What is Bone?
• Mineralized, rigid connective tissue
• Functions: o Supports soft tissues o Muscle attachment to move body o Protects vital organs o Surrounds bone marrow (blood cell formation) – don’t want to expose it to UV radiations of the sun that could cause mutations o Reservoir of calcium, phosphate, etc.
• Cells:
o Osteoblasts
o Osteocytes
o Osteoclasts
Bone Matrix
• Matrix contains a significant amount of inorganic material – matrix is very MINERALIZED
• Inorganic matter in matrix:
o Calcium, phosphorus, magnesium, potassium, etc.
o Calcium and phosphorus form hydroxyapatite crystals
• Organic matter in matrix
o Type I collagen
o Ground substance
• Metabolites can’t diffuse through the matrix, so CANALICULI allow
communications among cells (they are like tunnels)
How to make histological
slides
• Decalcified bone sections
o Acid solution removes
calcium salts and softens
bone – can’t just slice it
with a microtome like
other tissues
o Bone can then be
embedded and
sectioned
o Osteocytes are distorted• Ground bone sections
o Saw bone in slices and grind pieces down until very thin
o Cells are destroyed, but it will keep integrity of the matrix
Osteoblasts
- Matrix (osteoid) synthesis
- Basophilic cytoplasm
*ORIGIN - MESENCHYME
- Osteoprogenitor > preosteoblast > osteoblast
- Line surfaces of bone tissue (resemble simple epithelium)
• Cuboidal to columnar in shape when active in matrix synthesis; more
squamous when less active
- Become bone lining cells or osteocytes
- Functions:
o Synthesis of unmineralized bone matrix (osteoid)
o Osteoid resorption
o Osteoclast stimulating factors
o Involved in matrix mineralization
Osteocytes
o Derived from osteoblasts
(osteoprogenitors > turn
into Osteoblasts > turn
into Osteocytes)
o Matrix maintenance – do NOT secrete matrix
o Found in lacunae (surrounded by matrix)
*ORIGIN = MESENCHYME
• When osteoblasts surround themselves with matrix, they become
osteocytes
o Each osteocyte is in a space in the matrix called a lacuna
o Osteocytes have long cytoplasmic processes, these are in tunnels
called canaliculi
• Gas, waste, nutrient exchange can’t occur
through mineralized matrix
o The osteocyte processes contact each other via gap junctions
o Exchange using the gap junctions can provide nourishment for a
chain of about ten cells
• Functions:
o Maintain matrix
o Mechanosensory: detect mechanical loading and therefore need
for bone increase or decrease
Osteoclasts
see ppt for pictures
Matrix resorption
o Large, multinucleated (5-50) cells
o Also found on the surface of bone but look much different that
osteoblasts
- Have microvilli
- Sit in a cavity called Howship’s Lacuna as bone is removed
*ORIGIN = HEMATOPOIETIC
Joints see other card deck as well
• Functional classification:
o Synarthrosis – limited/no movement
o Amphiarthrosis – slightly moveable
o Diarthrosis – freely movable
• Structural classification:
o Fibrous joints: connected by dense CT
- Suture: non-movable, only in skull
- Syndesmosis: limited movement, e.g. between radius and ulna
- Gomphosis: limited movement, only dentoalveolar joint
o Cartilaginous joints: connected by cartilage
- Symphysis: fibrocartilage, e.g. pubic symphysis
- Synchondrosis: hyaline cartilage, temporary joint that will eventually fuse, e.g. between sphenoid & occipital
o Synovial joints: articular surfaces with hyaline cartilage, synovial fluid in a capsule; hyaline cartilage in synovial joints does not have perichondrium and are nourished by synovial fluid
Synovial Joints
- Ends of bones are lined by ARTICULAR CARTILAGE, hyaline cartilage with no perichondrium
- Joint held together by ligaments and a joint capsule
• Capsule is lined by a SYNOVIAL MEMBRANE that secretes a lubricating
fluid
• Synovial membrane does not line articular cartilage or discs/menisci (if
present)
Types of Bones
• Bone can be classified in several ways
o SHAPE: long, short, etc.
o MACROSCOPICALLY: compact vs. spongy
o MICROSCIPICALLY: woven vs. lamellar
o EMBRYOLOGICALY ORIGIN: type of ossification
Shapes of Bones
• LONG BONES: longer than they are wide (Most of the limb bones)
• SHORT BONES: approximately cube-shaped
(Mostly spongy bone like wrist and ankle bones)
• FLAT BONES: thin and flat
Two layers of compact bone, with one layer of spongy bone (diploe) between (sternum, ribs, skull bones)
• IRREGULAR BONES: none of the above (some skull bones, hip bones, vertebrae)
Long Bone Anatomy
From outside to inside:
periosteum > compact bone > endosteum > spongy bone
• Diaphysis: shaft
• Epiphyes: at ends
- Spongy bone at
epiphyses
• Metaphysis: between diaphysis and epiphyses
• Epiphyseal plate/line:growth
• Periosteum
- Outer fibrous layer (dense CT)
- Inner osteogenic layer
- On the outside of the
compact bone• Endosteum
o One the inside of the compact bone
• Articular cartilage (instead of periosteum) –
at joint surfaces
Periosteum and Endosteum
- Surfaces of bone covered by periosteum (external) or endosteum (internal)
- Periosteum has TWO layers
• SHARPEY’S FIBERS: periosteal collagenous fibers; bind periosteum to bone
o Type I collagen
• Endosteum is thinner than periosteum – it has ONE layer
Compact and Spongy Bone
• Compact bone (AKA cortical bone)
- Dense, looks smooth
- Surrounds the spongy bone
- Will have OSTEONS
• Spongy bone (AKA cancellous bone, trabecular bone)
- Small needle-like or flat pieces of bone (trabeculae)
- Open spaces between the trabeculae that are filled with bone marrow
- NOT penetrated by blood vessels: NO OSTEONS
Woven bone
- immature bone, primary bone
- Found in recently ossified bone
- Prenatal, growth and development, healing of a fracture
- First bone to form during development or bone repair
• Abundant osteocytes – lots of cells compared to
lamellar bone
- Irregular bundles of type I collagen
- Less mineralized than lamellar bone
- Cells appear to be scattered randomly
Four Lamellar Bone
1) Outer circumferential lamellae
2) Inner circumferential lamellae
3) Osteons (Haversian systems)
4) Interstitial lamellae
Outer Circumferential Lamellae
Just deep to
periosteum (Layers that go in a circle around the outside just deep to periosteum)
Contain sharpey’s
fibers (collagen bundles that anchor periosteum to bone)
Inner Circumferential Lamellae
Encircle marrow cavity (Layer that goes around in a circle around the INSIDE)
Spongy bone trabeculae extend from this layer into the marrow cavity
Osteons
(Haversian System)
§ Cylinders of lamellae
§ 4-20 concentric layers around a
blood vessel in a HAVERSIAN
CANAL
§ Haversian canals are connected
by VOLKMANN’S CANALS,
oriented perpendicular to
Haversian canals
§ Blood stays IN THE CANALS
§ Each osteon is bounded by a CEMENT LINE (calcified
ground substance, with little collagen)
§ Collagen bundles are parallel to each other within a
lamella, but perpendicular to those in adjacent lamellae
§ Cytoplasmic projections of osteocytes are connected via
gap junctions
§ Cell body sits in lacuna, and the canaliculus is the extension
of the lacuna that the projections of the cell go through so
they can exchange nutrients (can view empty spaces of the
lacuna/canaliculi histologically)
Osteons
(Haversian System)
- Cylinders of lamellae
- 4-20 concentric layers around a blood vessel in a HAVERSIAN CANAL
- Haversian canals are connected by VOLKMANN’S CANALS, oriented perpendicular to Haversian canals
- Blood stays IN THE CANALS
- Each osteon is bounded by a CEMENT LINE (calcified
ground substance, with little collagen) - Collagen bundles are parallel to each other within a
lamella, but perpendicular to those in adjacent lamellae - Cytoplasmic projections of osteocytes are connected via
gap junctions - Cell body sits in lacuna, and the canaliculus is the extension of the lacuna that the projections of the cell go through so they can exchange nutrients (can view empty spaces of the lacuna/canaliculi histologically)
Bone remodeling
• Tissue is removed by osteoclasts, replaced by osteoblasts
- Newer osteons replace older ones
- To replace woven with lamellar, and to replace old lamellar with new lamellar
- As bone is remodeled, osteoclasts resorb osteons and osteoblasts replace them
- Remnants of old osteons remain as INTERSTITIAL LAMELLAE, located between current osteons
• Bone remodeling is done by a basic multicellular unit (BMU), a wandering team of cells that dissolves an area of the bone surface and then fills it with new bone • Remodeling follows the A-R-F sequence o Activation o Resorption and reversal o Formation and mineralization
Ossification
- Bone formation; bone that appears first is woven bone, which is eventually replaced by lamellar bone
- Primary ossification center
- First area to start ossifying
- Usually during prenatal development
- Located in diaphysis (middle of bone) in developing long bones
• Secondary ossification center
- Appears after primary
- Usually during postnatal development
- Located in epiphyses in developing long bones
(top or bottom of bone)
Two Kinds of Ossification
o INTRAMEMBRANOUS OSSIFICATION:
- Develops from mesenchyme (i.e. no cartilaginous precursor; direct transition to mesenchyme)
- Direct mineralization of matrix secreted by osteoblasts
- Flat bones of the skull (frontal, parietal, etc.), lacrimal, nasal, palatine, vomer, maxilla, mandible (except condyle), some of clavicle
o ENDOCHONDRAL OSSIFICATION:
- Has cartilaginous precursor (template of hyaline cartilage)
- Cartilage does not become bone – the cartilage model is replaced by bone (cartilage cells die and disappear and bone tissue grows in its place)
- Usually long bone ossification as an example
Intramembranous Ossification
- Mesenchyme condenses and is
highly vascularized - Mesenchymal cells differentiate
into osteoblasts - Osteoblasts secrete bone
matrix (osteoid) to form
trabeculae
“osteoid” is a term for bone matric that does not yet have minerals in it
- Trabeculae fuse together to
form woven spongy bone (form the mineralized matrix) - The periphery of the spongy
bone will be remodeled into
lamellar compact bone; deeper
bone will be remodeled into lamellar spongy bone
Endochondral Ossification
- Hyaline cartilage model is formed
o See cartilage lecture for
cartilogenesis steps
2. Bone collar appears o Formed via intra-membranous ossification within the perichondrium o Hollow cylinder of bone around the mid-portion of the cartilage model (diaphysis in long bones)
o Note: the process of endochondral
ossification involves some
intramembranous ossification (to
make the bone collar)
- Chondrocytes in center hypertrophy, degenerate, and die
o Cartilage matrix is calcified - Empty spaces are invaded by the periosteal bud
o Vessels, mesenchymal cells, osteoprogenitor cells - Osteoprogenitors differentiate into osteoblasts, secrete osteoid on
surface of calcified cartilage - Bone collar increases in thickness and length, but osteoclasts resorb the
calcified cartilage and bone
o Results in marrow cavity which will be populated with marrow
cells - Secondary ossification centers form in a similar manner in the epiphyses
Growth in length and in width can occur – growth in length is only
possible while epiphyseal plate is present
*NOTE: Cartilage remains at EPIPHYSEAL PLATE (to
allow for growth in length
until adulthood) and at the
ARTICULAR CARTILAGE
