Bones Flashcards
What type of tissue is bone?
A specialised form of connective tissue
What are the cellular components of bone?
- Osteoblasts
- Osteocytes
- Osteoclasts
What is the function of osteoblasts?
- They synthesise unmineralised extracellular matrix called osteoid which will later be mineralised to form bone
What are osteocytes and what is their function?
- They are formed when osteoblasts become trapped in mineralised osteoid
- They are situated between lamellae in lacunae
- They regulate the deposition of minerals in the bone matrix
- They regulate bone remodelling by sensing mechanical workload and triggering osteoblasts or osteoclasts
What are osteoclasts and what is their function?
- Derived from monocytes and resorb (break it down) bone by releasing H+ ions and lysosomal enzymes
- Large and multinucleated
What is the extracellular matrix in bone?
- Molecules that provide biochemical and structural support to the cells
- Highly specialised - collagen and other proteins present plus mineral salts
- Matrix is organised into numerous thin layers known as lamellae
What makes the collagen in bone especially strong and rigid?
- Calcium hydroxyapatite crystals associate with the collagen fibres to strengthen them
What is immature (primary) bone?
- appears in embryonic development and fracture repair
- consists of osteoid and randomly arranged collagen fibres
- temporary structure as remodelling occurs to form mature bone
- resorption canals are the starting point of remodelling
What is mature (secondary) bone?
- bone of adult skeleton
- consists of highly organised sheets of mineralised osteoid
- organisation makes it stronger than immature bone
- can be further divided into 2 types - compact and spongy
What is peristeum?
An outer layer of connective tissue that covers the external surface of bone
What is endosteum?
A connective tissue that lines cavities within the bone
What is compact bone?
Dense and rigid outer layer of bone
What is spongy bone?
- Also known as cancellous bone
- Type of bone found in inner layers - softer and lighter than compact bone
How is compact bone organised?
- Layers of collagen fibres and associated calcium (lamellae) are organised in concentric circles where the collagen fibres within each layer are at a 45 degree angle to those in the next layer
- Lamellae surround a vertical Haversian canal
- Osteocytes located between lamellae within cavities known as lacunae
What are Haversian canals?
Vertical canals at centre of lamellae in osteon of compact bone where bundles of BV’s, LV’s, and nerve fibres are found
What are Volkmann’s canals?
Horizontal structures that connect to the Haverisan canals and contain smaller BV’s, LV’s, & nerve fibres
What is an osteon?
Refers to the functional unit of compact bone which is comprised of the lamellae organised in concentric circles surrounding a Haversian canal and containing osteocytes
How are osteocytes connected to one another?
Osteocytes sit in cavities known as lacunae and lacunae are interconnected by series of tunnels that join osteocytes of different levels together and then with the Haversian canal to allow for the movement of ions and nutrients across bone
How is spongy bone organised?
Matrix consists of a 3D network of fine columns that crosslink to form trabeculae which produces light porous bone with large spaces
NO CANALS
What is the importance of the lightness of spongy bone?
Allows the body to move - if we only had compact bone then it would be too heavy to move
What is the structure of peristeum?
- Attaches to bone by collagen fibres
- Has 2 layers
- Outer later is dense collagenous connective tissue
- Inner layer is loosely arranged and contains osteoprogenitor cells
What are osteoprogenitor cells?
Cells that differentiate into osteoblasts and are present in the inner layer of peristeum where they form the osteogenic layer close to bone
What is ossification?
Formation of bone
What are the 2 types of ossification?
Endochondral ossification
Intra-membranous ossification
Both form spongy bone that is later organised to form compact bone
What is endochondral ossification?
- Formation of long bones from a cartilage template (replace the cartilage)
- Continued lengthening at epiphyseal plates
- Appositional growth occurs which means growth at the edges so is in all directions
What is intra-membranous ossification?
- Formation of bone from clusters of mesenchymal stem cells in centre of bone
- Interstitial growth - growth in middle
- E.g. in flat bone
How is compact bone formed from spongy bone?
- MSC’s in BM convert to osteoblasts that line the recently formed trabeculae
- Osteoblasts lay down osteoid that is then mineralised
- Osteoblasts become trapped in bone tissue and form osteocytes
- Steps 1-3 repeat forming lamellae in concentric circles
- Central MSC’s convert into BV’s, LV’s and nerves - Haversian canal is formed
Why are bones remodelled?
Remodelling maintains strength of bones and is constantly occurring
How are bones remodelled?
- Osteoclasts make a wide tunnel, resorbing bone - known as cutting cone
- Osteoclasts anchor to bone surface and create an acidic micro environment in sealed zone beneath it that dissolves minerals in bone
- Lysosomal enzymes then released that hydrolyse collagen matrix
- Osteoblasts make a smaller tunnel behind forming new osteoid that is mineralised to form bone
What is the cement line?
Where newly formed bone tissue meets unmineralised osteoid
What is the role of osteocytes in bone remodelling? And how is this activity regulated?
- Detect mechanical load (physical stress) and regulate bone formation and resorption
- Can act as osteoblasts and law down scavenged osteoid in their lacunae - increased by oestrogen and thyroid hormone
- Can act as osteoclasts and slightly degrade bone around lacunae and this is know as osteocytic osteolysis - increased by parathyroid hormone
How is the activity of osteoblasts regulated?
Stimulated by calcitonin (hormone), growth hormone, thyroid hormone, and vitamin A
Calcitonin works by blocking action of PTH
How is the activity of osteoclasts regulated?
- Increased by parathyroid hormone that releases calcium ions into the blood
What is the importance of vitamin D in bone stability?
- Rigidity and stability of bone is dependent on association of collagen fibres with calcium ions
- Vitamin D3 produces calicitriol that aids calcium absorption
Name the types of bone
- Long bone
- Short bone
- Flat bone
- Irregular bone
- Sesamoid bone
What are long bones?
- Longer than they are wide
- E.g. femur and small bones in fingers
- Mostly appendicular skeleton
- Function - support weight of body and facilitate movement
What are short bones?
- About as long as they are wide
- E.g. carpals in wrists and tarsals in ankles
- Function - provide stability and some movement
What are flat bones?
- Somewhat flattened shape
- E.g. bones in skull, thoracic cage (sternum + ribs), and pelvis
- Functions - protects internal organs and provide large areas of attachment for muscles
What are irregular bones?
- Vary in shape and structure - often complex shape
- Function - protect internal organs e.g. vertebrae protect spinal cord
What are sesamoid bones?
- Bones embedded in tendons and shape often similar to sesame seed
- E.g. small round bones in hands, knees, feet and the patella in the knee
- Function - protect tendons from wear and tear
What are the bones of the axial skeleton?
- 80 bones
- Bones in skull, thoracic cage, and vertebral column
What are the bones of the appendicular skeleton?
- 126 bones
- Shoulder girdle, upper limbs, pelvic girdle, lower limbs
What bones are there that aren’t included in the axial or appendicular skeleton?
- 3 in inner ear (x2 as 2 ears)
- Hyoid bone
What are the 2 types of bone marrow?
- Red bone marrow
- Yellow bone marrow
Where is red bone marrow found?
In the spaces in spongy bone
Where is yellow bone marrow found?
In the medulla region of long bones
Give some characteristics of red bone marrow and describe its function
- Full of developing blood cells
- Has a rich blood supply
- Only found in spongy bone
- Function is to replenish cells in the blood via haematopoiesis
Give some characteristics of yellow bone marrow and describe its function
- Full of adipocytes
- Has a poor blood supply
- Function - shock absorber and energy source and can convert to red marrow
What is important about adipocytes in yellow bone marrow compared to adipocytes in other tissues?
They can be converted into red bone marrow so can differentiate into any type of blood cell
This cannot occur in any other adipocytes
How do maturing blood cells leave the bone?
Leave via sinusoids (fenestrated discontinuous capillaries)
What is oestoid?
Unmineralised bone matrix formed by osteoblasts
What are precapillary sphincters?
- Located between arterioles and arteries and control blood flow to the capillary beds
- Contain smooth muscle for contraction
- Controls fluid exchange between capillaries and tissues
How do larger cells bypass capillaries?
- Don’t fit through capillaries so bypass it using arteriovneous anastomosis which is where arteries and veins join together
Define vein
An elastic blood vessel that transports blood from various regions of the body to the heart
What are the 3 layers of veins?
- Tunica intima
-Tunica media - Tunica externa
What makes up the tunica intima in veins?
Endothelial cell layer that interacts with blood
What makes up the tunica media in veins?
Layer of elastic fibres and smooth muscle cells
What makes up the tunica externa in veins?
Elastic fibrous capsule
What is the function of valves?
Work against gravity to assist flow of blood to the heart preventing back flow
Name 4 types of vein
- Pulmonary veins
- Systemic veins
- Superficial veins
- Deep veins
What do pulmonary veins do?
Carry oxygenated blood from the lungs to the left atrium
What do systemic veins do?
Return deoxygenated blood from the body to the right atrium
Major vessel is vena cava
What are superficial veins?
Close to surface of skin and not located near corresponding artery
What are deep veins?
Located deep within tissue and typically located near corresponding artery - e.g. femoral artery and vein
Define arteries
Elastic blood vessels that transport blood away from the heart
What are the 3 layers in an artery?
- Tunica intima
- Tunica media
- Tunica externa (adventitia)
Briefly compare the structure of arteries and veins
Same general features but walls of arteries are thicker to deal with higher BP
Smaller lumen in arteries
What are collateral blood vessels?
- Also called communicating blood vessels
- Provide an alternative route for arterial blood flow for tissues where blood supply may become compromised
- Some generated due to chronic disease e.g. atherosclerotic plaque build up causing reduced blood flow
- Others produced in development e.g. in brain and joints
What is the importance of collateral blood vessels?
They ensure blood flow to an area even if artery leading to it becomes blocked
What is vasculogenesis?
- Formation of new blood vessels using angioblast precursors from the bone marrow
- Occurs in embryonic development, cancer, endometriosis
What is angiogenesis?
- Formation of new blood vessels from existing blood vessels
- Occurs in fetal development and in adults e.g. in formation of collateral blood vessels
Give a brief overview of vasculogenesis in embryogenesis
- Vascular endothelial growth factor is produced by endoderm
- Leads to formation of primary plexus which folds into a primary vessel
- Angiogenesis can then occur in one of 2 ways
What are the 2 types of angiogenesis?
- Sprouting
- Division of primary vessel
- Both types take place in utero and throughout adult life
What is sprouting angiogenesis?
- New BV grows from original one through degradation of basement membrane and proliferation of endothelial cells
- Pericytes then convert to smooth muscle and surround new BV
- Slow - can take days
Describe angiogenesis through the division of the primary vessel
- Primary vessel splits in 2
- Explains presence of neurovascular bundles where artery and vein are next to one another
- Quick process that can take minutes to hours
What are pericytes?
- Structure - immature cell similar to smooth muscle cell. Key component of capillaries that can contract and are involved in nerve cell communication
- Differentiate into endothelial cells, smooth muscle cells, fibroblasts
Functions - maintain tight capillaries (stop blood leaking where you really don’t want it e.g. brain) and prevents endothelial cell proliferation
What are lining cells?
Osteoprogenitor cells that line the surface of bone when it is not undergoing remodelling
