Musculoskeletal System 2 Lecture 28 Flashcards
Which type of bone cell is not in mineralised bone?
Osteons
What is periosteum?
The fibrous layer located on the outer surface of bones.
What is the structure of the fibrous component of periosteum?
Provides a robust protective layer for the bone.
What is the structure of the cellular component of periosteum?
Contains osteogenic (bone-forming) cells.
How does dormancy occur in periosteum?
When only osteogenic cells are present, without significant bone activity, the periosteum is considered to be in a resting or dormant state, awaiting activation.
Are there blood vessels on the periosteum?
Frequently found on the outer surface of bones, providing vascular supply.
What is after the periosteum?
Compact bone
What cells does the mineralised bone consist of and what are their functions?
Osteocytes: Mature bone cells trapped within the bone matrix in small spaces called lacunae.
They remain connected to other osteocytes and surface cells through tiny channels known as canaliculi.
Function: Osteocytes monitor and maintain the bone matrix, ensuring the bone’s health and integrity.
What is the function of Osteoblasts?
Responsible for building new bone tissue.
What is the function of Osteoclasts?
Responsible for breaking down or resorbing bone tissue.
As we move towards the _________ ______ (inner region of the bone), we encounter the __________.
As we move towards the medullary cavity (inner region of the bone), we encounter the endosteum.
What is the endosteum?
A thin membrane lining all internal bone surfaces, including the medullary cavity.
- Similar to the periosteum but much thinner and with fewer cells and fibers.
- Also contains osteogenic cells, which remain dormant unless activated for bone remodeling or repair.
What is the medullary cavity?
The innermost part of the bone, the medullary cavity, is highly vascular and contains blood vessels and bone marrow.
Bone marrow: The site of blood cell production and fat storage.
Bone Growth in Childhood to Adulthood
The diaphysis of a child’s bone needs to increase in diameter to match the size of an adult diaphysis.
This increase in diameter requires both the addition and removal of bone tissue.
What are the two types of ways connective tissues can grow?
Interstitial growth and appositional growth
What is interstitial growth?
Involves cells within the tissue dividing mitotically and producing more extracellular matrix.
This expands the tissue from within.
Limitation: This process requires the tissue to be deformable, and since bone is rigid, it cannot grow via interstitial growth.
Appositional Growth
Bone grows by adding new tissue to existing surfaces.
In this case, the bone increases in diameter by adding new bone to the outer surface, specifically in the periosteum.
This type of growth accommodates the increased loads as the child grows.
However, adding too much bone can result in the walls becoming overly thick, making the bone too heavy and unnecessarily strong.
What is bone resorption?
To prevent the bone from becoming too thick, bone is removed from the inside of the bone.
This process of removing bone tissue is called bone resorption.
Bone Remodeling
The processes of appositional growth and bone resorption occur continuously throughout life.
This ongoing cycle of growth and resorption is referred to as bone remodeling.
Bone remodeling adjusts based on the forces and loads experienced by the skeleton (or lack thereof).
When looking at the cross-section of the diaphysis, which two primary processes can we observe happening?
**Appositional growth
**: On the outer edge of the bone, adding bone tissue to increase the diameter.
Bone resorption: On the inner edge near the medullary cavity, where bone tissue is removed.
These two processes can be independent of each other, but in this example, they occur simultaneously. However, it is important to note they do not always have to occur at the same time.
Appositional Growth Process
Periosteum Activation:
The periosteum, which is in a resting state, becomes active when chemical signals (not fully understood) trigger the activation of osteogenic cells.
Osteogenic cells begin to divide, and some daughter cells differentiate into osteoblasts (plump, “fried egg”-shaped cells).
Osteoblast Activity:
The osteoblasts are positioned on the surface of the bone and begin secreting osteoid, the unmineralized bone matrix.
As the osteoid is secreted, it gradually becomes calcified, and some osteoblasts get trapped in the matrix, differentiating into osteocytes (mature bone cells).
Cell Communication:
The osteocytes and osteoblasts maintain contact through canaliculi, which are small channels that allow communication between cells in the bone matrix and those on the surface.
This process leads to appositional growth, where new bone layers are added, increasing the bone’s diameter.
Growth Termination:
As growth ceases, osteoblasts receive signals to stop secreting osteoid and finish calcifying the matrix.
Osteoblasts can either:
Convert back into osteogenic cells, returning the periosteum to a resting state.
Undergo apoptosis (programmed cell death).
When appositional growth stops, the periosteum contains only osteogenic cells, fibers, and blood vessels, but new bone has been added to the outer surface.
Bone Resorption Process
Osteoclast Formation:
While appositional growth occurs on the outer surface of the bone, osteoclasts begin to form on the inner surface near the endosteum.
Signals, some originating from osteocytes, trigger the formation of osteoclasts.
Monocyte progenitor cells (precursors to osteoclasts) migrate from the bone marrow’s blood vessels to the bone surface, where they fuse to form large, multinucleated osteoclasts.
Osteoclast Activity:
Osteoclasts start dissolving bone by secreting acids and enzymes that break down the bone matrix.
The dissolved bone material is released into the surrounding serum for transport.
As osteoclasts resorb bone, they create space in the medullary cavity.
Blood Vessel Growth:
Blood vessels grow into the newly resorbed space to provide nutrients and remove waste products.
The growth of blood vessels is crucial for maintaining cell health and keeping the bone environment dynamic and well-supplied.
Osteoclast Apoptosis:
Osteoclasts have a short lifespan and undergo apoptosis (self-destruction) once they finish their bone-resorbing task.
This controlled cell death ensures that the bone isn’t excessively resorbed, maintaining a balance in the bone remodeling process.
Once the osteoclasts die off, the endosteum returns to a resting state, ready for future remodeling as needed.
What does this diagram of the periosteum show?
Resting Periosteum:
The periosteum is in a resting state, meaning it is not currently active in bone growth or remodeling.
The P in the image represents the periosteum layer.
Periosteal Layer:
This is the outer layer of the bone surface, containing flattened cells that are osteogenic cells or osteoprogenitor cells (Op).
These cells are located right near the bone surface and can later differentiate into osteoblasts when bone formation or repair is required.
Osteogenic cells and osteoprogenitor cells are two names for the same cell type, responsible for initiating the formation of bone when triggered.
Fibrous Component:
The periosteum also contains a fibrous component, providing structure and support to the membrane that surrounds the bone.
Blood Vessel:
A blood vessel can be seen in the image, which has been cut mostly in a longitudinal section.
The periosteum is a highly vascularized membrane, meaning it contains many blood vessels that help provide nutrients to the bone and aid in the healing process when needed.
Bone Layer
Lacunae:
The small spaces within the bone are called lacunae, which house osteocytes (Oc).
These osteocytes, or mature bone cells, maintain the bone matrix and communicate with other bone cells to regulate bone maintenance.
Osteocytes:
The osteocytes appear as black dots in the lacunae, though they may have shrunken slightly due to artefacts during the preparation of the microscope slide.
Canaliculi:
The canaliculi, which are tiny channels connecting osteocytes to one another and to surface cells, aren’t visible in this particular image, but they are still present and are critical for cell communication and nutrient transport within the bone.
What does this diagram of the periosteum show?
Active Periosteum under the Microscope
This image depicts an active periosteum, highlighting the changes in cellular activity compared to the resting periosteum.
Thickened Cellular Layer:
The cellular layer in the active periosteum has become significantly thicker, indicating that the periosteum is active in bone formation or repair.
The outer edge of the periosteum still contains osteoprogenitor or osteogenic cells (labeled “P” for periosteum), which remain flattened.
Osteoblasts (Ob):
Osteoblasts (labeled “Ob”) are now present on the surface of the bone. These cells are noticeably fatter and plumper compared to the flattened osteogenic cells, signaling that they are actively involved in secreting osteoid.
These osteoblasts are supplied with the resources needed to secrete osteoid, the unmineralized bone matrix, and begin the process of calcification.
Zone of Calcification:
The lighter-colored zone just beneath the osteoblasts represents the area where osteoid is being calcified.
This lighter color is due to the fact that the calcification process is still underway, so the area hasn’t yet picked up the stain fully.
The progression of calcification can be seen, showing the osteoblasts actively depositing and calcifying the new bone matrix.
Osteocytes (Oc):
Some osteoblasts are in the process of getting trapped within the matrix, which is visible in the image.
These trapped osteoblasts will eventually become osteocytes (labeled “Oc”), maintaining contact with their neighboring osteocytes through canaliculi.
Masson’s Trichrome Stain:
This slide has been stained using Masson’s Trichrome, which provides a different contrast compared to H&E staining. It emphasizes the bone matrix and cellular activity, showing different zones of mineralization.
Comparison with Resting Periosteum
In contrast to the resting periosteum, the active periosteum shows a much thicker cellular layer with prominent, active osteoblasts.
The presence of the lighter, partially calcified zone under the osteoblasts indicates active bone growth or repair.
Additionally, the appearance of trapped osteoblasts suggests the ongoing formation of osteocytes, which will maintain the bone matrix once calcification is complete.
Histological section of bone tissue
Osteoblasts and Osteogenic Cells: Osteoblasts are the active bone-forming cells that secrete the bone matrix, while the flatter cells on the surface are osteogenic cells, which can differentiate into osteoblasts.
Osteoid Layer: The blue zone seen on the outer side of the bone represents the osteoid, which is the unmineralized portion of the bone matrix composed mainly of collagen and proteoglycans. It has not yet undergone calcification, hence it does not pick up the pink stain.
Artefact in the Slide: There is a space seen in the section that is an artefact, likely caused during the preparation of the slide. Such artefacts are common in histological studies.
Calcium and Staining: The mineralized part of the bone contains hydroxyapatite, which is why it stains pink, whereas the uncalcified osteoid stains blue.
Osteocytes and Lacunae: The osteocytes, which were once osteoblasts, become trapped in the bone matrix within spaces called lacunae. The blue zone around the lacunae indicates the potential for rapid calcium turnover, as osteocytes can quickly release calcium from the bone matrix.
Ground section of human bone
Osteocyte Lacunae: These are the small, dark voids that show where osteocytes (mature bone cells) would normally reside. The lacunae in this ground bone section are emphasized using Indian ink, which fills the voids, making them easier to see.
Canaliculi: These are the fine channels extending from the lacunae, appearing like small branches or threads in the image. The directionality of the canaliculi is significant because it shows that they grow as the bone itself is forming and growing in a particular direction (appositional growth). Since the osteoblasts can only extend the canaliculi during bone formation, this growth reflects the orientation of bone deposition.
Appositional Growth: The canaliculi’s arrangement helps us infer the direction of bone growth. Since the osteocytes cannot bore into existing bone to extend their processes, the canaliculi can only form in the direction of new bone growth, illustrating the expansion of the bone matrix.
Lattice Network: Even though the canaliculi follow the growth direction, they still form an interconnected lattice of channels. This network is crucial for monitoring and maintaining the bone matrix, allowing osteocytes to communicate with each other and the surrounding bone environment.
Rows of Osteocytes: In the image, the lacunae (osteocytes) tend to line up in rows, suggesting an underlying organizational pattern within the bone tissue, possibly related to the alignment of collagen fibers or the orientation of the bone growth process.
Faint Lines: The faint outlines of lines running vertically in the image could represent lamellae, layers of bone matrix that develop in concentric rings around the central canal in compact bone, contributing to the structural organization.
Scanning Electron Micrograph (SEM) Detail
SEM images provide greater detail of the bone’s microstructure, particularly how an osteoblast gradually transforms into an osteocyte as it becomes embedded in the osteoid. The “furry” appearance of the osteoid under SEM is due to visible collagen fibers, which are more pronounced in the uncalcified matrix. Once the bone calcifies, these fibers become obscured by the smooth mineralized layer of hydroxyapatite.
Osteocyte Processes: The osteoblast, as it transforms into an osteocyte, reaches out through the canaliculi to form connections with neighboring cells. This network is essential for bone health and homeostasis, allowing cells to monitor and adapt to changes within the bone matrix.
Osteoclast Activity: The diagram you referenced shows an osteoclast’s effect on bone tissue. Osteoclasts resorb (break down) bone, leaving behind empty lacunae where osteocytes once resided. The absence of the osteocyte could be due to cell death or, in some cases, the osteocyte reverting to an osteogenic cell, depending on the context.
Lacunae and Canaliculi: Even after osteoclast activity has removed the osteocyte, the lacunae and canaliculi remain, showing where cellular processes once extended. These channels remain important as they indicate the routes through which osteocytes communicated and regulated the bone matrix.
Rickets
Rickets occurs when there is insufficient calcium or vitamin D in a child’s diet, which are both essential for proper bone mineralization. Without these nutrients, the bones remain soft and malleable.
Calcium and Vitamin D:
Calcium is necessary for the mineralization of the bone matrix, specifically the calcification of the osteoid that osteoblasts lay down. In rickets, the osteoid is produced but remains unmineralized due to the lack of calcium.
Vitamin D is also crucial because it facilitates the absorption of calcium in the intestines. Without sufficient vitamin D, even if calcium is present, the body cannot absorb and use it effectively.
Bone Deformities: As seen in the images and the X-ray, rickets leads to characteristic bone deformities, such as bowed legs. This happens because the soft, pliable bones are unable to support the child’s weight properly. As the child grows and puts pressure on their legs and other weight-bearing bones, they bend and deform under the load.
