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Bone


Main component of the adult skeleton



Functions of bone tissue
o Provides support
o Protects vital organs
o Contains bone marrow, which produces blood cells
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com, Types of Bone Cells, available at http://study
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html

Bone cells
Osteoblasts (bone forming cells)


Osteoblasts synthesise and secrete organic components of bone matrix,
including type I collagen, proteoglycans and several glycoproteins



Mature osteoblasts are located exclusively at the surfaces of bone matrix,
side by side and resembling epithelial cells



During matrix synthesis osteoblasts, take a cuboidal/columnar shape
o However, when this synthesising activity is reduced they take on a
flatter structure
o Inactive osteoblasts represent most of the bone lining cells



Osteoblasts are polarised cells with matrix components being secreted on the
cell surface in contact with the existing bone matrix
o The layer of new material (which is not yet calcified) is called osteoid
o This process is completed by deposition of calcium salts into the new
matrix

Calcification


The calcification process is not completely understood



Secreted vitamin k-dependent polypeptide, osteocalcin, along with various
glycoproteins, bind Ca2+ ions and raise their concentrations locally

Osteocytes (mature bone cells)


Many osteoblasts are gradually surrounded by the matrix they secrete and
differentiate into osteocytes which are enclosed within lacunae that are
regularly spread throughout the matrix



In this cell transition the cells grow dendritic projections that also become
calcified



Metabolites can diffuse between osteocytes and blood vessels through a
small amount of extracellular fluid



Osteocytes can interact with each other through gap junctions in their
dendritic structures



These cells maintain the bony matrix, and their genes are different from other
bone cells, secreting proteins that help regulate bone modelling (sclerostin
and cytokines)



The network of osteocytes and their communication with all other bone cells
suggest an additional role in calcium homeostasis

Osteoclasts (bone dissolving cells)


Osteoclasts are very large motile cells which play a major role in matric
reabsorption and bone remodelling



In areas of reabsorption, osteoclasts are in enzymatically active cavities, also
known as Howship’s Lacunae



At a site of active bone resorption, the osteoclast forms a specialized cell
membrane, the "ruffled border," that opposes the surface of the bone tissue



Osteoclasts are characterized by a cytoplasm with a homogeneous, "foamy"
appearance



This appearance is due to a high concentration of vesicles and vacuoles
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Taken from St George’s, University of London, available at http://find
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ac
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Periosteum and Endosteum

Taken from Wikipedia
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wikipedia
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Lacunae with osteocytes
found between lamellae connected by canaliculi which connects with source
of nutrients and oxygen in osteonic canal



Outer boundary of each osteon is the cement line



Specific helical arrangement of collagen fibres provide the great strength of
secondary bones



In compact bone the lamellae also exhibit a typical organizational structure
consisting of external circumferential lamellae and sometimes internal
circumferential lamellae



Bone remodelling is continuous throughout life, both synthesis and removal



Resorbs parts of old osteons and produces new ones – involves osteoclasts

Osteogenesis


Bone can be formed in one of two ways
o Intramembranous ossification – osteoblasts differentiate directly from
mesenchyme and secrete osteoid
o Endochondral ossification – matrix of pre-existing hyaline cartilage is
eroded and replaced by osteoblasts producing osteoid



In both processes the bone tissue that appears first is primary (or woven)
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g
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com, Exam 3: Anatomy and Physiology 163, Fayettiville Technical Community
College, available at https://www
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com/notes/note/n/exam-3/deck/5975647

Endochondral ossification


Responsible for formation of short and long bones



Takes place within a piece of hyaline cartilage, which resembles a small
shape of the bone to be formed



Bone tissue appears as a collar surrounding the diaphysis of cartilage model
– collar produced by local osteoblast activity within surrounding perichondrium



Collar impedes diffusion of oxygen and nutrients into underlying cartilage
causing degenerative changes there



Chondrocytes produce alkaline phosphatase and swell up (hypertrophy),
enlarging lacunae



This compresses matrix into trabeculae and calcifies these structures



Chondrocytes die resulting in a porous 3D structure



Blood vessels from former perichondrium, now periosteum, penetrate through
bone collar bringing in osteoprogenitor cells to central region



Osteoblasts adhere to calcified cartilage matrix producing continuous layers of
cartilaginous matrix remnants



The process in the diaphysis forms the primary ossification centre



Secondary ossification centres appear later at the epiphyses



Both primary and secondary produce cavities that fill with bone marrow
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com, 7
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com/73474893/74b-endochondral-ossification-flash-cards/



Epiphyseal cartilage is divided into 5 zones:
o Resting zone – hyaline cartilage with typical chondrocytes
o Proliferative zone – chondrocytes divide rapidly forming columns of
stacked cells parallel to long axis of bone
o Hypertrophic cartilage – swollen chondrocytes containing glycogen;
hypertrophy compresses matrix into thin septa between chondrocytes
o Calcified cartilage zone – loss of chondrocytes by apoptosis
accompanied by calcification of septa by formation of hydroxyapatite
crystals
o Ossification zone – bone tissue first appears; capillaries and
osteoprogenitor cells from periosteum invade cavities left by
chondrocytes; osteoprogenitor cells form osteoblasts which deposit
osteoid

Bone growth, remodelling and repair


Osteogenesis and bone growth involve the partial resorption of bone tissue
formed earlier, while simultaneously laying down new bone at a rate
exceeding that of bone removal



Osteoblasts and osteoclasts work in tandem to resorb and reform bone tissue;
this allows the bone to be a dynamic tissue that is able to adopt to stresses
exerted on the bone

o E
...
orthodontic appliances push teeth towards one side, which resorbs
the site at which the teeth are pushed together


Bone turnover is about 200 times faster in young children compared to adults



Osteoblasts are generated from stem cells called ‘osteoprogenitor stem cells’
which can be found in the periosteum, endosteum and bone marrow, thus
bone has an excellent capacity for repair

Bone repair in fractures

Taken from apsubiology
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apsubiology
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htm



The repair of bone fractures occurs in a coordinated step-by-step process

The inflammatory stage


As the bone fractures, it destroys local blood vessels that quickly coagulate
and clot to form a large fracture hematoma around the fractured region



The hematoma acts as a connecting bridge between the two ends of the bone



At the same time important immune cells infiltrate the fracture sight in
response to prostaglandin signalling: macrophages, monocytes, lymphocytes,
neutrophils and fibroblasts

The repair stage


Fibroblasts begin to lay down a stroma that helps to support vascular ingrowth



In addition fibroblasts also lay down collagen proteins while osteoblasts
secrete osteoid, which is mineralized by the addition of calcium
hydroxyapatite



The activity of fibroblasts and osteoblasts lead to the formation of a soft callus
around the repair site (fibrocartilaginous callus)



At this stage of repair, due to the softness of the callus it is vital to immobilize
the injured bone to prevent damaging the callus

The remodelling stage


The remodelling stage of fracture repair is the process by which the bone
tissue cells act to resorb and reform the bone to its original shape



This is achieved by the action of osteoclasts and osteoblasts

The metabolic function of bone


The skeleton (hence bone tissue) serves as the body’s calcium ion reservoir,
containing 99% of the body’s total calcium in crystals of calcium
hydroxyapatite



The concentration of calcium in the blood and tissues is 9-10mg/dL and it is
kept relatively stable because of the interplay between blood calcium and
bone calcium



Ca2+ from calcium hydroxyapatite is the main source of bone derived Ca2+;
this is achieved in different ways:
o Parathyroid hormone (PTH) from the parathyroid glands


Act in bone to raise low blood calcium level



Travels to bones and stimulates osteoblasts to secrete paracrine
factors that stimulate osteoclasts to become more active



The activity of the osteoclasts is increased thus more bone is
resorbed to release Ca2+ ions

o Calcitonin produced in the thyroid gland


Reduces elevated blood calcium levels by opposing the effects
of PTH in bone



This hormone directly targets osteoclast to slow matrix
resorption and bone turnover

Taken from researchgate
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researchgate
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e
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the pubic symphysis

The synovial membrane


The synovial membrane (aka synovium) is the soft tissue found between the
joint capsule and the joint cavity of synovial joints (diarthroses)



The synovial membrane is concerned with the secretion of synovial fluid that
must fill the synovial cavity to keep the cartilage of diarthroses joints friction
free and lubricated

Taken from Wikipedia
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wikipedia
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Such enzymes lead to the
degradation of the synovial membrane and cartilage in the vicinity leading to
bone-bone contact



The bone-bone contact causes pain and swelling at joints
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Mescher
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Twelfth Edition

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