Development and Growth of Bones 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 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 two main types of bone?
Woven (immature) bone
Lamellar (mature) bone
*In adults woven bone is only found in repairing fractures
What are the features of bones?
hollow shaft
compact bone on outside
spongy bone in head
Describe 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)
*trabeculae, although irregularly arranged, are made of lamellar bone
Decribe the blood supply of bones?
rich blood supply
in long ones - often there’s an opening (nutrient canal)
large vessels can also penetrate in heads and smaller ones across surface
What is the periostium?
Fibrous and cellular layer:
- Key roles in bone growth and repair
- Vascular
- Good sensory nerve supply
Describe the development of bone cells?
osteogenic cells -> osteoblast (early stages forms a bone matrix) -> osteocyte (embedded and layered in lamella bone)
osteoclasts derived more from immunce cell type lineage - reabsorption and breakdown of bone matrix
When does the skeleton start to form?
6 weeks of foetal life -> some bones continue o grow till 25 years of age (e.g clavicle)
What are the two types of ossification (bone remodelling)?
Intramembranous:
- In existing vascular connective tissue
- Bone matrix (ostein) deposited around collagen
- Mineralises to form woven bone
- Remodels to lamellar bone
- e.g flat bones of skull
Endochondral:
- Within existing fetal cartilage models
- Cartilage calcifies and chondrocytes die
- Periosteal osteoclasts cut channels for sprouting vessels
- Osteoblasts enter with vessels to build bone round them
*Most long bones must support large forces while growing
These would disrupt terminal appositional growth
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
How are bones adaptable?
- 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
What allows bone 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
How do bones grow in 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
How do fractures heal?
Bleeding is an important part of the process.
Haematoma forms -> haematoma becomes infiltrated by fibrous matrix and invaded by cartilage/bone progenitors
How are calcium ion levels regulated by bones?
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:
- Parathyroid Hormone (PTH) released by chief cells of parathyroid gland
- osteoclast bone resorption activity promoted
- increases Ca2+ re-absorption by the kidneys
