Cellular Structure Of Bone Flashcards
What are the 5 functions of bone?
β Support and movement β Protection for internal organs β Bone marrow store β Mineral reservoir β Endocrine
Where is cortical compact bone found?
β On the outside
How is cortical bone organised?
β In sheet like structures
β Surrounding a hollow structure called the Haversian canal
What are the repeating units of cortical bone?
β Osteons
What are lacunae?
β cell sized holes within the bone matrix
What are the function of Haversian canals?
β Allow for blood vessel penetration
Where is trabecular bone found?
β Near the head of the bone
What is the structure of trabecular bone?
β spongy
β interlocking struts with spaces inbetween
What is the function of trabecular bone?
β Makes bone lighter
What is the composition of bone in %s?
β Protein : organic osteoid matrix 25%
β Mineral : 75%
β Cells
What is the function of the organic protein matrix?
β Gives flexibility and tensile strength
What is bone mineral?
β Hydroxyapatite
β Calcium and Phosphate
What is the function of bone mineral?
β Gives bone rigidity and compressive strength
What are the 4 types of bone cells?
β Osteoblasts
β Osteoclasts
β Osteocytes
β Bone marrow cells
What are the two subtypes of bone marrow cells?
β Mesenchymal cells
β Haematopoietic cells
What do mesenchymal stem cells make?
β Osteoblasts and osteocytes
Where are osteoblasts found?
β Lining the inner surface of bone
Where are osteocytes found?
β Encased within the bone matrix
What do haematopoietic stem cells give rise to?
β Lymphoid lineage - lymphocytes
β Myeloid lineage - RBC and macrophages
What are the 3 functions of osteoblasts?
β Bone forming cells
β Secrete osteoid collagen matrix of bone
β Promote mineralisation of the osteoid
What are the 2 functions of osteoclasts?
β Bone reabsorbing - digest old bone
β Dissolve bone mineral and have enzymes to digest the organic matrix
What is the life cycle of osteoclasts controlled by?
β Apoptosis
What is a protease that osteoclasts secrete?
β Cathepsin K
What do osteoclasts secrete?
β Acid and proteases that are active in acidic environments
What do osteoclasts look like?
β They are large and multinucleate
What are osteocytes?
β Terminally differentiated osteoblasts
What do osteocytes do?
β They lay down bone matrix (organic material that becomes mineralised)
How do osteocytes extend?
β Via multiple dendrites via minute canals in the bone matrix
What is the function of the lacunocanalicular system?
β Maintains communication between the bone surface and blood vessels
How is bone continually renewed?
β Old bone is continually resorbed and new bone is laid down
β Cycle between osteoclasts digesting old bone and osteoblasts laying down new bone
How does bone remodelling occur?
β The leading edge is where cells differentiate into osteoclasts
β Behind that the new osteoblasts differentiate into new bone
β As the osteoblasts lay down the new bone some of them will stay in it where they are encased in lacunae
What are the 3 phases of bone remodelling?
β Activation : promotion of differentiation of new osteoclasts
β Resorption : duration of osteoclast activity
β Reversal : osteoclast apoptosis, terminating activity
What are the 3 factors that control bone remodelling?
β Load bearing exercise
β Cytokines and local signals
β Endocrine
What is the role of estrogen in bone remodelling?
βinhibits osteocyte apoptosis
β Promotes osteoclast apoptosis
β essential for skeletal health
What does process does estrogen favor?
β Formation over resorption
What is osteoporosis?
β Loss of bone mass above a certain threshold
In what gender is osteoporosis more common in?
β Women
Why is osteoporosis more common in women?
β dramatic loss of estrogen due to menopause
What is delayed skeletal growth in men linked to?
β Aromatase deficiency
What is RANK?
β A surface receptor on pre-osteoclasts that stimulates osteoclast differentiation
What is the RANK ligand produced by and what does it do?
β Produced by pre-osteoblasts, osteoblasts and osteocytes
β binds to RANK and stimulates osteoclast differentiation
In what two forms can the RANK ligand exist in?
β Surface bounds
β Soluble form that is secreted from osteocytes
What does the RANK receptor binding promote?
β osteoclast differentiation
What is OPG?
β A soluble protein produced by osteocytes
What does OPG do?
β Acts as an antagonist
β binds to RANK ligand and prevents it from binding to the receptor
What ratio controls osteoclast differentiation?
β Ratio of RANK ligand to OPG
What are the steps for activating Wnt?
β The frizzled receptor
β and the co-receptor called LRP5/6 have to interace
What is the effector of the Wnt pathway?
β beta catenin
What happens after the Wnt pathway is activated?
β beta catenin is translocated to the nucleus where it causes transcription
What is the Wnt pathway regulated by?
β DKK and Sclerostin
Where is sclerostin expressed and what does it do?
β Expressed in osteocytes
β stops osteoblast differentiation
What do OPG and NO do?
β Inhibit the differentiation of osteoclasts
What does the RANK ligand promote?
β Promotes the differentiation of osteoclasts
What do PGE2, NO and ATP promote?
β Differentiation of osteoblasts
What do sclerostin, DDK and SFRP1 inhibit?
β Inhibit the differentiation of osteoblasts
What is a common bone disease?
β Osteoporosis
What is a rare bone disease?
β Osteomalacia
What is sclerosteosis caused by?
β Mutation of the SOST gene
β Inactivating sclerostin protein
What is osteopetrosis caused by?
β Inactivating the RANK ligand
β overgrowth of bone
What 2 changes occur during osteoporosis?
β Thinning of the cortical bone
β Widening of the trabecular bone
