Development and Growth of Bone Flashcards
What are the functions of bone?
Support of the body shape System of levers for muscle action Protection of internal organs Site of blood cell formation Mineral storage pool
What are the 2 distinct mechanical properties of bone?
- Cable-like flexibility and resistance to tension because framework is collagen + other bone proteins (= osteoid)
- Pillar-like stiffness and resistance to compression conferred by impregnation of collagen with crystalline mineral (hydroxyapatite – a complex calcium hydroxyphosphate)
What are the 2 main types of bone tissue? Which one is found in adults?
Two main types of bone tissue:
- Woven (immature) bone -> more random organisation
- Lamellar (mature) bone -> concentrical circles around a central canal
- In adults woven bone is only found in repairing fractures
What is the arrangement of lamellar bone?
- Outer hard layer of compact lamellar bone (cortical bone)
- Inner layer of interlacing struts of lamellar bone: cancellous bone (= spongy or trabecular bone) -> makes the bone lighter and provides space for BM
- (be careful not to confuse cancellous bone with woven (immature) bone)
What is the structural unit of compact bone?
Osteon
-> central canal with vessles and concentric lamellae (cells with lots of Ca2+ salt deposits)
What is trabecular bone?
= spongy / cancellous bone
- it is irregularity arranged but made up of lammetlar bone
- do not confuse with woven bone
What is the blood supply of bones like?
- rich
- veins run along with arteries
Long bones:
- the nutrient artery enters though a nutrient canal
- there are metaphyseal arteries
What is the periosteum?
- it is the outer surface of bone
- Fibrous and cellular layer:
- Key roles in bone growth and repair
- Vascular
- Good sensory nerve supply
Development of osteoclasts
- modified immune cells, develops from similar lineage
- its job is often to dissolve calcium
- often a target in older individuals, especially post-menopausal women
When do bones start and stop developing?
Start at 6w in utero, end at about 25 years of age in some bones
What are the 2 types of ossification and what do they involve generally speaking?
- intramembranous (flat bones e.g. skull or mandible)
- endochondral (within existing cartilaginous models, cartilage calcifies and chondrocytes die)
Intramembranous bone development
- In existing vascular connective tissue -> signals for bone to develop there
- Bone matrix (ostein) is deposited around collagen
- Mineralises to form woven bone
- Remodels to lamellar bone later
- in flat bones such as the skull or mandible
Endochondral bone development
- Within existing fetal cartilage models (first a cartilage model is made and then it calcifies and forms the bone)
- Cartilage calcifies and chondrocytes die
- Periosteal osteoclasts cut channels for sprouting vessels
- Osteoblasts enter with vessels to build bone round them
- more common
Endochondral ossification Growth in bone length - long bones must support a lot of weight while growing, how is this done?
- Most long bones must support large forces while growing
- These would disrupt terminal appositional growth
Solution? - Shaft ossifies first, followed by epiphyses
- Growth continues by ossification at growing cartilage plate between them
- Growth cessation when cartilage growth ceases and plate is over-run by ossification
- epiphyseal plate closes quite late
Bone elongation = epiphyseal growth
Age related changes in the appearance of normal bone (on x-ray)
- In child’s wrist (lower) epiphyses ossify in 2nd year
- Epiphyseal plates (dark) remain cartilaginous until growth ceases after puberty
- Not breaks, there are natural epiphyseal plates, places where growth takes place.
- In childen this is normal
- When seeing an X-ray like this ask for the age
- Cartilage is dark on x-ray
Adaptability of bone
- Can grow without compromising its support functions
- Increases or decreases bulk and density in response to pattern of use
- Can alter its external and internal shape in response to pattern of use – remodelling
- Can repair when fractured
Bone in a zero-gravity environment
- less force exerted on bone
- loss of bone mineral density
- also there is muscle loss
What are key characteristics of bone that allow growth and remodelling?
- Bone has a large blood supply – cells are never far from nutrients and O2
- Osteocytes maintain matrix but can activate osteoblasts for new bone building
- Osteoclasts are giant cells specialised for destruction of bone matrix
Growth in bone diameter
- Apposition – addition to exterior at periosteum
- Osteoblasts and osteoclasts create ridges and grooves on bone surface
- Blood vessels align in grooves
- Osteoblasts build new osteons round vessels
- Osteoclasts remove bone from endosteal surface
(see slide 29 in presentation)
What are the different types of fractures?
- transverse
- oblique
- spiral
- comminuted
- segmental
- avulsed
- impacted
- torus
- greenstick
Bone fracture healing
- 1st step: haematoma (it becomes infiltrated by fibrous matrix and invaded by cartilage/bone progenitors)
- you start to get woven bone which will remodel to mature lamellar bone -> fracture repair is via woven bone
- Late fracture repair reactive cartilage undergoing endochondrial ossification.
Bleeding is aan important part of the process.
What stages can fracture repair be classified into?
- early fracture healing
- Fracture repair phase: woven bone formation
- Late fracture repair reactive cartilage undergoing endochondrial ossification.
How is bone involved in the regulation of bone calcium levels?
- balance of bone absorption/resorption
- Blood Ca2+ levels high:
Calcitonin released by Parafollicular thyroid cells - breakdown of bone matrix by osteoclasts inhibited; uptake of Ca2+ into bone matrix is promoted.
- Blood Ca2+ levels low:
PTH released by chief cells of PT-gland; osteoclast bone resorption activity promoted; increases Ca2+ re-absorption by the kidneys.
Can woven bone be found in adults?
- only found in repairing fractures
(there are two main types of bone, woven is immature (random organisation) and lamellar (circle of cells around central canal) is mature).
Blood supply of bone
- bones have rich blood supply
- in big bones there is a nutrient artery entering through nutrient canal
Describe the outer surface of bone
- periosteum
- fibrous and cellular layer
Functions:
- key roles in bone growth and repair
- vascular
- good sensory nerve supply
When does the skeleton start and finish to form?
- starts 6w of foetal life
- growth continues in some bones until age 25
What is the most common cause of death in people aged 1-34? (different lecture and topic)
blood cancers
Terminal appositional growth?
- shaft ossifies firt
- epiphyses ossify at different ossification centres and you get growth continuing in cartilage plate between the ossification centres
- growth stops when the plate is overrun by ossification
- you get ossification in the shaft of the bone and later also in the epiphysis but in-between there is cartilage left (EPIPHYSEAL PLATE) -> place between primary and secondary ossification centres.
How are chondrocytes in the epiphyseal plate arranged?
they form columns
Appositional bone growth
- how bone grows in diameter
- addition to exterior periosteum
formation of ridges creating a groove for periosteal blood vessles -> forming an endosteum lined tunel
What is the term used for the way that bone grows in diameter?
appositional growth
Which cells release calcitonin?
parafollicular thyroid cells.
What is one of the first bones to become calcified during fetal development and one of the last bones to calcify?
clavicle
age 20/22
Which neck of the humerus is more commonly damaged/
the surgical neck
(anatomical neck is oblique. surgical neck is a circle that would be parallel to the ground when standing and more distal to the head of the humerus)
