Bone Flashcards
Define bone
A rigid & specialized connective tissue with a mineralized ground substance
Functions of bone
- Support soft tissue
- Protective encasement of soft tissue
- Reservoir for minerals (Ca2+)
- Levers for muscles
- House bone marrow (hematopoesis)
What are the organic components of the bone matrix?
- Collagen type 1
- Glycoproteins (osteopontin, sialoprotein, osteocalcin)
- Aggrecans
Osteopontin
Binding sites for mineral components and integrins of bone cells
Sialoprotein
Assist in binding bone cells to Matrix
osteocalcin
Binds Ca2+
Aggrecans
Smaller with fewer glycosaminoglycans side chains
Basophilic staining (Matrix appears to stain acidophilic due to the abundance of collagen)
Hydration shell
Forms around hydroxyapatite crystals, allowing ion exchange between the crystals and extracellular fluid
What would occur if bone were decalcified (hydroxyapatite crystals removed)?
It becomes extremely flexible (the hardness is due to hydroxyappetite’s association with collagen)
How does bone mineralization begin?
The secretory vesicles in the osteoblast
Name the two classifications of bone tissue.
- Compact AKA dense bone
- Spongy AKA cancellous bone
What are the spaces of spongy bone filled with?
Blood vessels and Marrow
(note the location of spongy versus compact bone)
What is this classification of bone?
Compact AKA dense bone
The outer layer of periosteum contains _____. The inner layer contains _____.
- dense irregular collagenous connective tissue
- osteoprogenitor cells
Endosteum
- Lines bone cavities
- Then connective tissue with a single layer of osteoprogenitor cells and osteoblasts
Cells of bone
- osteoblast
- osteoclast
- osteocyte
- osteoprogenitor cells
Histology of osteoblasts
Cube or column shaped when active; flat when inactive
Under processes connect osteoblasts with neighboring osteoblast (gap junctions in between)
Lacuna
Space in The Matrix which is occupied by an osteocyte cell
Canaliculi
- Small tunnels radiating from each Lacuna (space in matrix which is occupied by osteocytes cell)
- Occupied by slender extensions from a pair of neighboring osteocytes
(denoted as C in micrograph)
Function of canaliculi
Important for passage of ions, nutrients, hormones, and other molecules to cell deep within the matrix
Green: Canaliculi
Red: Haversian Canal
Blue arrow: lacunae (space containing osteocyte
L: Lamellae (concentric rings
Osteoprogenitor cells are derived from _____.
mesenchyme stem cells
(differentiate into osteoblast or chondroblasts; active during growth & remodelin after fracture)
Location of osteoprogenitor cells
- Inner layer of periosteum
- Endosteum
- Lining of haversian canals
(pale nuclei & little cytoplasm. they are primarily identified by location)
Function of osteoclast cells
Bone resorption
Phagocytic cells → erode bone
Howship’s lacunae
Small depressions of the bone where osteoclasts are found; where bone absorption occurs
Histology of osteoclasts
Very large with distinctive multinucleate cells (this occurs due to factors released from osteoblasts or osteoclasts)
What does the dotted yellow line represent?
Howship’s lacunae
(the flattened cells are inactive osteoblasts)
Paget disease
excessive osteoclastic activity, followed by new bone formation (creates immature woven bone → prone to fracture)
Which bone formation process develops flat bones?
Intramembranous ossification
(also increases growth of long bones)
Describe intramembranous ossification.
- In the primary ossification center mesenchyme cells differentiate into osteoblasts and secrete Matrix (osteoid)
- Ossification centers continue radially and fuse with one another
Osteoid
Initial Matrix laid down in intramembranous ossification that is not yet calcified
Long and short bones of the body are formed by ______ formation
endochondral
List the two general steps of endochondral formation of bone.
- Miniature hyaline cartilage model is formed
- As a cartilage model grows, it is simultaneously being reabsorbed and replaced by bone
The cartilaginous model of long bone consists of which two structural areas?
- Diaphysis
- Epiphysis
Endochondral ossification begins in ______.
the middle of the diaphysis
Bone collar (endochondral ossification)
- A tube of bone that forms at the outer region of the diaphysis
- Initial formation of ossified tissue
Describe endochondral ossification of bone.
- A tube of bone forms at the outer region of the diaphysis
- periosteum envelopes the bone
- osteoprogenitor cells divide and differentiate into osteoblasts
- osteoblast lay down bone and add to the width of the bone collar
- the cartilage Matrix becomes calcified (NOT ossified as in intramembranous ossification)
- chondrocytes undergo apoptosis
- osteogenic bud penetrates collar
- osteoblast form a continuous single layer of cells over the calcified cartilaginous matrix and begin to secrete bone matrix
What is taking place at number one?
Formation of the cartilage model
What is taking place at number two?
The periosteal / perichondrial collar of bone forms around the diaphysis (shaft of cartilage model)
What is taking place at number 3?
Cartilage Matrix begins to calcify (NOT ossify as in intramembranous ossification that occurs in flat bones)
What is taking place at number 4?
Blood vessels and connective tissue cells erode and invade the calcified cartilage creating a primitive marrow (remnant spicules of calcified cartilage remain at the two ends of the cavity)
What is taking place at number five?
- The primary ossification center develops. The endochondral bone is formed on spicules of calcified cartilage. The bones at the end of the developing marrow cavity constitutes the metaphysis.
- Periosteal bone continues to form. The periosteal bone is formed as a result of intramembranous ossification.
What are the key differences between intramembranous ossification and endochondral ossification
- Periosteal bone formation involves intramembranous ossification, which is not accompanied by local cartilage erosion, nor is the bone deposited on spicules of calcified cartilage
- The Matrix laid down is calcified and not ossified in endochondral ossification
What is happening in number 6?
Blood vessels on Perry vascular cells invade the proximal epiphyseal cartilage
What is happening in number seven?
A secondary Center of ossification is established in the proximal epiphysis
What is happening in number 8?
A secondary ossification center forms at the distal end of the bone.
What is happening in number 9?
An epiphyseal cartilage is this formed between each epiphysis and diaphysis. With continued growth of long bone, the distal epiphyseal cartilage disappears
What is happening in number 10?
With cessation of growth, the proximal epiphyseal cartilage disappears. The metaphysis then becomes continuous at the epiphysis
(epiphyseal lines remain where they have physio plate last existed)
Under H&E stain, central calcified hyaline cartilage is ____ & peripheral bone matrix is _____.
- basophilic
- acidophilic
Where is the secondary center of ossification in endochondral ossification?
Each epiphysis
What are the major differences in endochondral ossification between the diaphysis (primary) and the epiphysis (secondary)?
And secondary ossification centers:
- No bone collar
- No periosteum
*
yellow; A = articular surface
blue; M = marrow cavity
green; B= bone matrix
black; E = epiphyseal plate
At which point is no more growth in length of bone possible?
The bone becomes fully ossified when the primary and the secondary ossification centers meet.
(articular cartilage remains at the articular surface of the bone)
What is critical to the longitudinal growth of long bones?
An epiphyseal plate
What are the five zones of the epiphysial plate?
- Zone of Reserve cartilage / resting Zone
- Zone of proliferation
- Zone of hypertrophy
- Zone of calcification
- Zone of ossification
(beginning from the epiphysis side and going down tour the diaphysis side)
The zone of reserve cartilage (resting zone)
Cartilage randomly distributed throughout the Matrix; appears as typical hyaline cartilage
The zone of proliferation
Chondrocytes are rapidly dividing and form packed rows that are parallel to the direction of bone development.
The zone of hypertrophy
- chondrocytes mature and hypertrophy
- leads to reduced interlock on a matrix
The zone of calcification
- Chondrocytes undergo apoptosis
- The thin matrix walls of the lacunae require hydroxyapatite deposits
The zone of ossification
- Endochondral bone appears
- Osteoprogenitor cells differentiate into osteoblasts and begin to secrete bone matrix
- Osteoclast continue reabsorbing bone
Bone lengthening will continue as long as
The rate of mitotic activity in the zone of proliferation equals or exceeds the rate of reabsorption in the zone of ossification
(bone is fully ossified by 20 years of age)
Spicules are also known as _____.
trabeculae
(this is spongy bone and is present in the epiphysis)
Lamellae
Concentric rings of compact bone
Osteon is also known as
Haversian system
Haversian system runs ____ with the length of the bone, while the Volkmann’s canals run ____ to the length of the bone.
- parallel
- perpendicular
(both carry nerves and blood vessels. The volkmann’s canals connect haversian canals to one another.)
Define interstitial lamellae.
remnants of old osteons inter meaning between the complete osteons
What does this diagram depict?
Which bone is laid down very quickly? Woven or lamellar bone?
Woven bone
What are the main differences between thick and thin lamellae?
Thick: looser collagen fibrils, more mineral
thin: more collagen-rich
(collagen orientation is the same)
Define parallel - fibered bone
- Structurally intermediate between woven bone and lamellar bone
- Highly calcified
- Collagen fiber bundles more parallel then and woven them
How are haversian systems formed (this is the same process as bone remodeling)?
- Osteoclast form a cutting cone which advances leaving a cylindrical cavity
- Cavity is filled in with bone creating concentric lamellae
(this process takes two to four months to complete)
Define secondary osteon AKA drifting osteon
- Haversian systems
- Replace phone that has previously existed
Histological differences between primary and secondary osteons?
Secondary osteons: are surrounded by a cement sheet, appear to drill through pre-existing phone, without regard to structure, weaker
Primary osteon: lamellae are round and merge smoothly with surrounding bone
How is primary bone replaced by secondary bone?
The bone is either eroded away at its surface and new bone laid down
OR
Haversian systems can be formed
What are thre 4 organizations of bone?
- Lamellar bone
- Woven bone
- Fibrolamellar bone
- Secondary osteons (haversian systems)
What type of bone is found in fracture callus?
Woven bone
Define fibrolamellar bone
Alternates sheets of lamellar and woven bone with parallel-fiberered bone
