What do mesenchymal condensations do?
Generate cartilage and bone
Mesenchymal condensations in the embryo predict the location of the future
Skeletal system
Mesenchymal condensations can generate cartilage or bone. What determines their fate?
Proximity to vascularity
Condensations that arise in avascular parts of the embryo form
Cartilage
Condensations that arise in vascular parts of the embryo form
Bone
What is a more likely fate, that condensations form cartilage or bone?
Cartilage
Some embryonic cartilage remains cartilage, but most is transiently replaced by
Bone (does not become bone though)
Cells of condensation give rise to a type of cell that begins to make the ECM, called
Chondroblasts
Chondroblasts that get trapped in the ECM are called
Chondrocytes
Forms outer border of most cartilage
-Generates new chondroblasts around the perimeter of the forming cartilage
Perichondrium
New chondroblasts generated by perichondrium lead to appositional growth around the
Perimeter
What are the two distinct layers of the perichondrium?
- ) Fibroblastic (outermost layer)
2. ) Chondrogenic
Chondrocytes retain the ability to divide and generate
-bone can’t do this
Isogenous groups (clusters)
Grows from within (interstitial) and without (appositional)
Cartilage
Avascular and contains chondrocytes in lacunae
Cartilage
Cartilage is made up of which type of collagen?
Type II
What are the three types of cartilage?
Hyaline, Elastic, and Fibro
Hyaline cartilage is located in the
Nasal, laryngeal, tracheal, and articular regions
Articular hyaline cartilage has no
Perichondrium
Where is elastic cartilage found?
Ear, eustacian, and eppiglottis
A special feature of elastic cartilage is that it contains
Elastic fibers (requires a special stain)
Where is fibrocartilage located?
Symphyses, intervertebral disks, and menisci
Fibro cartilage is intermixxed with
Dense CT (no perichondrium)
Chondrocytes are secretory cells and are distinguished by
Lots of RER and prominent golgi
Stained with basophilic staining and is located right around the edge of chondrocytes
Territorial matrix
Acidophilic staining in the area between chondrocytes
Interterritorial matrix
Caps the bone at moveable joints
Articular cartilage
Articular cartilage is susceptible to osteoarthritis because it lacks a
-the reason it is ill equipped to initiate repair
Perichondrium
Erosion of the articular cartilage in osteoarthritis results in diminution or disappearance of the
Joint space of X-Ray
Made up of a delicate synovial inner layer and a tough fibrous outer layer
Joint capsule
Joints are discontinuous epitheliod intima with no tight junctions or a basement membrane. They are leaky and are comprised of
Loose CT and many vessels
Causes influx of immune cells into the synovium, which results in destruction of the joint
Rheumatoid arthritis
Is radiolucent, i.e. why we can’t see ears on an X-ray
Elastic cartilage
The annulus fibrosis is made up of
Fibrocartilage
Fibrocartilage is made up of which type of collagen?
Type I
Bone can form in two ways. Is the final product different depending on the mechanism?
No
Bone formation used for bones with a complex shape where a cartilage model is laid down first and then replced by bone
Endochondral bone formation
What are the two ways the bone can form?
- ) Endochondral (complex shape)
2. ) Intramembranous (flat bones)
Some of the flat bones of the face and skull develop via the
Intramembranous route
Generates osteoblasts
Periosteum
Prominent in developing bone
Osteoblasts
Osteoblasts trapped in the matrix become
Osteocytes
The matrix of bone is primarily
Type I collagen
The unmineralized bone matrix is called the
Osteoid
Mineralization (CaPO4) of the osteoid yields
Bone
Membranous bone enlarges only through
Appositional growth
The calcified matrix of bone is less than 10% H2O so the diffusion of nutrients and waste removal is
Poor
To fix this, each osteocyte sits in an unmineralized
Lacunae
The lacunae are connected to neighbors and blood supply by processes and
Gap junctions
Processes, nutrients, and wastes pass through tunnels in the bone matrix called
Canaliculi
Has appositional and interstitial growth
Cartilage
Has appositional growth only
Bone
Employs a transient cartilage scaffold that is ultimately replaced by bone
Endochondral Bone
Takes advantage of cartilage’s interstitial growth to creae complex shapes and lengthen bone
Endochondral bone formation
Exist as a bone/cartilage hybrid from prenatal development to early adulthood
Most long bones
Endochondral chondrocyte life cycles overlap, with those furthest from the resting zone being the most
Advanced
Endochondral zones are maintained at the
Growth plate
In endochronal bone formation, the initial bone product is
-highly cellular and disorganized
Poor quality (woven bone)
Soon after the initial bone is formed, osteoblasts and osteoclasts remodel the bone, forming
Lamellar bone
Has fewer osteocytes and is highly organized
Lamellar bone
A multinucleate bone destroyer that plays an essential role in bone maturation, remodeling, repair, and Ca2+ balance
Osteoclasts
What do osteoclasts make to
- ) Dissolve minerals
- ) Degrade collagen
- ) HCL
2. ) Cathepsin K
The divot osteoclasts make when breaking down bone is called
Howships Lacunae
Covers the hollowships lacunae
-new osteoblasts are generated and new bone is produced
Endosteum (analogous to periosteum)
Spongy bone is in contact with blood supply and the trabeculae are lined with
Endosteum
Compact bone has a system of channels called
Volkmann Canals and Haversian canals
To remodel, osteoclasts invade compact bone and create a perpendicular tunnel called a
Volkmann canal
Then, what enters into the volkmann canal?
Endosteum and blood vessels
Osteoclasts then change direction and continue through the long axis of bone creating
Haversian Canal
The Haversian canal is then lined with endosteum that initiates the
Rebuild and repair process
Generates osteoblasts on outer aspect (bone side)
Endosteum
Produce concentric layers of new bone and push the endosteum toward the center of the canal
Osteoblasts and osteocytes
This process continues until the Haversian canal is nearly obliterated by
New bone formation
Occupy the center of the remaining canal
Vessels and nerves
New bone is populated by osteocytes in their lacunae, connected via processes through canaliculi in the matrix. The whole structure is called the
Osteon
In the process of creating a new osteon, and old one is
Destroyed
Remnants of an old osteon lost to the remodeling process
Interstitial lamellae
Defines the outer edge of each osteon
Cement line
The advantage in bone thickness you get from exercising will disappear after
20 years of inactivity
Imbalance between bone reabsorption and bone production
-affects primary trabecular bone
Osteoporosis
Osteoporosis is probably due to diminished osteoblast activity, but most treatments target
Osteoclasts
A discontinuous epithelial intima with no tight junctions or basal membrane
-Very leaky
Synnovial inner layer
What are the two layers of a joint capsule?
- ) Delicate synovial inner layer
2. ) Tough fibrous outer layer