The Cellular Structure Of Bone Flashcards by Carys Hooper

What are the functions of bone?

Structure, mechanical, protective, metabolic

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What is the structural function of bone?

Give structure and shape to the body

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What is the mechanical function of bone?

Sites for muscle attachment

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What is the protective function of bone?

Vital organs and bone marrow

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What is the metabolic function of bone?

Reserve of calcium and other minerals

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What are the proportions of inorganic and organic bone?

65% inorganic, 35% organic

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What makes up the inorganic part of bone?

Phosphorus, sodium, magnesium, calcium hydroxide

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What makes up the organic part of bone?

Bone cells and protein matrix

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What are the types of anatomical bones?

Flat, long, short, irregular and sesamoid

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What are examples of flat bone?

Sternum/ skull

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What is the function of flat bone?

Protective

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What is the distinguishing structure of long bones?

Longer than they are wide

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What is the function of long bone?

Allow us to make big movements

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What are some examples of short bones?

Carpals in hands or tarsals in feet

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What is the function of short bone?

Provide stability in joints and allow us to make smaller movements

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What is the distinguishing feature of short bone?

As long as they are wide

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What is the function of irregular bones?

Complex shapes that protect specific organs like the pelvis or vertebrae

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What are sesamoid bones?

Embedded in muscle or tendons and act like a pulley to reduce stress

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What are the two macroscopic types of bone?

Trabecular and cortical

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What are the features of trabecular bone?

Thin, honeycomb-like 
Highly metabolic
Adds strength
15-25% calcified
Large SA

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How much of the skeleton is made up of trabecular bone?

20%

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Which bit of the bone is made up of trabecular bone?

The axial bits (Epiphysial)

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What are the features of cortical bone?

Thick bone that makes up the shaft of the bone

80-90% calcified

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What are the functions of cortical bone?

Mainly structural, mechanical and protective

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Which bit of the bone is made up of cortical bone?

Shaft

How much of the Skeleton is made up of cortical bone?

80%

What are the two types of microscopic bone structure?

Woven and lamellar

When is woven bone usually around?

During initial bone development

Is woven bone mature or immature?

Immature

Is lamellar bone mature or immature?

Mature

In what type of bone does endochondral ossification happen in?

Long bones

In what type of bone does intramembranous ossification happen in?

Flat bones

What is intramembranous ossification?

Direct differentiation of osteoblasts from connective tissue

What is endochondral ossification?

Bones forming from a cartilage model

What bones fuse together last (and at what age)?

Clavicle growth plates at 20 yrs

How does endochondral ossification work?

Cartilage model is made in the womb. Blood vessels invade the scaffold, bringing precursors for bone cells Precursors differentiate into bone (called primary ossification centre) First occurs in the diaphysis and moves towards the epiphysis Secondary ossification centre formed in the epiphysis

When is bone growth complete?

When the primary and secondary ossification centres fuse together

What do growth plates allow for?

Rapid linear growth

What are growth plates made of?

Cartilage which contains chondrocytes organised into a structure with three distinct layers

What are the three layers of growth plates called?

Reserve zone, proliferative zone and hypertrophic zone

What does the reserve zone contain?

Stem cells

What happens in the reserve zone?

Stem cells proliferate very slowly to maintain their population

How close is the reserve zone to the blood supply?

Very

What happens in the proliferative zone?

Chondrocytes become highly proliferative and form distinct column-like structures. As more get made, they move further away from the blood supply, causing hypertrophic differentiation

What happens in the hypertrophic zone?

Chondrocytes undergo apoptosis, leaving behind a mineral matrix which then calcifies and is ossified by bone cells

Do cells move in the growth plate?

What are the three types of bone cells?

Osteocytes, osteoclasts and osteoblasts

Which are the mature bones cells?

Osteocytes

Which are the immature bone cells?

Osteoblasts

What do osteoblasts do?

Produces osteoid to form new bone

What do osteoclasts do?

Secrete acids and enzymes that resorb/remove bone

How are osteoclasts formed?

Fusion of macrophages

How do osteoclasts work?

Sealing off a portion of bone beneath them by forming an actin ring

What are osteocytes?

Mechanosensory network embedded in the lacunae of mature bone

How are osteocytes connected?

Processes through cannilicular channels

What are the three overall steps in bone remodelling?

Microfracture Resorption Formation

What is a microfracture?

Tiny cracks forming in bone from stresses and strains in older bone tissues

What detects microfractures?

Osteocyte mechanosensory network

What happens when the Osteocyte mechanosensory network senses a microfracture?

Signals to the osteoclast to come in and resorb the old, damaged bone

What happens in the resorption step in the bone remodelling cycle?

Osteoclasts differentiate at the microfracture and resorb away the old bone and then dedifferentiate into individual mononuclear cells

What happens in the formation step of bone remodelling?

Osteoblasts secrete osteoid to repair the resorbed bone

When does bone disease happen?

Either when the formation doesn't return the bone to its original width or when there is excess formation

What would cause the formation to not return the bone to its original width?

Excess resorption or not enough formation

What would cause excess bone formation?

Either due to insufficient resorption or excess formation

What is it called when bone formation doesnt return to its original width?

Bone loss or osteoporosis

What's it called when there is excess bone formation?

Bone gain or osteosclerosis

What are the endocrine controls of bone remodelling?

Estrogen, thyroid hormone, PTH

What are the paracrine controls of bone remodelling?

RANKL, Wnt signalling

What is RANK receptor activation required for?

Osteoclast differentiation and survival

What is RANK ligand produced by?

Osteocytes and osteoblasts

How does RANK ligand work?

Binds to RANK receptor causing the signalling cascade resulting in the fusion, differentiation and activation of the osteoclast

What does OPG stand for?

Osteoprotegerin

What is OPG?

Decoy receptor for RANKL also produced by osteocytes and osteoblasts

What is an example of a synthetic OPG?

Denosumab

What does denosumab work to do?

Prevent bone loss by osteoclasts

What does wnt signalling stimulate?

Osteoblast differentiation

What does wnt signalling inhibit?

Sclerostin and DKK-1

How does wnt signalling work?

Wnt binds to the frizzled receptor and sets off a signalling cascade that results in the translocation of beta Katenin causing a change in gene expression and leading to osteoblast differentiation

What does the frizzle cell membrane receptor have to be bound to for it to work?

LRP 5 and 6

What do osteocytes secrete?

OPG and sclerostin

What is osteopetrosis?

Excess bone formation

What are the two situations systemic osteopetrosis could occur in?

LRP5 activating mutations: Van Buchems and SOST

What is the non-systemic reason osteopetrosis would occur?

Locally at the site of prostate metastasis

What is van buchems?

No production of sclerosis by the osteocytes so excess osteoblast differentiation

What are the characteristics of van buchems?

Enlarged mandible, skull and generalised bone overgrowth

What are the symptoms of van buchems?

Nerve palsies and headaches because of the increased intercranial pressure

What is the definition of osteoporosis?

Bone mass < 2.5 standard deviations below average peak bone mass

How many people does osteoporosis affect?

1:2 women and 1:5 men

What is primary osteoporosis caused by?

Menopause or aging

What is secondary osteoporosis caused by?

Drugs, disease or lifestyle

How does the menopause cause osteoporosis?

Loss of Estrogen causes a temporary increase in osteoclast activity causing a sudden loss of bone mass that lasts until the body gets used to the new oestrogen levels