MSK Flashcards
What is the purpose of the skeletal system/bone?
Store minerals (hydroxyapatite crystals- calcium and phosphate)
Protect vital organs
Forms joint lever system for movement
Houses bone marrow (site of haematopoiesis)
Maintains posture and body position
What are the 5 different types of bones?
Long bones - Tubular shape with hollow shaft, has an expanded end for articulation with other bones (humerus, radius, ulna, metacarpals, phalanges)
Short bones- Cuboidal in shape (carpals in the wrist - scaphoid, lunate, triquetral)
Flat bones- plates of bone, often curved and have a protective function (skull, pelvis, thoracic cage)
Irregular bones- appear in various shapes (vertebrae, bones in ears)
Sesamoid bones- round, oval nodules in a tendon - protect tendons from wear and tear (patella, two sesamoid bones at the thumb metacarpophalangeal joint)
What are the 2 microscopic appearances of bone?
Describe them
Woven bone
- Made quickly (in fetal bone)
- Disorganised
- No clear structure
Lamellar bone
- Made slowly
- Organised
- Has a clear layered structure
What is the composition of bone?
50-70% minerals - hydroxyapatite
20-40% organic matrix - Type 1 collagen (90%), non-collagenous protein (10% of all protein)
5-10% water
What are the 2 macroscopic appearances of bone?
Describe them
Cortical (Compact)
- Dense outer shell of bone
- Lamellae are organised into concentric circles which surround vertical haversian canals.
- Vertical haversian canals are connected by horizontal volkmann’s canals
- Osteocytes are located in lacunae (cavities in between lamellae) and these are connected by canaliculi
Trabecular (spongy)
- Makes up the interior of bone
- Contains many large spaces - giving it a honeycombed appearance
- Spaces between trabeculae are filled with bone marrow
Where is yellow and red bone marrow found?
Diaphysis- yellow bone marrow (adipocytes)
Epiphysis- red bone marrow (haematopoietic stem cell)
What is the process of endochondral ossification? What type of bone does it form?
Forms long bones
Primary ossification centre- diaphysis
Secondary ossification centre- epiphysis
1) Chondrocytes produces the cartilaginous precursor.
2) The perichondrium forms around the cartilage model and has blood vessels which contain nutrients that diffuse into the cartilage precursor.
3) Osteoblasts form a bone collar around the diaphysis of the bone, preventing nutrients from diffusing into the cartilage, leading to chondrocyte cell death.
4) Chondrocyte cell death forms small cavities which blood vessels penetrate to bring in cells like osteoblasts which deposit bone, changing the perichondrium to a periosteum.
5) (At birth?) Secondary ossification occurs at the end of the long bones. The primary and secondary ossification centres have a thin cartilage between them called the EPIPHYSEAL GROWTH PLATE.
6) Chondrocytes in this plate continue to proliferate to form new cartilage which are replaced by bone until early adulthood (when they reach their maximal length)
What is intramembranous ossification? What type of bone does it form?
Flat bones
1) Mesenchymal stem cells divide into osteoblasts which aggregate at the ossification centre and secrete osteoid.
2) Whilst most osteoblasts remain on the surface of the bone, some get trapped within the matrix and become osteocytes
3) At the edge of the bone, osteoblasts continue secreting osteoid which surround blood vessels to form trabeculae. –> blood vessels eventually form red bone marrow
4) Periosteum forms around this and trabecular bone is replaced by compact bone
5 steps of bone remodelling
Activation, resorption, reversal, formation, quiescence
Describe bone remodelling
Activation
- Osteoblasts secrete RANK ligands which bind to osteoclast progenitors to stimulate the differentiation into osteoclasts
Resorption
- Activated osteoclasts release acids and cathepsin K to degrade collagen. Resorption pits are formed.
- Calcium is released into the blood for use and osteoclasts disappear
Reversal
- Osteoblasts are activated
Formation
- osteoblasts release osteoid into the resorption pits which then form a soft non-mineralised matrix
- That is mineralised with calcium and phosphate (to form bone)
Quiescence
- Osteoblasts are inactive and there are resting lining cells
- Osteoprotegrin mops up the rank ligands to prevent unnecessary remodelling
What mops up RANK ligands in the quiescence stage of bone remodelling?
Osteoprotegrin
What is Wolff’s law?
It states that bones will adapt to the degree of mechanical loading - so an increase in mechanical loading will cause the bone to strengthen (denser bone)
(tennis players have a higher load on their serving hand)
Function of Parathyroid hormone on calcium and phosphate?
Calcium
1) Increases bone resorption - releases calcium into the blood stream
2) Increases reabsorption of calcium in the kidney (DCT)
3) Increases synthesis of calcitriol which in turn increases intestinal absorption of calcium.
Phosphate
- Decreases reabsorption of phosphate (and possibly excretes phosphate)
What is the pathway of vitamin D synthesis?
1) 7 dehydrocholestrol –> vitamin D3 (via UV light in the skin)
2) vitamin D3 –> 25 hydroxyvitamin D3/Calcidiol (via 25 hydroxylase in the liver)
3) 25 hydroxyvitamin D3 –> 1,25 dihydroxyvitamin D3/Calcitriol (via 1 alpha hydroxylase in the kidneys)
What is the action of calcitriol?
On calcium and phosphate
Increases intestinal absorption of calcium and phosphate
