Musculoskeletal System 3 Lecture 29 Flashcards
Suppose the wall of a diaphysis was carrying on with appositional growth out here in the periosteum putting down more layers or circumferential lamellae. What would happen if these layers became more than 0.2 of a millimetre away from the periosteum?
Osteocytes in the deeper layers risk being too far from a blood vessel to survive.
This is why we need osteons in the compact bone, as the center of each osteon contains a blood vessel that supply nutrients to the osteocytes.
How and when are primary osteons formed?
Primary osteons form during bone growth and increase in diameter, particularly in the diaphysis (shaft of long bones).
The process:
Step 1: Appositional growth occurs outwards as the bone grows.
Step 2: Around blood vessels located in the periosteum, the periosteum’s growth slows behind the vessel but speeds up on either side.
Step 3: Crests form on either side of the blood vessel, growing upwards and eventually enclosing the blood vessel, creating a tunnel.
Step 4: The periosteum that surrounds the tunnel becomes the endosteum once it is no longer on the bone surface.
After the tunnel forms, the endosteum stays active, and osteoblasts inside the tunnel continue to deposit concentric lamellae on the tunnel walls.
This process continues until only a blood vessel or a small amount of connective tissue remains.
At this point, the osteoblasts either die or revert to osteoprogenitor cells, leaving a resting endosteum and completing the new osteon.
What are the differences between periosteum and endosteum?
Periosteum (outer layer of bone) is slightly thicker and more protective.
Endosteum (inner layer lining the tunnels) forms once the tunnel is enclosed and remains active, contributing to bone growth by laying down concentric lamellae inside the tunnel.
The tunnel is filled with concentric layers until only a blood vessel or a small amount of connective tissue remains.
What is the limitation on Osteon Size?
The diameter of an osteon can’t exceed 0.4 mm, as the osteocytes on the outer edge would be too far from the blood vessel in the center for nutrient access.
Detailed overview of how primary osteons are formed
1. Initial Bone Growth and Periosteal Ridges:
- Osteoblasts in the periosteum (outer layer of the bone) are responsible for bone formation.
- As the bone grows outward through appositional growth, periosteal ridges begin to form on either side of a blood vessel located on the outer surface of the bone.
- These osteoblasts secrete bone matrix, creating new circumferential lamellae (layers of bone that wrap around the outer surface of the bone).
2. Tunnel Formation Around the Blood Vessel:
- As bone growth continues, the periosteal ridges on either side of the blood vessel gradually grow upwards, forming crests.
- These ridges eventually fuse over the blood vessel, enclosing it in a tunnel.
- Once the tunnel is formed, the inside of this tunnel becomes lined with endosteum (the inner bone layer), which was previously periosteum on the surface before the ridges enclosed the blood vessel.
- This tunnel marks the beginning of a new osteon formation.
Filling the Tunnel with Concentric Lamellae:
- After the tunnel forms, the osteoblasts in the endosteum remain active, laying down more bone in the form of concentric rings called lamellae.
- These lamellae are deposited inward, towards the center of the tunnel, gradually narrowing the space within the tunnel.
- This process creates the distinctive concentric ring structure seen in osteons (also known as the Haversian system).
- Eventually, only a small central canal remains, which houses a blood vessel (the central/Haversian canal), ensuring a nutrient supply for the surrounding osteocytes.
Ongoing Growth and Osteon Formation:
- As bone growth continues, the periosteum continues to deposit more circumferential lamellae around the bone’s outer surface.
- New ridges form over other blood vessels, repeating the process of osteon formation.
- This means that as bones grow in diameter, new osteons are continually being formed, ensuring that all osteocytes remain close enough to a blood supply for survival.
- The maximum diameter of an osteon is limited to around 0.4 mm because osteocytes further from the center would not receive enough nutrients from the central blood vessel.
Difference between primary and secondary osteons
Secondary osteons are different from primary osteons because they are created by removing and replacing existing bone tissue deep within the bone.
Formation of Secondary Osteons
- Secondary osteons require the use of osteoclasts to bore out tunnels within existing bone, removing older bone tissue that may no longer be viable.
- The reason for this process is that the periosteal blood vessels are insufficient to establish all the required osteons through primary osteon formation alone.
- This process is not unique to growing bones but happens continuously throughout life. Roughly 10% of the skeleton is replaced each year, primarily through secondary osteon formation.
What are the triggers for secondary osteon formation?
- Osteocytes are the primary cells responsible for monitoring the bone matrix. They are interconnected, forming a lattice that detects the bone’s health and integrity.
- Nutrient deficiency: If osteocytes are too far from a nutrient source (such as a blood vessel), they are unable to maintain themselves and will die. When osteocytes die, the bone surrounding them becomes weaker and can no longer be repaired or maintained properly. This initiates the formation of a secondary osteon.
- Stress and micro-damage: Osteocytes also monitor mechanical stress on bones, such as during weightlifting or other physical activities. When micro-damage occurs due to stress, osteocytes initiate the process of forming a new osteon in the area to repair and strengthen the bone.
Orientation of Secondary Osteons
- Secondary osteons often orient themselves in the plane of greatest stress experienced by that particular region of the bone. This is part of the body’s natural remodeling process to ensure bones are strong in the areas that need it most.
- This is why, when observing mature lamellar bone, you may notice that osteons are aligned with the forces acting on the bone. They are remodelled in this orientation to compensate for stress and micro-damage over time.
Osteoclast activity forming a secondary osteon
Osteoclasts Begin the Remodeling Process:
Osteoclasts form and gather in areas of the bone that require remodeling. They begin boring through the existing bone, creating a tunnel known as the “cutting cone”.
This tunnel is formed inside the bone, which distinguishes secondary osteons from primary osteons. In primary osteons, the tunnel is formed on the surface as the bone grows.
Osteoblasts Begin the Formation of New Bone:
After the tunnel has been created by osteoclasts, osteoblasts move in to line the walls of the tunnel, forming the new active endosteum.
Osteoblasts deposit osteoid (the unmineralized bone matrix) onto the tunnel walls, which is later calcified to form new lamellae.
A blood vessel will eventually grow into the newly formed tunnel to supply nutrients to the cells (note: the blood vessel is not shown in the diagram).
Filling the Tunnel with New Lamellae:
The osteoblasts continue to deposit layers of concentric lamellae inside the tunnel, gradually filling it.
This active area behind the osteoclasts is called the “closing cone”.
Some of the osteoblasts become trapped in the new lamellae and differentiate into osteocytes, which are the mature bone cells that maintain the bone tissue.
Completion of the Secondary Osteon:
The tunnel is reduced to the size of a typical Haversian canal as the osteoblasts finish depositing lamellae.
The remaining osteoblasts lining the Haversian canal either die or become bone-lining cells (osteogenic cells) and contribute to the resting endosteum.
At this point, a new osteon has been fully formed.
A line called the “cement line” sometimes forms between the outermost lamella of the new osteon and the surrounding older bone.
Observing Osteons Under the Microscope
Microscopic Observation of Osteons:
When viewing polished bone under the microscope with ink smeared over it, you can observe the outline of a new osteon, particularly in cross-section.
Though the cement line isn’t visible in this instance because it hasn’t been stained for, it is still present.
The layers (lamellae) of collagen in the lamellar bone can be made out, with alternating orientations in each layer.
Haversian (Central) Canal and Blood Vessels:
The Haversian Canal (also called the central canal) is the hollow space within the osteon, where blood vessels would reside to supply nutrients to the bone cells.
This entire structure, including the lamellae surrounding the Haversian Canal, constitutes the osteon.
Osteocytes and Lacunae:
The osteocytes, which are trapped within the bone matrix, can be observed in their small spaces called lacunae.
The direction of bone growth can be inferred by looking at the lacunae and canaliculi.
Direction of Bone Growth:
The direction of growth is evidenced by the canaliculi (tiny channels) that radiate from the osteocytes towards the central canal.
Since canaliculi cannot be established in pre-existing bone, they only form in the direction that the bone is growing.
This shows how the new bone forms concentrically around the central canal, with osteocytes embedded in successive layers.
Nutrient and Waste Flow:
Nutrients (such as oxygen) and essential materials move from the central canal outward through the canaliculi, providing sustenance to the osteocytes.
Waste products produced by the osteocytes travel in the opposite direction, passing from one osteocyte to the next through the canaliculi, and eventually back into the blood vessel in the central canal.
Osteon Size Limitation:
The lecturer reiterates that if the osteon grows too large, the outermost osteocytes may not receive enough nutrients, which is why osteon size is typically limited to about 0.4 mm in diameter.
Osteon Size Limitation and Nutrient Supply:
If an osteon becomes too large, the osteocytes on the periphery (outermost layer of the osteon) run the risk of running out of nutrients.
However, this typically does not happen because of the size limitation of osteons (around 0.4 mm in diameter).
Identifying Compact Bone:
Compact bone can be identified by the presence of osteons.
An osteon has a central (Haversian) canal, which houses blood vessels that supply nutrients to the osteocytes.
Using polarized light on a thin slice of bone makes it easy to see the lamellae (layers within the osteon). This is because certain layers of collagen fibers align with the wavelength of the polarized light, while others do not, creating a visible pattern.
Therefore, the definition of compact bone is based on the presence of osteons. If osteons are present, you have compact bone. If osteons are absent, it may indicate spongy (trabecular) bone.
Circumferential Lamellae and Bone Growth:
On the surface of the bone, there may be circumferential lamellae instead of osteons.
The bone grows outward in a process called appositional growth, where new layers of bone are added on the surface.
As the bone continues to grow in thickness, the osteocytes near the deeper layers may eventually lose access to nutrients. When this occurs, secondary osteons will form by boring out old bone and replacing it with new osteons to maintain nutrient supply.
Spongy (Trabecular) Bone and Bone Marrow:
Spongy bone is found in areas with bone marrow, which is likely hematopoietic tissue in this context.
In spongy bone, the spaces between the trabeculae (the bony matrix) contain fat and bone marrow.
Red bone marrow contains a higher percentage of hematopoietic cells, responsible for producing blood cells, while yellow bone marrow contains more fat cells. Both types of tissue are typically present, but the higher concentration of one type determines whether the bone marrow is classified as red or yellow.
Osteons are absent in which bone?
Spongy (both compact bone and spongy bone have lamellae, but only compact bone has osteons)
How to identify secondary osteoclasts
Secondary osteons are relatively easy to identify by observing the beginning of the tunnel where osteoclasts are located.
Osteoclasts and the Cutting Cone
Osteoclasts – Large Cells:
Osteoclasts are giant cells that play a critical role in bone remodeling by breaking down bone tissue.
Under the microscope, osteoclasts are significantly larger than normal cells.
The little dots seen around the osteoclasts represent normal cells, which are much smaller in comparison.
The large size of osteoclasts is due to them being a syncytium, meaning they are formed by the fusion of multiple precursor cells, which results in multiple nuclei within a single cell.
Cutting Cone:
Osteoclasts form a collection of these giant cells that work together to create the cutting cone.
The cutting cone is responsible for boring through the bone, breaking it down as part of the remodeling process.
The green arrow in the diagram indicates the direction in which the cutting cone is moving as it resorbs bone.
Closing Cone:
If you move further back in the osteon, closer to the closing cone, you can see a cross-section (TS) of the tunnel.
The cement line forms at the junction between the newly formed bone and the older bone, marking a clear boundary.
The older bone (further from the tunnel) is more calcified, while the newer bone (closer to the tunnel) is still being calcified and doesn’t pick up stain as strongly.
The active endosteum is where osteoblasts are depositing new bone. These cells fill the tunnel inward, creating new lamellae layer by layer.
The blood vessel is located at the center of the osteon, supplying nutrients as the tunnel is filled in from the outside towards the center.
Difference between cutting and closing cones of osteon?
The cutting cone is responsible for removing old bone through the action of osteoclasts, creating a tunnel for new osteon formation.
The closing cone is the phase where osteoblasts fill the tunnel with new bone, forming the layers of the osteon.
Different levels of calcification as evidenced by the areas where X-rays pass through
Osteons and Bone Calcification:
X-ray Visualization:
The bone slice is visualized using X-rays.
Whiter areas indicate more calcification (denser bone); darker areas indicate less calcification (less dense bone).
X-rays pass more easily through less calcified areas (darker regions).
Microprobe Experiment:
A microprobe with a pyramid-shaped tip was used to test the hardness of the bone.
Same force applied to different areas of the bone.
Whiter (more calcified) areas = smaller indentation (harder).
Darker (less calcified) areas = larger indentation (softer).
Osteon Development and Remodeling:
New osteons are generally less calcified and darker.
Older osteons are more calcified and show more resistance (lighter in the X-ray image).
Secondary osteons form by eroding into older osteons, leading to overlapping bone remodeling.
Surface Bone Appearance:
On the surface of the bone, multiple secondary osteons may overlap.
Hard to identify the origin of highly calcified areas (could be older osteons or circumferential lamellae being replaced).
Constant remodeling process, where newer osteons replace older ones.
What unit is in spongy bone?
Trabecula
Describe the unit formation
Grows outwards
Where is spongy bone located?
Inside bones, epiphysis of long bones
What is the blood supply in spongy bone?
Blood vessels in medullary cavity
What is the function of spongy bone?
To support the outer cortex of compact bone in areas where forces occur from multiple directions. This is to help reduce the weight of bone. Rapid turnover of Ca and P.
What is the unit of compact (cortical) bone?
Osteon (haversian system)
Describe the unit formation in compact bone
Which 4 common features do all synovial joints have?
- Articular cartilage
- Articular capsule (Fibrous layer + synovial membrane)
- Joint cavity which contains
- Synovial fluid
Under the microscope
ECM: Ground substance - GAG + PG
Provides the swelling and hydrating mechanism for the proper function of cartilage
Part of the solid component that is fixed inside the tissue
Structure of articular cartilage: Surface zone
Surface Zone (Superficial Zone)
This is the outermost layer of the cartilage.
Low PG (proteoglycan) content: This zone has a low concentration of proteoglycans, which are important for water retention and load-bearing properties.
The collagen fibers are oriented parallel to the surface, which helps resist shear forces.
Chondrocytes (cartilage cells) in this zone are flattened and aligned parallel to the surface, reflecting the role of the surface zone in maintaining smooth articulation between joint surfaces.
Structure of articular cartilage: Middle zone (transitional)
Moving deeper into the cartilage, the PG content increases.
Collagen fibers are arranged more randomly in this zone, providing greater resistance to compressive forces.
Chondrocytes are rounder and more dispersed than in the surface zone. This zone is a transition between the superficial and deep zones, playing a role in both load distribution and resistance to mechanical stress.
Deep Zone
The highest PG content is found here, contributing to water retention and the ability to withstand compressive loads.
Collagen fibers are oriented perpendicular to the surface, maximizing the cartilage’s ability to bear compressive forces.
Chondrocytes are arranged in columns and are larger in size. This zone is essential for providing the cartilage with compressive strength.
Tide Mark
The tide mark is a boundary between the deep zone and the calcified cartilage. It is often visible in histological sections and represents the junction between the non-calcified and calcified regions of the cartilage.
Below this mark, the tissue begins to calcify, providing an interface with the subchondral bone.
Calcified Cartilage
This region contains low PG but is rich in hydroxyapatite, a mineral found in bones, which gives it strength and rigidity.
This zone anchors the cartilage to the underlying bone.
Chondrocytes here are often smaller and surrounded by calcified matrix.
Osteochondral Junction
This is the junction between the cartilage and the bone, which ensures that the cartilage is firmly attached to the underlying bone (subchondral bone). This structure provides support and stability.
Subchondral Bone
Located below the calcified cartilage, the subchondral bone supports the cartilage and absorbs mechanical loads.
