Bone Flashcards
What are the two types of bone?
Compact (dense)
Spongy (cancellous/trabecular)
What is ossification?
Ossification in bone remodelling is the process of laying down new bone material by cells called osteoblasts.
How do most flat bones develop?
Intramembranous ossification - directly from mesenchymal tissue
How do long bones develop?
Require a cartilage template in order to develop by endochondral ossification
Where does intramembranous ossification take place?
Within condensations of mesenchymal tissue and not by replacement of a previous hyaline cartilage template
What are mesenchymal cells?
Mesenchymal cells are able to develop into the tissues of the lymphatic and circulatory systems, as well as connective tissues throughout the body, such as bone and cartilage.
What does intramembranous ossification also contribute to?
The thickening (not lengthening) of long bones, at their periosteal surfaces (appositional growth)
What are the stages of intramembranous ossification?
- A small cluster of mesenchymal stem cells form a tight cluster of cells (a nidus)
- The mesenchymal stem cells become osteoprogenitor cells and each develop more Golgi and RER
- These osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing Type I collagen (an osteoid)
- The osteoid mineralises to from rudimentary bone spicules which are surrounded by osteoblasts and contain osteocytes
- The spicules join to form trabeculae which merge to form woven bone, which is finally replaced by the lamellae of mature compact bone
What is compact bone?
Compact bone forms the external surface of the bone and comprises around 80% of the body’s skeletal mass
What is cancellous bone?
Cancellous bone forms a network of fine bony columns or plates to combine strength with lightness. The spaces are filled with bone marrow
Which type of ossification is involved when bones develop directly from mesenchymal tissue?`
Intramembranous ossification
What do the Haversian and Volkmann’s canals carry?
Blood vessels, lymph vessels and nerves
What are some examples of flat bones that develop via intramembranous ossification?
Skull, clavicle, scapula and pelvic bones
What is a nidus?
A small cluster of mesenchymal stem cells
What are osteoprogenitor cells?
These are like the stem cells of bone, source of new osteoblasts
What do osteoprogenitor cells become?
Osteoblasts
In the third step of intramembranous ossification, what do the osteoblasts lay down?
Extracellular matrix containing type I collagen (osteoid)
What is osteoid?
Osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue.
What does the osteoid mineralise to form?
Rudimentary bone tissue spicules, which are surrounded by osteoblasts and contain osteocytes
What do the spicules join to form?
Trabeculae
What do trabeculae merge to form?
Woven bone
What finally replaces woven bone that is formed by the merging of trabeculae?
The lamellae of mature compact bone
What is an osteocyte?
This is a mature osteoblast that has become embedded in the material it secreted
What is the difference between an osteoblast and an osteoclast?
Osteoblasts are responsible for the formation of bone while osteclasts are responsible for resorbing (breaking down) bone
Do osteoclasts have a single nucleus or are they multinucleated?
Multinucleated
What is the periosteum?
A layer of vascular dense irregular connective tissue that covers the outer surface of all bones (except at the joints of long bones)
What is the endosteum?
A thin vascular layer of connective tissue that lines all internal cavities within the bone
What are the functions of the periosteum and endosteum?
Nutrition of osseous tissue and provision of a continuous supply of new osteoblasts for repair or growth of bone
What cells do the periosteum and endosteum contain?
Osteogenic cells
What are bone spicules?
Isolated, newly formed pieces of bone
How do the arrangement of osteocytes differ between immature and mature bone?
Immature bone has osteocytes which are randomly arranged while mature bone has osteocytes arranged in the concentric lamellae of osteons.
What are canaliculi?
Bone canaliculi are microscopic canals between the lacunae of ossified bone.
How are nutrients passed between osteocytes?
Osteocytes have cytoplasmic processes which reach out to adjacent osteocytes via canaliculi. These processes connect via gap junctions and nutrients can be exchanged.
What are the canaliculi also believed to be connected with?
The central Haversian canal
What does spongy bone consist of?
Trabeculae, with the presence of osteocytes lying between lamellae. There are no Haversian or Volkmann’s canals.
In spongy bone, what does each trabeculum consist of?
Numerous osteocytes embedded within irregular lamellae of bone.
Which types of cells lie in the cavities of interconnecting trabeculae of cancellous bone?
Adipose and haemopoietic cells
What do osteons consist of?
Each osteon consists of lamellae of compact bone tissue that surround the Haversian canal
Why does bone resist fracture?
It has a great tensile and compressive strength as well as a degree of flexibility
Why are bones thought to have a degree of flexibility?
It is thought that the lamellae are able to slip relative to each other before excessive load causes fracture
Why is a haematoma formed between the broken bone ends?
When a bone breaks, there is bleeding from multiple broken blood vessels
What is granulation tissue?
Granulation tissue is new connective tissue and microscopic blood vessels that form on the surfaces of a wound during the healing process
What is the first stage of fracture repair?
A haematoma is formed in which granulation tissue arises
In the second stage of fracture repair, the procallus of granulation tissue is replaced by what?
A fibrocartilaginous callus in which bony trabeculae are developing
What is the third stage of fractue repair?
Endochondral and intramembranous ossification give rise to a bony callus of spongy bone
In the final stage of fracture repair, what is cancellous bone replaced by?
Compact cortical bone until remodelling is complete
Why do bone cells at the edge of the fracture die?
No blood supply as blood vessels in bone and periosteum break
Which cells remove dead and damaged tissue in a bone fracture?
Phagocytc cells and osteoclasts
What eventually removes the blood clot formed in a bone fracture?
Macrophages
What are the stages of bone fracture repair?
- Haematoma fomation
- Fibrocartilaginous callus formation
- Bony callus formation
- Bone remodelling
In bone fracture repair, what do fibroblasts do?
Produce collagen fibres that span the break and differentiate into chondroblasts that give rise to a sleeve of hyaline cartilage
How long does it take after a fracture for new bone trabeculae to appear in the fibrocartilaginous callus?
Within a week
What is the fibrocartilaginous callus converted to?
Hard (bony) callus of spongy bone
What do endochondal and intramembranous ossification do in a bone fracture repair?
Ultimately, endochondral ossificationr eplaces all cartilage with spongy bone but intramembranous ossification also produces new spongy bone in the area
In young people, how soon do the processes of endochondral/intramembranous ossification in fracture repair begin?
Within 2 days
How long does bony callus formation continue for?
About two months iuntil a very firm union is formed
Where is cancellous bone especially remodelling into compact bone during bone fracture repair?
Especially in the cortical region - the region of the formed bone shaft walls
How long does the process of bone remodelling (final stage of fracture repair) go on for?
Several months
What removes the material bulging from the outside of the bone and inwads into the medullary cavity?
Osteoclasts
Why is the final shape of the remodelled area the same as that of the original unbroken bone?
This is because it responds to the same set of mechanical stressors
If a fracture involves the loss of bone fragments, what is not possible?
Bony union and callus formation
What does osteoporosis reflect?
Enhanced bone resorption relative to formation
What is osteoporosis?
A metabolic bone disease in which mineralized bone is decreased in mass to the point that it no longer provides adequate mechanical support
What characterises the disease of osteoporosis?
Depletion of bone mass
In osteoporosis what happens to the trabecular bone?
Loss of mass/thinning - increased susceptibility to fracture
What does osteoporosis associated with aging result from?
Incomplete filling of osteoclast resorption bays
When does bone mass peak?
Between 25-35 years
When does bone mass begin to decline?
In the 5th or 6th decade
Amongst whites, what is the difference in risk of hip fracture between men and women?
Women have 2 times the risk of hip fracture than men
Amongst whites, what is the difference in risk of vertebral fractures?
Women have 8 times he risk of vertebral fractures as men
What is Type I osteoporosis?
Occurs in postmenopausal women and is due to an increase in the number of osteoclasts as a result of oestrogen withdrawal
What is Type 2 osteoporosis?
Occurs in elderly people of both genders, often after age 70 and is due to attenuated osteoblast function
What are some risk factors associated with osteoporosis?
Genetics (peak bone mass higher in black people)
Insufficient calcium intake
Vitamin D deficiency
Exercise
Smoking
What is the difference between osteoporosis and osteomalacia?
Osteoporosis involves loss of bone mass
Osteomalacia involves softening of bones
Why should a fractured bone be immobilised?
Protection and alignment
