MSS: Cellular Structure Of The Bone Flashcards
What does the skeleton consist of?
- Axial and Appendicular skeleton
- Axial: Skull, neck, vertebral column, Thoracic cage
- Appendicular: Upper and Lower limbs, Pectoral and Pelvic girdle
- Articulated by cartilaginous joints
What is the importance of bones?
- Physical support, maintain posture, space, and delineation of spaces
- Protection of vital structures, e.g. brain, heart, lungs
- Mechanical basis of movement. Attachment for muscles and tendons, allows for leverage
- Storage of salts, including calcium, resource sink
- Haematopoiesis, location of red bone marrow, new blood cells
What is a bone made out of?
- Connective tissues, covered by hyaline cartilage or periosteum
- Organic framework layered with inorganic minerals
- Combination of organic/inorganic gives strength and flexibility
What is the organic components of the bone?
Organic (35%): collagen fibres (ossein) and some ground substance forming the bone matrix.
What is the inorganic components of the bone?
Inorganic (65%): Mostly calcium hydroxyapatite (Ca10(PO4)6(OH)2) crystals which calcify the ossein
- Main storage of calcium, phosphorus
- Minerals are removed and added as the body needs
Describe the organic components of the bone in more detail
- Type 1 collagen (85-90%), some collagen V, trace III, XI, XIII
- Ground substance from non collagenous proteins
- Key to development, remodelling, Growth, Repair
4 major groups - Proteoglycan macromolecules
- Multiadhesive glycoproteins
- Bone-specific, vitamin K–dependent proteins
- Growth factors and cytokines
What are the 3 classifications of the bone?
Anatomical, Microscopic structure and Macroscopic structure
List some of the anatomical classifications of the bone
- Flat
- Long
- Short
- Irregular
- Sesamoid
List some of the macroscopic structures of the bone
- Cancellous (spongy, medullary)
- Cortical (compact)
List some of the microscopic structures of the bone
- Woven (primary, immature)
- Lamellar (secondary, mature)
Describe the microscopic structure of the bone in more detail (PART 1)
Woven bone (immature, primary bone)
- Random assortment of collagen fibres interlacing
- Temporary bone, in fetuses or where the bone is being repaired after a break
- Can also be found where remodelling happens, attachments of tendons and alveolar sockets of teeth
Describe the microscopic structure of the bone in more detail (PART 2)
Lamellar bone (mature, secondary)
- Organised concentric layers (lamellae)
- Organised into osteons
- Circular in compact bone
- Fewer cells and ground substance, more mineralised
Describe the Macroscopic structure of the bone in more detail
Cortical (compact) bone forms exterior
- 80% of bone (by mass)
- Very dense
- Most of the strength of the bone
Cancellous (spongy) bone deep to the cortical, forming the interior
- Network of bony trabeculae, lighter, supports cortical bone while minimising weight
- Spaces filled with marrow
- Medullary cavity in long bones, lined with cortical, full of marrow
Describe the structure of the cancellous bone
- Lighter, weaker and more flexible
- Set up in trabeculae, when mature these run in the direction of stress
- Higher surface area to volume ratio, more suitable for metabolic activity
- Space between the trabeculae filled in with bone marrow, red or yellow
What are some anatomical bone classifications?
Check lecture video
What are the Periosteum anatomical properties of the bone?
Lines external surface of bones
•Exception: where articulating, then hyaline cartilage
Inner cellular layer
- Contains osteoprogenitors (stem cells) if actively growin/remodelling bone
- In grown bone, contains periosteal cells
Outer fibrous layer
- Collagen fibres arranged parallel to bone surface
- Exception: where ligaments and tendons attach, extend oblique/right angles and continuous with collagen in these
What are the bone cavities anatomical properties of the bone?
- Lined with endosteum, usually 1 cell layer of bone lining cells and osteoprogenitors
- Marrow cavity, and spaces between spongy bone trabeculae
- Contain red and yellow marrow
- Red marrow: centre of haematopoiesis
- Yellow marrow: in adults less new blood cells needed, adipose tissue. Can revert
- In adults, red marrow in limited locations
Describe the osteoprogenitor bone cells
- From mesenchymal stem cells in bone marrow
- On external and internal surfaces, and vascular canals
- Runt-related transcription factor 2 (RUNX2) triggers differentiation
What is the use of IGF 1 and IGF 2 in Osteoprogenitor cells?
- IGF 1 and IGF 2 stimulate osteoprogenitor proliferation + differentiation to osteoblasts
- Also bone morphogenic factors
- Also Pulsed electromagnetic stimulation (witchcraft)
Describe the role of Osteoblasts in bone cells
- Secretes type 1 collagen and bone matrix proteins (form osteoid)
- Involved in the calcification process
Secretes small matrix vessels
- Single layer in contact with forming bone
Some become embedded in the forming bone
- Specialise into osteocytes
Describe the role and structure of Osteocytes in bone cells (PART 1)
- Mature bone cell enclosed by bone matrix it secreted as an osteoblast
- Smaller, more processes
- Following mineralisation, it occupies a lacuna (space) conformed to its shape
Describe the role and structure of Osteocytes in bone cells (PART 2)
- Cytoplasmic processes extend through canaliculi
- Communiocate with other osteocytes/bone cells via gap junctions
- Metabolically active, maintain bone, actively detect and respond to mechanical forces
- Flexing of bone moves interstial fluid through canniculi
What are bone lining cells?
- Also osteoblast derived
- Flat cells, few organelles
- In sites where no bone remodelling is taking place, cover the bone surface
- Periosteal cells on external surface
- Endosteal cells on internal surface
- Maintain and nutritionally support embedded osteocytes
- Regulate calcium/phosphate flow
What are osteons?
- The functional unit of compact bone
- Lamellae arranged in concentric circles around central canal, containing ANV
- Canniliculi diffuse nutrients through interstitial fluid to osteocytes
- Also Volkmann canals link up Haversion canals
- One central canal and all surrounding circles of lamellae are one osteon
- Arranged in direction of stress
- Interstitial lamellae fill gaps (old osteons)
What are packets also known as?
What do they rely on?
- Trabecular osteons called packets
- Rely on diffusion from bone marrow through canaliculi
- no need for haversian canals
Describe Osteoclasts as bone cells
- Found where bone is being resorbed, directly on the bone in resorption bays
- Large, multinucleated, acidophilic
- Not related to osteoblasts, formed by fusion of macrophage progenitor cells
- Ruffled border where absorption takes place
- Clear zone forms a ring around resorption zone and seals
- Carbonic acid decalcifies, lysosomes break down bone proteins
How does Osteoclast activation work?
- Osteoclast precursors express RANK receptor molecule
- Reacts to RANK ligand (RANKL), from stromal cell surfaces or activated T lymphocytes (inflammation)
How is Osteoclast activation compromised?
- RANK-RANKL pathway blocked by osteoprotegerin (OPG)
- Produced by osteoblasts, regulated by many bone metabolic regulators
- Therefore, little osteoclasts when new bone being made
- OPG/RANKL system mediates bone resorption/remodelling
What is the theory about bone remodelling?
- Dynamically changing organ, small portions constantly being remodelled
- Every 7 year, whole skeleton replaced. Process called bone remodelling
Watch lecture to view how bone remodelling takes place
What are the 2 different types of bone formation?
- Intramembranous ossification
- Endochondral Ossification
What is the Intramembraneous ossification method of bone formation?
Intramembraneous ossification
- Simple method, bone forms straight from mesenchyme
- Flat bones of skull and face, mandible, clavicle
What is the endochondral ossification method of bone formation?
- Uses cartilage precursor as a framework
- Axial and weight bearing bones
- Only refers to initial setting down of bone
- Remodelling by appositional growth
- Long bones later use intramembranous as well
Describe Intramembraneous ossification in more detail (Part 1)
- Within embryonic connective tissue precursor, the mesenchyme
- Ossification centre appears, aggraegation of osteoprogenitors
- Osteoblasts accumulate at periphery, produce ostoid inwards, trapping osteocytes
Describe Intramembraneous ossification in more detail (Part 2)
- Forms woven bone, in two plates (outer and inner) with thick trabeculae
- Spaces lined with osteoblasts and endothelial cells
- Woven bone eventually replaced by lamellar bone. Blood vessels carry bone marrow cells to spaces between trabeculae
Describe Endochondral Ossification in a bit more detail
- Mesenchymal cells differentiate into chondrocytes
- Produce hyaline cartilage model of bone
Describe what occurs during Endochondral Ossification bone formation (PART 1)
- Cuff of bone forms (around diaphysis in long bone)
- Chondrocytes continue to grow and mature (zone of hypertrophy)
- Chondrocytes in midregion become hypertrophic, then synthesise TNAP
- Causes calcification of cartilage to occur (not mineralisation)
- Calcified cartilage matrix inhibits diffusion, chondrocytes die off
Describe what occurs during Endochondral Ossification bone formation (PART 2)
- Calcified cartilage matrix inhibits diffusion, chondrocytes die off (zone of calcified cartilage)
- Osteoprogenitor cells and blood vessels from periosteum invade, proliferate and differentiate into osteoblasts, which start to lay down bone matrix (resorbtion zone).
Describe what occurs during Endochondral Ossification bone formation (PART 3)
- Secondary ossification centres form after birth in epiphysis
- Continues until only physis (growth plate) is left of original cartilage framework
- New cartilage produced (zone of proliferation) and resorbed (zoned od resorption) to grow bone
- Plate maintained until maximal growth achieved, then epiphysial closure takes place (epiphyseal and diaphyseal marrow cavities become confluent)
