Bone Flashcards
How do long bones develop?
Endochondral ossification
How do flat bones develop and what do they develop from?
Intramembranous ossification, directly from mesenchymal tissue
Where does intramembranous ossification take place?
Within condensations of mesenchymal tissue - NOT by replacement of a pre-existing hyaline cartilage template
What are some examples of flat bones?
Skull, clavicle, scapula, pelvic bones
Does intramembranous ossification contribute to lengthening of long bones?
No, it contributes to thickening of long bones at periosteal surfaces (appositional growth)
Outline the stages of intramembranous ossification
A small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells (a nidus).
The MSCs become osteoprogenitor cells (each developing more Golgi apparatus and rough endoplasmic reticulum).
The osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing Type I collagen (osteoid).
The osteoid mineralises to form rudimentary bone tissue 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 do spicules connect to form?
Trabeculae
Name 3 types of cell which remodel bone
Osteoblasts, osteocytes, osteoclasts
What is the main unction of osteoblasts
To deposit new bone
What is the main function of osteoclasts?
To resorb bone (and they hence lie in a depression)
What are the 2 types of bone?
Compact (cortical) and Cancellous (trabecular)
Where is compact (cortical) bone found?
Compact bone forms the external surfaces of bones and comprises ca. 80% of the body’s skeletal mass.
Describe cancellous (trabecular) bone
Cancellous bone forms a network of fine bony columns or plates to combine strength with lightness. The spaces are filled by bone marrow.
What do Haversian and Volkmann’s canals contain?
Blood vessels, lymph vessels, and nerves
What is an osteon
The fundamental functional unit of much of compact bone. Roughly cylindrical structures.
How do the canals lie in relation to the osteon?
Haversian - through the osteon (parallel)
Volkmann - across osteon (perpendicularly)
What are osteocytes?
Osteoblasts which have become embedded in the material they have secreted
How are osteocytes arranged?
Immature bone - randomly arranged
Mature bone - concentric lamellae of osteon s
Where are resorption canals found?
Parallel with the osteons long axis
How are nutrients passed between osteocytes?
The osteocytes have very slender cytoplasmic processes, which reach out to those of adjacent osteocytes, via canaliculi. These processes connect via gap juctions such that nutrients can be passed between osteocytes.
Describe the structure of cancellous (trabecular) bone
The internal histological structure of the trabeculae is similar to that of compact bone, with the presence of osteocytes lying between lamellae. There are however no Haversian or Volkmann’s canals.
Which cells are found in cancellous (trabecular) bone and where are they found?
Each trabeculum consists of numerous osteocytes embedded within irregular lamellae of bone. Osteoblast and osteoclasts on their surfaces act to remodel them.
What lies in the lacunae between trabeculae
Adipose and haemopoietic cells
Briefly describe how bone is resorbed
A cutting cone is boring a tunnel through the bone by the action of osteoclasts, which release H+ ions and lysosomal enzymes.
How does bone resist fracture?
Bone resists fracture because it has great tensile and compressive strength, but also because it has a degree of flexibility.
The lamellae are thought to be able to slip, relative to each other, before excessive load causes fracture.
What is a haematology and how can it form?
A solid swelling of clotted blood within tissues
When bone breaks there is bleeding from multiple broken blood vessels, resulting in a haematoma between the broken bone ends.
Give a brief overview of fracture repair
1- A blood clot (haematoma) is formed in which granulation tissue arises.
2- The procallus of granulation tissue is replaced by a fibrocartilaginous callus in which
bony trabeculae are developing.
3- Endochondral and intramembranous ossification give rise to a bony callus of spongy/cancellous bone
4- Cancellous bone is replaced by compact cortical bone until remodelling is complete
Describe haematology formation including the types of cells involved
Blood vessels in bone and periosteum break.
A mass of clotted blood (haematoma) forms.
Bone cells at the fracture edge die (no blood supply).
Swelling and inflammation occur.
Phagocytic cells and osteoclasts begin to remove dead and damaged tissue.
Macrophages will eventually remove the blood clot.
Describe fibrocartilaginous callus formation
New blood vessels infiltrate the fracture haematoma. A procallus (soft callus) of granulation tissue (i.e. tissue rich in capillaries and fibroblasts) develops.
Fibroblasts produce collagen fibres that span the break. Others differentiate into chondroblasts that give rise to a sleeve of hyaline cartilage. An externally bulging, fibrocartilaginous matrix thus splints the broken bone.
Concurrently, and more centrally, osteoblasts from the nearby periosteum and endosteum, (and multipotent cells from the bone marrow) invade the fracture site and begin bone reconstruction by forming spongy/ trabecular bone.
Describe bony callus formation
As a result, within a week, new bone trabeculae begin to appear in the fibrocartilaginous callus.
The trabeculae develop as the former fibrocartilaginous callus is converted to a hard (bony) callus of cancellous bone.
Ultimately endochondral ossification replaces all cartilage with cancellous bone, but intramembranous ossification also produces new cancellous bone in the area.
These processes begin as soon as two days after fracture in young people.
Bony callus formation continues for about two months until a very firm union is formed.
Describe bone remodelling
As soon as it is formed the callus of spongy, cancellous bone begins to be remodelled into compact bone, especially in the cortical region (i.e. in the region of the former bone shaft walls).
This process continues for several months.
The material bulging from the outside of the bone, and inwards, into the medullary cavity, is removed by osteoclasts.
The final shape of the remodelled area is the same as that of the original unbroken bone because it responds to the same set of mechanical stressors.
Why are bone banks necessary?
If a fracture involves loss of bone fragments, then bony union and callus formation is not possible.
Since the 1970s, bone banks have become available to supply viable bone for grafting purposes. Bone fragments are frozen and used by orthopaedic surgeons.
Define autograft, homograft and heterograft
If a fracture involves loss of bone fragments, then bony union and callus formation is not possible.
Since the 1970s, bone banks have become available to supply viable bone for grafting purposes. Bone fragments are frozen and used by orthopaedic surgeons.
What is osteoporosis?
Osteoporosis is a metabolic bone disease in which mineralized bone is decreased in mass to the point that it no longer provides adequate mechanical support.
What is the main characteristic of osteoporosis?
Depletion of bone mass always characterises the disease. Loss of mass within the trabecular bone is particularly relevant to increased susceptibility to fracture.
What does osteoporosis reflect?
Osteoporosis always reflects enhanced bone resorption relative to formation
The outer surfaces of the bony trabeculae of cancellous bone are regularly remodelled by osteoclast resorption and osteoblast deposition.
How is this different in osteoporosis?
Osteoporosis associated with aging results from incomplete filling of osteoclast resorption bays.
At what stage of life does bone mass peak?
Bone mass peaks between 25 – 35 years.
It begins to decline in the 5th or 6th decade.
What is the cause of type 1 primary osteoporosis and who is affected?
Type 1 occurs in postmenopausal women.
Type 1 is due to an increase in osteoclast number, a result of oestrogen withdrawal.
What is the cause of type 2 primary osteoporosis and who is affected?
Type 2 occurs in elderly persons of both sexes (senile osteoporosis).
Type 2 generally occurs after age 70 and reflects attenuated osteoblast function.
Name some risk factors of osteoporosis
Genetic: Peak bone mass is higher in blacks than in whites or asians
Insufficient calcium intake: recommended value for postmenopausal women
is 800 mg/day.
Exercise: immobilization of bone (prolonged bed rest or application of a cast) leads to accelerated bone loss. Physical activity is needed to maintain bone mass. The weightlessness experienced by astronauts can result in osteoporosis.
Cigarette smoking in women has been correlated with increased incidence of osteoporosis.
