SDL: Development of the skeleton Flashcards
What is the extracellular matrix?
Large network of proteins and other molecules that surround, support, and give structure to cells and tissues in the body
How is the skeletal system organised?
Skeletal system is highly specialised connective tissue system
Comprised or paucicellular matrix together with cells that manufacture the matrix
Consists of bones, cartilage, joints + ligaments
What is the skeleton divided into?
Axial- vertebrae, ribs, pelvic bones and ribs
Appendicular- upper and lower limb
What are the most common types of bones
- Long (humerus, femur) and flat (sternum, ribs, pelvic bones)
What is long bone divided into?
- Epiphysis
- Metaphayses
- Diaphysis
What is the epiphysis?
How do they initially grow?
End part of long bone, initially growing separately from the shaft
What is the metaphyses?
Neck portion of a long bone between epiphysis and diaphysis. Contains growth plate, part of bone that grows during childhood, as it grows it ossifies near the diaphysis and the epiphyses (there are 2 epiphyses on long bones, one at each end of the bone
What is the diaphysis?
What is it comprised of?
Shaft, central part of long bone, long tubular structure in all long bones, comprised of medullary cavity, which contains bone marrow and blood vessels
What is bone comprised of?
- Mineralised bone matrix and bone cell
- Over 90% of bone is comprised of matrix, most abdundantly type I collagen with other non-collagenous proteins.
- Unmineralised bone matrix called osteoid
What are the functions of bone?
- Provide structural support (e.g. axial skeleton)
- Protect vital organs (e.g. skull, ribs)
- Enable movement (long bones of upper and lower limbs)
- Store calcium and participate in calcium homeostasis
What are the types of bone?
- Cortical
- Cancellous
Where is cortical bone found?
Lies in outer position of long bones + vertebrae
What is cortical bone composed of?
Long parallel columns called osteons
What are osteons made of?
Concentric rings of bone, lamellae, surrounding a central Haversian canal containing blood and lymphatic vessels
What bone cells are buried within bone and what do they lie within?
Osteocytes, which lie within lacunae
What are osteocytes differentiated from?
How do they connect to other osteocytes?
Osteoblasts, and they have a cell body and cell processes that connect with other osteocytes
Where are the cell processes?
In canaliculi
What is cortical bone covered by?
What does this consists of?
Periosteum, this consists of fibrous connective tissue
What is the medullary cavity and what is it composed of?
Centre of bones, composed of cancellous bone, with intervening marrow
What is cancellous bone comprised of?
Network of bony plates or struts called trabeculae
What are trabeculae?
What do they connect with?
Thin columns and plates of bones that create a spongy structure
These connect with each other and to the endosteum of cortical bone
In adults, what is the space between trabeculae filled with?
Haemopoetic bone marrow or adipose tissue (fat)
How is cancellous bone different from compact (cortical) bone?
This type of bone is lighter and less dense than cortical bone, and has irregular cavities that contain red bone marrow
What are the 3 types of bone cells?
- Osteoblasts
- Osteocytes
- Osteoclasts
What is the function of osteoblasts?
Where are osteoblasts found?
- Make osteoid (unmineralised, organic portion of bone matrix, forms prior to maturation of bone tissue)
- Synthesise bone matrix, make bone, lie on surface of bone, when they’re actively synthesising bone matrix they appear cuboidal with small nuclei, when they become inactive they have the appearance of flat bone-lining cells
- Found on cartilage/ fibrous tissue via ossification
Describe osteocytes
What is the function of osteocytes?
Derived from osteoblasts
Linked by thin processes of cytoplasm connected by cell junctions
Nuclei spaced far apart
Buried within cortical and cancellous bone.
Mature bone cells (osteocytes)
Function is thought to be mechanosensation (transduction of mechanical force
What is the function of osteoclasts?
Where are osteoclasts found?
- Remove/ Reabsorb bone, they’re large, often multinucleate
- Bone removed by dissolving mineral and breaking down matrix, this is recycling of bone governed by hormones, vitamins and available calcium
- Found on surface of bone
- Remodel bone in response to weight etc.
What is cartilage made of?
- Chondrocytes
- Cartilage consists of chondrocytes set in matrix, which is rich in proteoglycans (proteins present in connective tissue), hence why cartilage is solid but flexible
What are the 3 types of cartilage?
-Hyaline/articular cartilage
-Fibrocartilage
-Elastic cartilage
What is the most common type of cartilage?
Where is this cartilage found?
Hyaline, which is found in napal septum, larynx, tracheal rings, most articular surfaces of joints
Describe hyaline cartilage
Contains few fibres, most common type of cartilage, found in:
- napal septum
- larynx
- tracheal rings
- most articular surfaces of joints
Describe fibrocartilage
Where is it found?
Contains abundant collagen fibres
Found in intervertebral discs and some joints such as pubic symphysis
Describe elastic cartilage
- Contains elastic fibres
- Occurs in external ear, external auditory meatus, epiglottis, parts of laryngeal rings
What is the name of the process in which long bones elongate?
Endochondral ossification
What happens in endochondral ossification?
When a partially mineralised cartilaginous matrix formed at the growth plate becomes reabsorbed and replaced by woven bone initially, which is then remodelled into lamellar bone
What is woven bone?
Bone in which collagen fibres are randomly arranged, and this can be seen under polarised light.
This is immature bone that’s produced when bone is formed rapidly
Woven bone = spongy
What is lamellar bone composed of?
Lamellar bone is composed of sheets or lamellae of parallel collagen.
This can be visualised under polarised light
Almost all bones in healthy adults consists of lamellar bone, this bone is stronger than woven
Describe cartilage formation at the growth plate
- Growth plate enlarges by appositional growth, as mesenchymal cells differentiate into matrix-secreting chondrocytes
- These continue to divide with proliferative zone, increasing cartilage mass by interstitial growth
- Matrix produce by chondrocytes contains poorly organised type II and type IX collagen fibrils and several non-collagenous proteins including proteoglycans, chondrocalcin, osteonectin and matrix gla protein
- Chondrocytes progressively enlarge, form hypertrophic chondrocytes, which produce a matrix permissive for calcification in the hypertrophic zone by degrading type II collagen and increasing synthesis of type X collagen and osteopontin
- Once calcified, cartilage partly reabsorbed by osteoclasts in zone of vascular invasion (invading capillary loops from the metaphysic break through the last transverse septum of mineralised cartilage to enter the hypertrophic chondrocyte lacuna), then replaced by woven bone to form primary spongiosa
- Primary spongiosa remodelled and replaced by lamellar bone forming secondary spongiosa
What pattern does cartilage formation at the growth plate follow?
Sequential pattern
What is intramembranous bone formation?
- The process of modelling and remodelling at trabecular and cortical surfaces in the developing and adult skeleton
How does intramembranous bone formation differ from endochondrol ossification?
- There’s no cartilaginous template formed before bone deposition, therefore intramembranous bone can only form by appositional growth
What sequence does formation of intramembranous bone follow?
Temporal
Describe the formation of intramembranous bone
- Stromal + bone lining cells differentiate, form mature osteoblasts, secrete unmineralised organic matrix (osteoid) onto bone surface
- Composed of mainly type I collagen fibrils, aggregate to form collagen fibres, and in turn larger fibre bundles. Higher organisation varies between types of bone tissue, reflect how matrix was formed
- In woven bone, (formed during periods of rapid formation, e.g, developing foetus, repair after injury etc.) fibres loosely packed with ordered spatial arrangement, contrasting more orientated arrangement in lamellar bone, in which matrix formed in successive thin layers
- Variety of other non-collagenous proteins also secreted by osteoblasts + form part of osteoid
- Post osteoid formation, matrix undergoes maturation, creates environment permissive for mineralisation
- Calcium + phosphate deposited in organic matrix in form of hydroxyapatite
What is H&E?
Staining of tissue sections with haematoxylin, which stains cell nuclei blue
What happens to the calcified bone and osteoid in decalcified H&E sections?
Calcified bone and osteoid appear pink
What happens to the calcified bone and osteoid in Eosin?
Calcified bone and osteoid appear pink
Why does the calcified bone and osteoid in decalcified H&E and Eosin stain pink?
Because calcium has been removed during histological processing
What do small amounts of calcium that remain change the colour of mineralised bone to?
A dark blue-purple colour
What is bone remodelling?
Specialised form of intramembranous bone formation that takes place throughout life in adult skeleton
What is the purpose of bone remodelling?
Remodel bone so it remains structurally adapted to mechanical needs of body, in addition to calcium homeostasis
Outline bone remodelling
- Follows strict temporal + spatial sequence
- Bone’s removed in small packets by osteoclasts
- Osteoid laid down by osteoblasts in same location, filling resorption cavity and replacing the bone
Describe osteoporosis
- Total amount of bone reduced, but bone fully and normally mineralised
- Results in thinner trabecular which are poorly connected with each other
- Causes structural and mechanical integrity of bone to be diminished, so bone fractures on minimal trauma
Describe osteomalacia
Total amount of bone preserved but amount of mineralised bone is reduced and there’s increased amount of osteoid (unmineralised bone matrix)
What is closely packed, ‘compact’ bone?
Cortical bone
What is calcified, spongy looking bone?
Cancellous bone, in some bones spaces in spongy bones contain bone marrow
What does cortical bone look like microscopically?
Orderly arrangement with concentric cylinders of bone laid down around central blood vessels containing osteocytes. More blood vessels connect the central vessels horizontally to form a complex mesh of small arteries and veins.
These cylindrical structures are Haversian systems or ‘osteons’
How are osteons packed to form cortical bone?
Closely together
How are osteons arranged in cancellous bone?
As irregular plates
