Cartilage and bones

What is a chondrogenic cells?

Cells that forms chondroblasts, which secrete matrix in the inner layer of perichondrium

Classify bonemarrow

Red bone marrow is the active bone marrow, in which blood cells are formed. Yellow bone marrow is the inactive bone marrow, which has fat cells.

Why does remodeling happen?

Remodeling is done to optimize bone use. That is, reorganize according to load. It repairs small cracks and replaces dead bone, as well as maintaining calcium homeostasis.

What is meant by remodeling of bone?

Remodeling is where "old" bone is replaced by "new" bone.

What is the difference between cortical and trabecular bone remodeling?

Remodeling takes place in both the cortical and trabecular bone, the only difference is how the process happens, that is, how the osteoblasts "eat" bone. In the trabecular bone, grooves are made into which the osteoblasts fill. In the cortical, boreholes/cylinders are made which the osteoblasts then fill. In the cortical bone, the haversian systems are seen as cylinders. Thus, remodeling of the cortical bone involves the formation of new cylinders on top of old ones. The trabecular bone still has bone units, but not cylinders - they have a more flattened shape. It is not called haversians systems, but just bone units / osteons.

What is a lacuna?

Spaces/cavities in the matrix where chondrocytes are located. Visible when cells chrink during histological preparation (artefact). Cells usually fill the entire lacuna.

Describe spongious bone tissue

Spongious/cancellous bone is lighter and less dense then compact bone. It is a cylindrial and plate-shaped structure that can be seen more like a mesh. consist of trabecular osteons, which contains lamellae, osteocytes and canaliculi etc.

Name the different zones of the epiphyseal plate

(from top to bottom of the epiphyseal plate): 1. The zone of reserved cartilage: superficielt, grows slowly in all directions. 2. The zone of proliferation: cells are arranged in columns (columneformation), hyperplasia (large number of cells divide -> proliferation) 3. The zone of hypertrophy: cells increase in size 4. The zone of calcified cartilage: begining of the green color, calcium (crystals) depoists in the matrix 5. The zone of resorption: bone formation from cartilage skeleton. The mineralized cartilage matrix is transformed into solid mineralized bone tissue. The osteoclasts first resorb the calcified cartilage matrix, after which the osteoblasts use the remaining calcified cartilage matrix as the foundation for bone formation.

Describe how long bones are constructed. Name the different components.

- Epiphyses are the ends - Epiphyseal discs are where longitudinal growth occurs - Metaphyses are the transition between the diaphysis and the epiphysis - The diaphysis is the long part (shaft) - Articular hyaline cartilage covers the ends of epiphyses - Periost is thin connective tissue layer and cells that line the outside of the bone - Endost is thin layer of cells that line the inside of the bone

Which two ways can bone tissue be formed/ossificated?

1. Endochondral ossification is where bone is built from a cartilage model. Bone is formed by replacing hyaline cartilage skeleton. This is also known as indirect ossification. This type of ossification applies to the extremities as well as the axial skeleton. 2. Intramembranous ossification is where bone is built from embroynic mesemchymal tissue (simple connective tissue). This is known as direct ossification. This type of ossification is seen for the flat bones, eg. cranial bones.

Mention the step of the remodeling process.

1. Resorption: The osteoclasts resorb bone, ie. removes bone. It happens differently depending on whether it is trabecular or cortical. 2. Reversal: Resorption stops and vessels are introduced (vascular ingrowth). The osteoblasts form the cement line. 3. Formation: The osteoblasts form osteoid, which are mineralized. The osteoblasts then becomes either lining cells or osteocytes. The result of the remodeling process is thus formation of new bone with Haversian systems/or osteons.

Describe the two ways cartilage can grow

1. interstitial growth where new cartilage is formed by mitotic division of chondrocytes within already existing cartilage (expansion from within). This type of growth occurs in the early fetal life (at the primary cartilage formation in the fetus) and later at the epiphyseal disc of long bones and articular cartilage. 2. Appositional growth where new cartilage is formed on the surface of an already existing cartilage layer (increase in width). Chondrogenic cells -> chondroblasts -> secrete matrix and differntiate to chondrocytes. This type of growth take place throughout life in the perichondrium.

How many zones are the epiphyseal disc divided into?

5

What makes up a Haversian system and what are these systems also called?

A Haversian system is also called a osteon. It consists of a central haverisan canal with blood vessels, lamellae, lacunae with osteocytes, and canaliculi. The osteons are interconnected via Volkman canals, which provide the osteons with vessels and nerves. A Haversian system is limited by the cement line, which can be seen as a dark line surrounding the system.

What is meant by isogenous groups?

An isogenous group is a cluster of chondrocytes (up to 4 cells). When chondrocytes are present in isogenous groups, they represent recently divided cells, and are all formed through division of a single progenitor cell. These groups can be found in hyaline and elastic cartilage.

What is the function of osteoblasts?

Bone formation. Osteoblasts come from osteoprogenitor cells, and can be seen as cubic cell on the surface of bone tissue. They create osteoid, and are thus bone-forming cells. After bone formation two things can happen; either the osteoblasts are incorporated into new bone as osteocytes or become a lining cell.

What is the function of osteoclasts?

Bone resorption. Osteoclasts come from monocytes/macrophage cell line. They are large cells with multiple nuclei found in places where bone is removed. They rest directly on the bone tissue where reabsorption takes place. As a result of osteoclast activity, a shallow deepening called resorption bay (Howship lacuna) can be observed in the bone directly below the osteoclast.

What is the function of bone tissue?

Bone tissue is supportive tissue, it is both harder and stronger than cartilage. Its function is to support and protect soft/vital organs (by enclosing). It also provides levers for the muscles to act on. It contains bone marrow (blood cell formation) as well as calcium stores, ie. the way the body can release calcium.

What is the functions of cartilage?

Cartilage is supporting tissue that can withstand heavy loads. It can be seen as the scaffold on which most bones build. It has a stiffening and supportive effect It ensures that bone and joints can move smoothly against each other (e.g. the knee joint). Makes up majority of fetal/immature skeleton

What are canaliculi?

Canaliculi are small canals into which the small processes of osteoblasts project into. They can be found between lacunae and they connect to other canaliculi, and vasucular canals. They can be seen as a passageway through the bone matrix for lacunae to the Haversian canal .

How are the groundsubstance functionally important in cartilage?

Cartilage consists of a lot of water and ground substance. The many proteoglycans will make the ground substance hydrophilic, which draws liquid into the cartilage. Thereby, the cartilage is filled with water until the fibers do not allow further expansion. This gives the cartilage a resilience, and this also allows cartilage to act as a shock-absorber.

How does cartilage develop?

Cartilage develop by mesenchyme differentiations, that is, the mesenchymal stem cells can become chondroblasts.

Cartilage is a _____________ structure?

Cartilage is an avascular structure; therefore, the composition of the extracellular matrix (high ratio of GAG to type II collagen fibers) is crucial for diffusion of substances between chondrocytes and blood vessels in the surrounding connective tissue. Also, it has limited ability to repair.

Mention the cells of cartilage

Chondroblasts lie in oval/oblong lacunae close to the perichondrium. Chondrocytes lie in more rounded lacunae located deeper in the cartilage. They also synthesize and secrete the matrix material.

What are the different types of bone tissue?

Compact/cortical bone covers the outer surface of the bone, makes up 80 % of all bone tissue. Spongious/trabecular bone covers the inner surface of the bone, makes up 20 % of all bone tissue.

Describe compact bone tissue

Compact/cortical bone is strong/dense and resistant. It consist of Haversian systems, which are small longitudinal cylinders. In each system a central (haversian) canal contains nerves and blood vessels. Around the central canals are rings of concentric lamellae. Interstitial lamellae surrounds the Haversian systems (between lamellae) and consists of residues from old haversian systems. Circumferential lamellae run parallel to the surface. Volkman canals interconnect the Haversian systems as well as connect them to surface or the inside. Between lamellae are lacunae (small cavities), which are connected by canaliculi. Bone cells (osteocytes) and their multiple extensions fill these spaces.

What are the three types of lamellaes in compact/cortical bone?

Concentric lamellae: surrounds the central canal Interstitial lamellae: surrounds the Haversian systems Circumferential lamellae: run parallel to the surface or the inside

What happens during bone growth?

During bone growth the reserved cartilage of the epiphyseal plate grows/divide slowly in all directions. At the same time, the epiphyseal plate remains the same size (balance between new formation of cartilage and removal) but is moved further up like the osteoblasts.

Describe elastic cartilage and its matrix

Elastic cartilage is characterized by elastic fibers and elastic lamellae in addition to the matrix material in hyaline cartilage (collagen type II fibers, etc.). The presence of elastin in the extracellular matrix gives elastic cartilage great flexibility to withstand repeated bending. It is always surrounded by perichondrium. The elastic cartilage matrix does not calcify during aging.

Describe fibrocartilage

Fibrous cartilage acts as a transition between connective tissue and hyaline cartilage, and can thus be seen as a combination of this and dense regular connective tissue. It consists of chrondocytes scattered among the collagen fibers either individually, in rows or in isogenic groups. It's matrix contains collagen type II and I. This type of cartilage do not have perichondrium.

What type of cartilage is most flexible?

From most to less flexible: 1. Elastic cartilage 2. Hyaline cartilage 3. Fibrous cartilage

What is the color of mineralized bone tissue?

Green. The combination of both bone matrices are called mineralized bone. When only the organic matrix is present, the tissue is red, however, when both the organic and inorganic matrix are together, the tissue is green.

List the different types of cartilage

Hyaline cartilage Fibrous cartilage Elastic cartilage

Describe hyaline cartilage and its matrix

Hyaline cartilage acts as a scaffold for bone growth. It provides a low-friction surface and participates in the lubrication of synovial joints. It is characterized by a matrix containing collagen type II fibers, GAGs, proteoglycans and mutli-adhesive glycoproteins. It has perichondrium. It is seen e.g. on the bone surface at synovial joints, in the trachea and bronchi.

Where does lengthening of the bone take place?

In the cartilage of the epiphyseal plate. The epiphyseal plate is between the epiphysis and diaphysis.

Where is fibrocartilage found?

In the intervertebral disc of the spine and pubic symphysis.

What is lining cells?

Lining cells are cells that cover all inactive bone surfaces, that is, surfaces where no remodeling takes place. They can also be called bone-covering cells or surface-osteocytes.

Classify bones according to shape

Long bones are tubular with two ends (proximal and distal epiphysis) and a long shaft (diaphysis). Short bones are almost identical in length and diameter Flat bones are flat and consist of two thin layers of compact bone with an intermediate layer of spongious bone. Irregular bones have a shape that does not fit into any of the three groups. The shape may be complex, or the bone may contain airways or sinuses.

Differentiate between megakarocyt and osteoblast.

Megakaryocytes give rise to platelets, they are related to sinusoids (vessels). They have one "lapdelt" core (they stick together) and a more stringy edge. Osteoclasts are related to bone. They have up to 10 separate cores.

What is the combination of the two bone matrices called?

Mineralized bone.

State the osteoblasts cell line, how does it differentiate?

Osteoblast cell line: mesenchymal stem cell -> osteoprogenitor cells -> preosteoblast -> osteoblasts -> lining cells -> osteocytes

Which of the five cells of bone tissue can be seen in the microscope and which cannot easily be seen?

Osteoblasts and osteoclasts are visible, whereas Osteoprogenitor cells, osteocytes and linig cells are not easy to detect.

State the osteoclasts cell line, how does it differentiate?

Osteoclasts cell line: hemapoietic stem cell -> osteoclast precursor cell -> mononucleated osteoclast -> multinucleated osteoclast -> mature osteoclast

What does osteoclasts have which make them more effective?

Osteoclasts have brushborder or wavy egde towards the bone, which increase the surface area, and thus their effectivity. The space between the wavy egde of the osteoclast and the bone is called the "subosteoclasts space".

What is osteocytes?

Osteocytes are mature bone cells located in Haverisan osteons embedded in the lacunae. They have many processes/projections in the canaliculi. They have a mechanosensory function. They help in regulation of formation of osteoblasts and bone (where bone must be reabsorped an remodeled).

What is the organic matrix in bone tissue also called?

Osteoid.

Mention the five different cells of bone tissue

Osteoprogenitor cells Osteoblasts Osteocytes Osteoclasts Lining cells

What can osteoprogenitor cells differentiate into?

Osteoprogenitor cells originate form the mesemchymal stem cell and can differentiate into osteoblasts. They are in both periost and endost, but are hardly visible and difficult to distinguish from fibroblasts.

State the different components of cartilage

Perichondrium, is a connective tissue layer that surrounds cartilage (except articular cartilage of joints). It is dense irregular connective tissue. Chondroblasts are immature cartilage cells that lies beneath the perichondrium. Chondrocytes are mature cartilage cells that lies in lacunes. Lacunes are small cavities with chondrocytes. Extracellular matrix, which contains fibers and ground substance.

What are periost/periostedum and endost/endosteum?

Periost cover all outer surfaces of bone. It has an outer layer and an inner layer. It is anchored to the bone via sharpey fibers. It can be seen as a connective tissue membrane that connects to the outer surface of sharpey fibers, which in turn extend into the lamellae. Endost cover all inner surfaces of bone. It consist of a single layer.

What is the functional unit of cartilage tissue?

The extracellular matrix

What does the inorganic matrix in bone tissue consists of?

The inorganic matrix consists of two salts (along with phosphate and calcium), which creates hydroxyappatite crystals. These crystals are deposited between collagene fibers, providing the hardness of bone. It is calcified, and is never seen by itself, but always seen embedded in the organic matrix.

What happens during bone resorption?

The inorganic matrix is broken down by pH, while the organic matrix is broken down by enzymes. Osteoclasts start by removing and breaking down the inorganic matrix and then the organic bone matrix. Hydrochloric acid is formed by the osteoblasts so the pH decrease. This causes the inorganic matrix and the crystals to dissolve. The now pure organic bone matrix is now ready for degradation. The lysosomes that the osteoclast has lying in vesicles in the cytoplasm degrade collagen and the organic bone matrix dissolves. At a given signal from osteoblasts, the osteoclast activity stops and the osteoclasts migrate to other sites where bone resorption is to take place or they undergo apoptosis.

What is the interterritorial matrix?

The interterritorial matrix surrounds the territorial matrix and fill out the space between isogenous groups. It is more lightly stained (acidophilic?)

When does mineralisation of the bone usually happen?

The mineralisation process is typically seen in relation to remodelering of the bone.

Describe the mineralisation process

The organic matrix (osteoid) is produced by the osteoblasts. Once osteoid is formed, the osteoblasts initiate the mineralization process. The primary mineralization process occurs inside the matrix-vesicles. The osteoblasts secrete vesicles that are filled with calcium and phosphate. This forms insoluble hydroxyapatite crystals, which accumulate in the matrix vesicles. The hydroxyapatite crystals are secreted into the surrounding extracellular matrix and deposited between the collagen fibers. The secondary mineralization process takes place outside the matrix vesicles, it is a much more gradual process with further maturation of the mineral component which leads to full/maximum mineralization.

What does the organic matrix in bone tissue consists of?

The organic matrix consist mainly of collagene fibers (type I). In addition, it also contains ground substance (proteoglycaner). These particularly the collagene fibers provide elasticity and tensile strength to the matrix, which give the bone ability to resists stretch. It can further be seen as the skeleton of the inorganic matrix. It is not calcified.

What type of connective tissue are the two layers of periost? and does endost also consists of connective tissue?

The outer layer of periost is fibrous, which resembles dense connective tissue and contains vessels and nerves. The inner layer is a more cellular layer, like loose connective tissue. The inner layer contains progenitor cells which can differentiate into osteoblasts. The collagen fibers of the periost are arranged parallel with the surface of the bone, like a capsule. The single layer of endost can also be seen as connective tissue, it contains different lining cells:

What two layers does the perichondrium consists of? And in what types of cartilage is it not seen?

The perichondrium consists of two layers: an outer fibrous layer and an inner cellular layer. The outer fibrous layer contains fibroblasts and collagen type I, whereas the inner cellular layer has chondrogenic cells that give rise to chondrocytes. Fibrocartilage as well as articular hyaline cartilage does not have perichondrium.

What is the different time intervals for the mineralization proces?

The primary mineralization takes 3-4 days, and make up 80 % of the mineralization, whereas the secondary mineralization can take place over 3-4 month -and constitute the last 20 %.

What is the territorial matrix? and how is it identified?

The territorial/capsular matrix is the area surrounding the lacunae. It is more basophilic, because chondrocytes in the lacuna are active (protein synthesis) and there is a higher concentration of proteoglycans.

How are Volkman's canals different?

They do not have concentric lamellae

What is hematopoietic cells?

They give rise to blood-producing cells, eg.: - Erythrocytes (red blood cells) - Leukocytes (white blood cells) o Granulocytes and lymphocytes - Trombocytes (platelets)

What happens when bone growth stops?

When the growth stops, the first zones in the epiphyseal plate stop proliferating, but the bone formation continues until the entire epiphyseal plate is transformed by osteoblasts into bone, where it becomes the epiphyseal line.

Ossification is the process of __________?

bone tissue formation.

Where is elastic cartilage found?

external ear, auditory tube (middle ear), epiglottis of larynx (hypopharynx)