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Flashcards in ToB - Bone Deck (21)
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What is the composition of bone?

65% hydroxyapatite crystals
23% type 1 collagen
10% water
2% other proteins


What are the functions of bone?

Support and protection
Mineral storage (calcium and phosphate)


How do PTH and calcitriol affect bone?

They cause osteoblasts to release cytokines which stimulate the activity and differentiation of osteoclasts.
This means the bone is broken down more, releasing the Ca2+ required.


What are the steps in repairing a fracture?

1). Haematoma formation and bone cells near haematoma die. Phagocytes and osteoclasts remove dead cells.
2). Infiltration of new blood vessels means a procallus of granulation tissue forms, which converts into a fibrocartilagenous callus.
3). Fibroblasts produce collagen to span break and some hyaline cartilage.
4). Bony callus formed as endochondral ossification replaces hyaline cartilage with spongy bone, and intramembranous ossification creates new spongy bone.
5). Remodelling converts some of the spongy bone into compact.


What occurs during bone remodelling?

A cutting cone forms, where the osteoclasts are at the front, secreting acid and lysosomal enzymes onto the bone to break it down. The osteoblasts follow behind in the resorption bays, and fill in the gap created.


What is intramembranous ossification?

Develops flat bones and contributes to the thickening of long bones.
A primary centre of ossification forms from mesenchymal tissue and mineral deposit radiates outwards from that point (the mesenchymal cells differentiate into osteoblasts and secrete matrix which is then mineralised.)


What is endochondral ossification?

A hyaline cartilage precursor is mineralised to form bone.
1). Compact bone forms collar periosteal bone on the shaft.
2). Central cartilage calcifies and a nutrient artery supply develops in diaphysis = aid to osteogenesis cells
3). Medulla becomes spongy bone whilst cartilage calcifies to form the epiphysial growth plate with their own secondary ossification centres.
4). Epiphyses ossify and growth plates move apart until epiphyseal plates calcify upon adulthood.


What are the 5 zones of the epiphyseal plate?

1). Reserve cartilage - no cell proliferation or matrix production.
2). Proliferation - chondrocytes divide and form a column, whilst secreting extra cellular matrix.
3). Hypertrophy - chondrocytes enlarge, compressing the cartilage between them into columns,
4). Calcified cartilage - chondrocytes degenerate
5). Resorption - calcified matrix is in contact with marrow, so CT and blood vessels can invade where chndrocytes died = spicules.


What is the effect of growth hormone on bone?

Prepubertal, an excess will cause gigantism by promoting the activity of the epiphyseal plates. A deficiency will cause pituitary dwarfism.

In adulthood, no epiphyseal plates, so an excess will cause acromegaly by promoting periosteum growth.


What is the effect of thyroid hormone on the bones?

A deficiency will cause congenital neonatal hypothyroidism (also known as cretinism).


What is the effect of sex hormones on the bone?

Testosterone and oestrogen both influence the development of ossification centres.
If an excess occurs in childhood (e.g. Tumour) it fuses the epiphyses = retarded growth.
If there is a deficiency, the person often grows very tall due to epiphyseal plates persisting later into life.


What is osteogenesis imperfecta and its symptoms?

It is an autosomal dominant disorder due to a mutation in the gene for type 1 collagen.
Symptoms include thin, blue sclerae, disfigured and fragile bones and bad hearing due to the ossification of auditory bones.


What is osteoporosis?

This is where the osteoblasts don't fill the resorption bays left by the osteoclasts, meaning the bone mass decreases due to demineralisation of the trabeculae and breakdown of the collagen framework.


What are the risk factors for osteoporosis?

1). Old age as bone mass is decreasing already.
2). Being female as oestrogen inhibits the work of osteoclasts whilst stimulating the osteoblasts. After the menopause, the fall in oestrogen means osteoclasts activity increases.
3). Poor Ca2+ and Vitamin D in the diet means that the body begins to break down bone in order to release serum Ca2+ levels.
4). Immobility as bone only remodels under stress, so if immobile, bone loss occurs.
5). Long term steroid use, as these inhibit the action of osteoblasts.
6). Hypogonadism because testosterone and oestrogen both act to maintain the bone, so if these aren't present this doesn't occur.


What is achondroplasia?

It is an autosomal dominant disorder which leads to a mutation in the fibroblast growth factor (central to CT).
Decreased endochondral ossification
Decreased proliferation and hypertrophy of chondrocytes in growth plate
Decreased cartilage matrix production



What is rickets and its symptoms?

This is a deficiency in Ca2+ or vitamin D in childhood, resulting in a failure of the bone matrix to calcify.
Symptoms are:
Soft, malformed bones
Bossing of skull
Enlargement of costochondral junctions of ribs


What is osteomalacia and its symptoms?

This is Ca2+ or vitamin D deficiency in adulthood, meaning the trabeculae have an abnormal amount of unmineralised bone (osteoid).
This leads to muscle weakness and back pain.


What are the differences between immature/woven bone and mature/lamellar bone?

In immature, collagen fibres are irregular but they are organised in mature.
In immature, blood vessels run randomly, in mature they are parallel to osteons in the Haversian canals.
Immature is usually temporary, mature is permanent.


What cells are in bone and what are their function?

Osteoblasts - synthesise new bone matrix
Osteoclasts - multihull eater cells which remodel and help resorb bone
Osteocytes - osteoblasts which have 'trapped' themselves in lacunae after secreting osteoid which has mineralised.
- communicate via cytoplasmic processes that run down canaliculi and connect via gap junctions.


What is the bone matrix made of?

Ground substance of proteoglycans and glycoproteins.
Minerals such as calcium and phosphorous hydroxyapatite crystals
Type 1 collagen


What are the 2 connective tissues in bone and their function?

Periosteum - Lines external bone surface formed by CT containing collagen, fibroblasts and some osteoprogenitor cells.

Endosteum - Lines internal bone surface, formed by a single layer of thin CT and osteoprogenitor cells.