopoiesis.
How do maturing cells leave the bone?
Sinusoids
Irregular tubular space for the passage of blood, taking the place of capillaries and venules in the liver, spleen, and bone marrow.
Incomplete basement membrane, with intercellular gaps between endothelium. Kupffer cells and other macrophages can fit through these gaps to travel.
Vasculogenesis
Formation of new blood vessels from angioblast precursors in bone marrow
eg embryo development, newly formed cancers, endometriosis
Angiogenesis
Formation of new blood vessels from existing blood vessels
eg during fetal development, collateral arteries, postnatal lung development
Collateral blood vessels
These are usually arterioles and provide protection for tissues that may become compromised (not enough O2).
They provide an alternate path for arterial blood flow.
- Can be generated due to chronic disease eg ischaemia, in coronary arteries (atherosclerosis)
- Some produced during development eg brain and joints
Sprouting angiogenesis
FGF produced by mesenchymal cells; pericytes convert into smooth muscle cells.
Slow - takes hours to days
Intussusceptive angiogenesis
Twinned vessels from primary vessel (splitting one into two).
Needs multiple growth factors.
Quick - takes minutes to hours.
Pericyte
Immature smooth muscle-like cell.
Key component of capillaries.
Have contractile properties.
Involved in nerve cell communication.
Differentiate into endothelial cells, smooth muscle cells or fibroblasts.
Function: prevent endothelial cell proliferation; maintains tight capillaries eg blood-brain barrier, retina.
Calcification
Deposition of calcium.
Mineralisation
Deposition of hydroxyapatite in bone matrix.
Bone fracture healing
Haematoma formation
Fibrocartilaginous (soft) callus formation
Bony callus formation
Bone remodelling
(endochondral ossification)
