Skeletal System Flashcards
Structure
- Long
- Flat
Long Bone
- Diaphysis: shaft
- Epiphysis: one end of a long bone
- Metaphyses: the areas between the epiphysis and diaphysis
- Endosteum: lining of marrow cavity
- Periosteum: tough membrane covering bone by not the cartilage
- Epiphyseal line: the growth plate
Bone Architecture
Within long bones there are two major components:
- Trabecular (spongy): gives supporting strength to the ends of the weight - bearing bone
- Cortical (solid): cortical bone on the outside forms the shaft of the long bone
How do long bones form: Endochondral Ossification
Step 1: cartilage: stem cells > chondrocytes, make cartilage
Step 2: growth of cartilage: cells in centre burst causing pH shift, triggering calcification
Step 3: primary ossification centre: nutrient artery penetrates centres of cartilage - bone mineral matrix covers the calcified cartilage forming spongy bone
Step 4: Medullary cavity: bone mineral is reshaped and remodelled to form medullary cavity
Step 5: Secondary ossification centre: blood vessels enter the epiphyses (around time of birth), spongy bone is formed but no medullary cavity
Step 6: formation of cartilage on the joints: cartilage on ends of bone remains as articular cartilage
Bone Growth in Length
- Occurs at the epiphyseal plate, or cartilage growth plate
- Cartilage cells are produced by mitosis (proliferation) on epiphyseal side of plate
- Cartilage cells are destroyed and replaced by bone on diaphyseal side of plate
- between ages 18-25, epiphyseal plates close
i. e. cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line)
What is bone made up of? Components
1) Organic collagen matrix: 10% of adult bone mass is collagen, provides flexibility
2) Mineral: 65% of adult bone mass is called hydroxyapatite, an insoluble salt of calcium and phosphorus
3) Water: Approx. 25% of adult bone mass
4) Trace amounts of magnesium, sodium, bicarbonate
Organic Collagen Matrix
- Collagen is in bone to give it strength and flexibility
- Collagen is formed from:
> chains (like short pieces of thread)
> triple helices (like strings)
> fibrils (like ropes) - The fibrils then are arranged in layers, and mineral crystals are deposited between the layers (like concrete)
What cells remodel bone and make bone?
Osteoblasts: produce collagen and form mineral
Osteoclasts: resorb (dissolve) bone mineral
Osteoblasts
- Bone forming cells found on the bone surface
- Make the collagen chains that will form the mature organic collagen matrix
- Control deposition of mineral (calcium + phosphate) around this new collagen matrix
- When finished making new bone, some become surrounded with matrix and differentiate into osteocytes others will remain on the surface of the new bone and become lining cells. The rest undergo apoptosis (cell suicide)
Osteoclasts
- Bone break down, resorbing cells
- Are large cells with many nuclei
- Form sealed compartments next to bone surface and separate acids and enzymes which degrade bone
- Once finished they undergo apoptosis
Osteocytes
- 46 Billion in skeleton, long lived, live inside the bone (lacunae)
- Extend long branches called canaliculi to communicate with other osteocyte and the bone surface
- Sense any mechanical strain and micro-fractures
- Respond and secrete hormonal factors which regulates osteoblasts and osteoclasts to remodelling bone
Lining Cells
- Are former osteoblasts which have become flat and pancake shaped
- Line the entire surface of the bone
- Contribute to bone remodelling process by creating a canopy over an area of bone remodelling
Why is it necessary to remodel bone?
1) To repair micro-fractures
2) To heal major fractures
3) To adopt the skeleton to the demands of the structure
4) To supply mineral (calcium and phosphate) as required for maintaining homeostasis
Why is it necessary to remodel bone? To heal major fractures: 1) Formation of fracture hematoma
- Damaged blood vessels produce clot in 6-8 hrs, bone cells die
- Inflammation brings in phagocytic cells for clean up duty
- New capillaries grow into damaged area
Why is it necessary to remodel bone? To heal major fractures: 2) Formation of fibrocartilagenous callus formation
- Fibroblasts invade the procallus and lay down collagen fibres
- Chondroblasts produce fibrocartilage to span the broken ends of the bones
Why is it necessary to remodel bone? To heal major fractures: 3) Formation of bony callus
- Osteoblasts secrete ‘spongy’ bone that joins 2 broken ends of bone
- Lasts 3-4 months
Why is it necessary to remodel bone? To heal major fractures: 4) Bone remodelling
- Compact bone replaces the spongy in the bony callus
- Surface is remodelled back to normal shape
What factors regulate the balance and rate of bone remodelling?
- Nutrition
- Vitamin D
- Sex steroids
- Other hormones
What factors regulate the balance and rate of bone remodelling? Nutrition
Adequate levels of minerals and vitamins
- calcium and phosphorus for bone growth
- vitamin c for collagen formation
- vitamins K and B12 for protein synthesis
What factors regulate the balance and rate of bone remodelling? Vitamin D
- Promotes calcium and phosphorus absorption at the intestine to supply the growing skeleton
- Regulates osteoblast and osteoclast function
What factors regulate the balance and rate of bone remodelling? Sex steroids
- At puberty, estrogen and testosterone stimulate growth
- At menopause, a lack of estrogen accelerates bone resorption
Changes with Ageing
- Lose bone with age
- Exercise, nutrition, hormones, genetics contribute
Rickets/osteomalcia
- “soft bone”
- not enough mineral deposits in the osteoid due to not enough Ca/Phos supplied to the bone
Rickets/osteomalcia: Cause
Vitamin D deficiency due to:
- Lack of sunlight exposure
- Liver or kidney disease reducing the ability to make active vitamin D
- Not enough active vitamin D, reduced intestinal calcium and phosphate absorption
Rickets/osteomalcia: Clinical Features
- Bowed legs in children
- Low serum calcium and phosphate level
- High serum parathyroid hormone (PTH)
Rickets/osteomalcia: Treatment
Calcium/phosphate and vitamin D supplements
Osteoporosis
- A condition in which the holes in the bones becomes larger
- Imbalance of bones resorption and bone formation
Osteoporosis: Causes
- Genetic conditions, hormonal and nutritional abnormalities, and other medical diseases
- Women more likely
- Bone mineral density is determined by a combination of heredity and lifestyle factors (e.g. diet, exercise, smoking and various medications)
Osteoporosis: Treatments
- Surgical for fractures (new implant)
- To reduce further bone loss: vitamin D and Calcium, estrogen replacement, anti-resorptive drugs
- To promote bone gain
Osteogenesis Imperfecta
- Brittle bone disease
- People are often short, have hearing problems and abnormal teeth
- If disease is severe, infants die because ribs can’t support lungs
Osteogenesis Imperfecta: Cause
- Inherited mutation in the gene for collagen
- Disorganised collagen leads to weak bone strength
X-linked Hyperphospatemic Rickets
- Genetic disorder within osteocytes
- Produce excessive hormone (called FGF23)
- Results in excessive renal phosphate excretion
- Not enough phosphate for mineral to be formed
Treatments: no effective treatments without side effects
