Cartilage & Bone (Exam II) Flashcards
Cartilage
- Is classified as a specialized connective tissue Is usually avascular
- Provides support & allows rapid growth
- Forms the fetal skeleton
- Persists wherever great resiliency is needed, e.g., articular (joint) surfaces, ear, nasal cartilages, larynx, auditory tube
Organization of Cartilage
Composed of:
- Chondrocytes in lacunae
- Matrix
- Perichondrium (absent in fibrocartilage & articular hyaline cartilage)
Can be distinguished from bone because:
- Growing cartilage has isogenous groups; bone never does.
- An isogenous group is a cluster of closely spaced chondrocytes derived from a single cell by mitosis
- Cartilage lacks the canaliculi that characterize bone
- There are usually “no” blood vessels in cartilage vs. many in compact bone (in Haversian & Volkmannʼs canals)
Perichondrium
- Nutrients diffuse from blood, through perichondrium, into the matrix
- Divided into 2 layers:
- Fibrous layer is outermost - contains fibroblasts, collagen fibers (mainly type I)
- Chondrogenic layer is in contact with cartilage matrix - contains stem cells that can differentiate into chondroblasts - cells are called chondrocytes when completely surrounded by matrix
- Chondrogenic layer is only clearly identifiable by LM when appositional growth is actively occurring (otherwise the chondrogenic cells are flat like fibroblasts)
Cartilage Matrix
- There is the territorial matrix (capsular/pericellular matrix) that immediately surrounds each lacuna. Contains more GAGs. More basophilic/PAS if GAGs have been conserved.
- There is the interterritorial matrix which is the remainder. Contains more collagen.
What does the cartilage matrix contain?
- Collagen (type II in hyaline & elastic cartilage; type I in fibrocartilage).
- GAGs, proteoglycans, and proteoglycan aggregates.
- Smaller adhesive “glycoproteins” such as chondronectin which promotes adherence of chondrocytes to matrix.
- Elastic fibers (in elastic cartilage)
Chondrocytes
- Defining characteristic: Are completely surrounded by matrix
- Round to oval cells that often shrink during fixation
- Produce most components of the matrix
- Receive poor oxygen supply
- Limits the thickness of a cartilage
- Makes cartilage slow to heal if damaged
- Often accumulate lipid droplets in cytoplasm as they age
Interstitial Growth of Cartilage
Defined as addition of new molecules to the matrix by chondrocytes in the interior of the cartilage
Often involves mitosis of chondrocytes to form isogenous groups New matrix is then laid down between cells, which gradually pushes cells of isogenous group further apart
Appositional Growth of Cartilage
Stem cells in chondrogenic layer of perichondrium differentiate and add new matrix to the outer surface of the cartilage
Hyaline Cartilage
- Most common type. Matrix looks homogeneous or glassy (= hyaline) because collagen type II fibrils are so thin they are not visible by LM
- Has a perichondrium (except for articular cartilage)
- May calcify with age
- Adult locations: Articular cartilage Costal cartilages (of ribs) Most laryngeal cartilages Tracheal rings & bronchi (as irregular cartilage plates)
- Other locations: Fetal skeleton Epiphyseal plates
Elastic Cartilage
- More cellular than mature hyaline cartilage
- Less likely to calcify
- Matrix contains:
- Collagen type II
- Elastic fibers visible by LM
- Has a perichondrium
- Found in locations such as (think of the letter E): Auricle (pinna) of the ear Cartilaginous part of external auditory canal Wall of auditory (Eustachian) tubes Epiglottis Corniculate & cuneiform cartilages of larynx
Fibrocartilage
- Interterritorial matrix is highly fibrous
- Contains many thick collagen I fibers visible by LM
- Territorial matrix is more homogeneous
- Contains some type II collagen
- Chondrocytes are often arranged in rows between fibers
- No perichondrium
- Found in: Annulus fibrosus of the intervertebral disks Symphysis pubis Some bone-ligament or bone-tendon junctions
How to distinguish between cartilage types in EM:
- The matrix of:
- Fibrocartilage contains bundles of banded collagen type I fibrils
- Elastic cartilage contains elastic fibers
- Hyaline cartilage contains only a fine feltwork of thin collagen type II fibrils
Functions of Bone
- Support and protect fragile tissues and organs (e.g. brain, spinal cord)
- Contain hematopoietic tissue in marrow cavities
- Form system of levers and pulleys with muscles that makes movement possible
- Store calcium
Bone Tissue
- Bone tissue is a specialized connective tissue composed of cells and extracellular matrix:
- Bone cells:
- Osteoprogenitor cells
- Osteoblasts (and bone-lining cells)
- Osteocytes
- Osteoclasts
- Bone matrix, which consists of:
- Organic components (collagen type I, GAGs, proteoglycans, & glycoproteins such as osteonectin, osteocalcin & osteopontin)
- Inorganic components (minerals, mainly calcium salts such as hydroxyapatite)
Bone (as an organ)
- Includes other tissues in addition to bone, such as: - connective tissue (in the periosteum)
- Usually some cartilage (epiphyseal plates &/or articular cartilage)
- Hematopoietic tissue & adipose tissue (in marrow cavity)
- Nerve tissue
- Smooth muscle (in blood vessel walls)
Decalcified Bone
- Most of the inorganic components are removed Organic components (including cells) remain
- Can be embedded, sectioned, & stained with conventional histologic stains such as H&E
- NOTE: Even “decalcified” bone tissue will usually still contain some calcium salts
- Visible by EM as black crystals They help distinguish bone tissue from cartilage by EM
Ground Bone
- Is mechanically ground down to make specimens so thin that they are translucent
- Organic components are lost
- Inorganic components remain
- Usually stained with India ink, which fills empty spaces (canaliculi, lacunae, Haversian canals)
Osteoprogenitor Cells
- Can differentiate into osteoblasts
- Found in two locations:
- Inner (osteogenic) layer of periosteum
- Endosteum
- Difficult to identify because they resemble the fibroblasts of the periosteum and the bone-lining cells of the endosteum
Osteoblasts
- Are uninucleate cells derived from osteoprogenitor cells
- Cuboidal to columnar depending on their level of activity
- Basophilic cytoplasm due to extensive RER Found on any surface of bone (periosteum or endosteum) where matrix is being actively deposited
- May line up side by side so they resemble a simple epithelium
- Communicate with each other & with osteocytes via gap junctions
- Produce the organic components of bone matrix (osteoid)
- Release matrix vesicles into the matrix
- The alkaline phosphatase contained in matrix vesicles is important for mineralization
- On surfaces (periosteal & endosteal) where matrix deposition is not occurring, osteoblasts become squamous cells called bone-lining cells
- Bone lining cells are indistinguishable by ordinary means from inactive osteoprogenitor cells in periosteum or endosteum
Osteocytes
- Uninucleate cells derived from osteoblasts
- When an osteoblast becomes completely surrounded by bone matrix it is called an osteocyte
- Lacuna = the space within bone matrix occupied by an osteocyte
- Canaliculi (“little canals”) = tunnels that run through the matrix & connect neighboring lacunae
- Contain cytoplasmic processes from neighboring osteocytes
- Gap junctions connect the processes
- Osteocytes function to maintain bone matrix
- Probably help maintain blood Ca++ levels by resorbing matrix from a narrow zone surrounding the lacuna, thus releasing Ca++ into the blood (= osteocytic osteolysis)
Osteoclasts
- Large multinucleated cells (2-50 nuclei)
- Derived from uninucleate cells that fuse
- Differentiate from the same bone marrow precursor as monocytes (i.e., not from osteoprogenitor cells)
- Acidophilic cytoplasm due to numerous lysosomes & mitochondria
- Found on any surface of bone (periosteal or endosteal) where bone resorption is occurring
- Function is to resorb bone matrix for modeling or remodeling purposes by secreting: Lysosomal enzymes (e.g., collagenase) to digest organic matrix Acid (HCl) to solubilize inorganic components of bone matrix
- Found near ruffled border = highly invaginated area of plasma membrane where bone resorption is occurring Increases surface area for active transport of ions (e.g. H+) and for endocytosis of matrix degradation fragments
- Clear zone (sealing zone) forms a ring around ruffled membrane and seals plasma membrane tightly to bone Contains abundant actin filaments
- Resorption produces a depression in the bone surface directly under the osteoclast (= a resorption bay or Howship’s lacuna)
Periosteum
- Periosteum covers outer surface of a bone except at the articular surfaces
- Has an outer fibrous layer and an inner osteogenic layer
- Fibrous layer contains collagen fibers & fibroblasts
- Osteogenic layer contains osteoprogenitor cells - can also contain osteoblasts, bone-lining cells & osteoclasts
Sharpey’s Fibers
- Are bundles of collagen fibers that penetrate the periosteum & enter bone matrix at an angle
- They anchor tendons, ligaments, or teeth, as well as the periosteum to the bone
Endosteum
- Endosteum lines most inner surfaces of bone
- It lines Haversian canals, Volkmannʼs canals, & the marrow cavity (including inner surface of compact bone & the outer surface of all trabeculae)
- It DOES NOT line lacunae or canaliculi
- Can include osteoprogenitor cells, osteoblasts, bonelining cells & osteoclasts
Spongy (Cancellous) Bone
- Has thin anastomosing plates (trabeculae) of bone surrounded by interconnected spaces that are part of the marrow cavity
- Has a “honey-comb” appearance
- Spicules contain no blood vessels (are thin enough that osteocytes within them can be nourished by diffusion from vessels in the marrow cavity)
- Spongy bone is found in interior of bones, especially: at epiphyses & metaphyses of long bones in the diploë of flat bones
Compact (Cortical) Bone
- Has a more solid appearance with the naked eye; no trabeculae
- Found on outer surface of all bones
- Contains blood vessels that often run within canals (e.g., Haversian canals, Volkmannʼs canals)
Woven Bone
- Characterized by random orientation of collagen fibers, creating an irregular or “woven” appearance
- Is more cellular than lamellar bone
- Lacunae are distributed & oriented more randomly than in lamellar bone
- Formed before lamellar bone in embryonic development & in repair of fractures
- Normally replaced later by lamellar bone
- Less mineralized than lamellar bone
- Can be compact or spongy
Lamellar Bone
- Characterized by layers (lamellae) of bone matrix
- Collagen fibers within an individual lamella are oriented in the same direction
- Fibers in one lamella are oriented at an angle to those in the neighboring lamellae (“plywood” arrangement for added strength)
- Osteocytes lie in the plane between successive lamellae & hence have an orderly distribution & orientation
- Lamellar bone forms as a result of bone remodeling
- Can be compact or spongy
5 Locations where lamellar bone is found
- Outer circumferential lamellae
These encircle the outer surface of the entire bone just deep to the periosteum
- Inner circumferential lamellae
Encircle the inner surface of compact bone just superficial to the endosteum & the marrow cavity May not be present in areas where spongy bone lines the marrow cavity (e.g., near metaphyses or epiphyses)
- Concentric lamellae
Make up the osteons surrounding Haversian canals
- Interstitial lamellae:
Remnants of old lamellae that were partially removed by osteoclasts during remodeling
- Trabeculae of mature spongy bone (where woven bone has been replaced by lamellar during remodeling)
Haversian Systems (Osteons) include:
- Haversian canal (= central canal) is an endosteum-lined channel containing blood vessels & loose connective tissue
- Osteocytes lie in lacunae that are layered between the lamellae
- Canaliculi that connect lacunae
- Cement lines that separate neighboring osteon
Volkmann’s Canals
- Endosteum-lined channels that interconnect Haversian canals
- They also connect with vessels from periosteum & marrow cavity
- Oriented transverse or oblique to the Haversian canals
- Cut across the concentric lamellae of osteons
Bone Tissue: Extracellular Matrix (Organic Matrix)
- Type 1 Collagen which forms fibrils
- Ground substance (proteoglycans like GAG and linkages to hyalurionic acid as well as glycoproteins like osteocalcin, osteonectin, osteopontin, and sialoproteins).
Bone Tissue: Extracellular Matrix (Inorganic Matrix)
Ca10(PO4)6(OH)2 crystals, similar to hydroxyapatite.
Crystals deposited in gaps between collagen fibers, following in their orientation.
Calcification lags behind fiber formation
When bone first deposited at site of growth or repair it lacks mineral component = osteoid.
As the matrix mineralizes, it gets its characteristic rigidity.
Role of PTH in controlling bone cell activity
- With increased PTH, osteoblasts stimulate maturation of osteoclasts through the release of macrophage colony stimulating factor (M-CSF) and RANKL molecules.
- With decreased PTH, Osteoblasts release Osteoprotegerin - this can bind RANKL to prevent differentiation of osteoclasts.
Role of Estrogen in controlling bone cell activity
Higher estrogen levels increase osteoblast production of osteoprotegerin; suppresses RANKL production and prolongs life of osteoblasts.
Vascular supply of bone
Supply through:
(1) Nutrient artery
(2) Periosteal vessels
(3) Epiphyseal/ metaphyseal vessels
Vessels in periosteum and medullary cavity continuous with those within the bone tissue (e.g. vessels within primary vascular channels; Volkmann’s canals; Haversian canals)
Chondroblasts
- Found within inner layer of perichondrium
- Responsible for secreting cartilage matrix (appositional growth)
- When they have secreted sufficient matrix to become completely surrounded, they are then known as chondrocytes
Chondrogenic Cells
- Found within the inner layer of perichondrium
- Differentiate into chondroblasts
Degeneration of Cartilage
- Degeneration of cartilage occurs when chondrocytes deep in thick cartilage cannot get sufficient access to nutrients in perichondrium
- Chondrocytes begin to atrophy; water content decreases.
- Cartilage may calcify, further impeding nutrition, leading to cell death.
- Process important as part of endochondral bone formation.
- Also occurs as cartilage ages; important in arthritic changes at joint surfaces
Regeneration of Cartilage
Regeneration: ability of cartilage to regenerate is very limited:
- Chondrogenic activity of perichondrium limited in adulthood; chondrocyte ability to produce matrix too slow to repair damage.
- Acute injuries in adult instead replaced by vascularized CT (granulation tissue); may eventually lose vascularity and persist as fibrous tissue.
Role of cartilage in skeletal formation
- Serves as a precursor to bone (cartilage model) in endochondral ossification
- Maintenance of a cartilaginous growth plate in long bones allows for continued bone growth in areas loaded in compression, and without disruption of joint function
Role of cartilage in osteoarthitis
- Osteoarthritis can initially be detected with changes in the structure of the articular cartilage.
- Articular hyaline cartilage surface becomes undulating in configuration, and little shred-like projections appear.
- Loss of protective articular cartilage will affect underlying bone, and chronic inflammation will affect the synovial fluid and membrane.
Why do proteoglycans cause the matrix of cartilage to stain basophilic?
They have many sulfate and carboxyl groups that impart a net negative charge to the matrix. Tissue component with net negative charges are basophilic.
Why do chondroblasts have more RER and Golgi cisternae than mature chondrocytes?
Chondroblasts are more active in synthesizing and secreting matrix proteins than chondrocytes.
Why don’t you see large secretory granules in osteoblast cytoplasm?
Osteoblast secretion is constitutive rather than regulated, i.e., secretory product is packaged into small vesicles and secreted as soon as it is synthesized rather than being stored in large secretory granules until a secretory stimulus is received.
What is the major type of collagen found in the matrix of hyaline cartilage? Of elastic cartilage? Of fibrocartilage?
In hyaline and elastic cartilage the main type of collagen is type II. In fibrocartilage it is type I. The larger fiber bundles that are typical of type I collagen account for the more fibrous appearance of the matrix in fibrocartilage.
Why do you think articular hyaline cartilage does not repair as well as hyaline cartilage in other locations?
Articular hyaline cartilage lacks a perichondrium and thus does not have a good source of new chondroblasts that could aid in repair.
Which of the following surfaces in bone tissue is/are lined or covered by endosteum: Outer circumferential lamellae, inner circumferential lamellae, Haversian canal, Volkmann’s canal, trabeculae, lacunae?
All the above except outer circumferential lamellae and lacunae represent surfaces that are lined or covered by endosteum. Outer circumferential lamellae are on the outer surface of bone rather than the inner surface, so they are covered by periosteum rather than endosteum. Lacunae have no lining layer at all– neither endosteum nor periosteum. They only contain osteocytes.
Why can’t bone grow interstitially?
Interstitial growth occurs in cartilage but not bone. It is carried out by the cells that already reside in lacunae. They bring about growth of the tissue by secreting additional matrix, and often by dividing to produce isogenous groups. Bone cannot grow interstitially because the matrix is calcified. There is no room for large amounts of new matrix molecules, and no way that the cells in their rigid lacunae can divide.
What is the difference between bone tissue and the organs known as bones?
An organ by definition is composed of more than one tissue type. Most bones, for example, include bone tissue, cartilage, nerve, hematopoietic tissue in the bone marrow, smooth muscle in blood vessels, connective tissue in the periosteum, etc. In contrast, bone tissue consists of the matrix and the cells that make it, maintain it, and remodel it (osteoblasts, osteocytes, and osteoclasts).
What is the difference between woven bone and lamellar bone?
The major difference is that the collagen fibers of the matrix are arranged irregularly in woven bone, but are laid down in a very regular pattern in lamellar bone. Thus, in woven bone the fibers are oriented in many different unpredictable directions, while in lamellar bone the fibers within an individual lamella have a single predominant orientation. The direction of this predominant orientation is different in adjacent lamellae, thus building up a plywood-like structure that is very strong. There are other differences as well. For example, woven bone is deposited much more quickly than lamellar bone, and therefore is more cellular. It has more loosely packed and poorly organized mineral crystals, and is generally less strong relative to lamellar bone. However, because it can be laid down quickly, it is found in areas of new and rapid bone growth, and is the first type of bone tissue formed in the skeleton of the developing fetus.
What is the difference between spongy bone and compact bone?
The difference has to do with how much of the volume in a given area is occupied by bone tissue. In compact bone nearly all the volume is filled by bone tissue, whereas in spongy bone the bone tissue occupies a much smaller percentage of the volume. Spongy bone exists as trabeculae that form an open meshwork. The abundant space that surrounds each trabecula is continuous with the marrow cavity.
Is it true that some bones are composed completely of compact bone and others are composed completely of spongy bone?
No. Every bone has both spongy and compact portions. The compact bone tissue is on the outer surface of the bone, while the spongy bone tissue occupies part of the interior of the bone. In a mature long bone for example, spongy bone tends to be found near the epiphyses. There is relatively little of it in the diaphysis, where the marrow cavity is directly surrounded by compact bone.
Can spongy bone be woven bone? Can it be lamellar bone?
When the trabeculae of spongy bone first form during bone development they are composed of woven bone, but as new bone is added to the surface of each trabecula during growth or remodeling, that new bone tissue is laid down as lamellar bone (i.e., in layers whose collagen fibers are regularly arranged). In other words, spongy bone can be either woven or lamellar.
Can compact bone be woven bone? Can it be lamellar?
Just as with spongy bone, compact bone can also be either woven or lamellar. When compact bone first forms during bone development, it is woven, but the bone tissue that is added to it during growth and remodeling is lamellar.
What cells give rise to osteoclasts?
Osteoclasts come from the same precursor cells in the bone marrow that give rise to monocytes and macrophages.
What purpose does the ruffled membrane serve in osteoclasts? Is it found over the entire surface of an osteoclast?
The ruffled membrane (also called the ruffled border) is another mechanism for increasing the membrane surface area of a cell. Other cell types accomplish this by having microvilli or basal invaginations of the plasma membrane (as in the cells of striated ducts), but osteoclasts use the more irregular folds and surface projections that we call the ruffled membrane. This increased surface area is needed to accommodate many copies of an ion pump that transports hydrogen ions out of the cell. These ions are secreted into the extracellular compartment called a Howship’s lacuna or resorption bay. The increased acidity in this limited space helps to solubilized the mineralized matrix of the adjacent bone surface. The ruffled membrane is not found over the entire surface of an osteoclast. It is limited to the surface that is in immediate contact with the bone, and is bounded by a “sealing zone” (also called the clear zone) where the cell membrane is closely adherent to the bone. The clear zone acts like a gasket to seal off the acidified Howship’s lacuna from the rest of the extracellular space.
What are each of the arrows pointing to in this electron micrograph?
- Osteoprogeintor cells
- Osteoblast
- Osteoid
- Mineralized bone matrix
- Osteocyte
What is the red arrow pointing to? What is the blue arrow pointing to? Where can you find the Haversian Canal? What about the Volkmann’s Canal?
- Red: periosteum
- Blue: endosteum
- # 3: Haversian Canal
- # 5: Volkmann’s Canal
What are the arrows pointing to?
Top pair of arrows: Osteoblasts
Adjacent pair of arrows: Osteocytes
Bottom pair of arrows: Osteoclasts
What are each of the black lines pointing to?
- Top: Concentric lamellae
- Middle: Haversian canal
- Bottom: Lacuna
What are the arrows pointing to? What kind of tissue is this?
This is cancellous bone.
The first bottom arrow: Developing marrow cavity
The second bottom arrow: Trabecula
What type of structure is encircled here? What is this structure’s purpose?
Cement line. Furthest extent to which osteoclast removed bone and then osteoblast came in and started laying down bone.
What do the 3 arrows point to?
- Top arrow: perichondrium
- Middle arrow: territorial matrix
- Bottom arrow: interterritorial matrix
Which tissue is the first arrow? Which tissue is the second arrow?
First arrow: cortical/compact bone
Second arrow: cancellous/spongy bone
What type of tissue is this?
Cortical/Compact Bone
Identify the type of preparation used for this bone: decalcified or ground? What is the advantage of this preparation?
Decalcified bone preserve organic components.
What type of tissue is this? How do you know?
Elastic cartilage. Note the weird black blobs or elastic fibers.
What type of cartilage is this? How do you know? Where is it found?
This is elastic cartilage due to the presence of elastic fibers, perichondrium and Type II Collagen Fibers.
Elastic cartilage is found in the pinna of ear, external auditory canal, Esutachian tube, and epilglottis.
What type of tissue is seen here? How do you know?
Fibrocartilage. Note the collagen at #6 and the Type I collagen at #7 (transversely cut).
Distinguish the two types of tissues.
Fibrocartilage (left) and dense connective tissue (right). Chondrocytes in fibrocariltage tend to be more rounded than fibroblasts in dense CT
What type of tissue is this? How do you know?
Fibrocartilage. Has very fibrous matrix and often has linear isogenous groups. Has collagen Type I
What type of preparation was used for this bone? What are the advantages? What do the arrows indicate?
Ground bone preserves inorganic components. Right arrow: canaliculi and left arrow: lamellae.
Why is this slide significant in terms of fibrocartilage?
Fibrocartilage lacks perichondrium so it often grades into hyaline cartilage or dense connective tissue.
What type of tissue is shown here? How do you know?
Hyaline cartilage because of the homogenous matrix and lipid droplets.
What type of tissue is shown here?
Hyaline cartilage.
What do the arrows indicate?
Concentric lamellae (first arrow on the left) and interstitial lamellae (arrow on the right)
Identify the type of tissue.
Lamellar cancellous bone.
Identify the type of tissue.
Lamellar cortical bone.
What type of tissue is this?
Fibrocartilage in the annulus fibrosis.
Identify the cells with the arrows
Osteoblast & Osteoclast (self-explanatory…)
What cell is this?
Osteoblast.
Deminerlaized (look at bones of crystal)
Osteoid (organic component)
Lots of rough ER to make collagen type 1
What type of cell is this? What is the significance of the ruffled border?
Osteoclast (lol) and the ruffled border is at a site of active bone resorption,
What cell type is shown? What are some properties of this cell type? Where are they situated?
Osteoclasts.
Multi-nucleated
Anatomical Syncytia
Eosinophilic (lysosomes, mitochondria)
Sitting on Howship’s Lacunae
What type of cell is this?
Osteocyte. Note the ostoid (#5) and how the cell is surrounded by bone.
What is this SEM image of?
Cancellous bone.
What are these fibers? What is their significance?
Sharpey’s fibers. Anchors periosteum, tendons, ligaments and teeth to bone
What is the structure shown? What does it do?
Volkmann’s Canal. Volkmann canals are any of the small channels in the bone that transmit blood vessels from the periosteum into the bone and that communicate with the Haversian canals. The perforating canals provide energy and nourishing elements for osteons
Identify the type of tissue.
Woven cancellous bone.
Identify the type of tissue.
Woven cortical bone.
Which is put down first?
Woven bone is laid down first; then lamellar bone.
