2.21, 2.22 Bone and Joints 1 and 2

Now bone repair. What are the two types of healing, and which is the most common? Explain the less intense one.

Direct - the ends of the bone are close together and stabilized, allowing for usual remodeling process to heal fracture (very uncommon) Indirect healing - ends of fracture are farther apart or repetitively moving; healing by new bone formation must occur which is similar to endochondral ossification.

Explain Bone re-modeling. (overview)

During life, old bone will be resorbed and new bone will be produced. Must recruit osteoclasts, then apoptosize them, then recruit osteoblasts to replace bone

Bone is a specialized connective tissue consisting of cells and a mineralized extracellular matrix. What are the inorganic and organic components of bone?

Essentially, inorganic component is hydroxyapatite, which is a mineral deposit, and the organic component is mostly type I collagen (90% of bone matrix) and other ECM components.

Indrect healing. First, what forms at the site of fracture? What will then grow into the injury site? What role do chondroblasts play next? What role do osteoprogenitor cells play next, and what do they replace? How long does this take?

Hematoma formed at site of fracture and neutrophils and macrophages will clean up site of injury Fibroblasts and capillaries will proliferate and grow into injury site forming granulation tissue (loose connective tissue found in wound healing) Chondroblasts will differentiate from periosteal cells, form cartilage matrix, replaces granulation tissue Osteoprogenitor cells of periosteum will differentiate into osteoblasts, begin depositing new bone. Eventually replaced fibrocartilagnous portion made here (usually replaces woven bone) Takes about 6-12 weeks

How can we classify joints?

Joints are junctions between 2 or more bones, classified histologically as synovial or synarthrosis (has fibrous/cartilaginous tissue connecting bones)

How is the matrix laid down? As the matrix surrounds osteoblasts, they will become... What is appositional growth? What bones are commonly produced this way?

Matrix will be laid down in spicules, and then spicules will enlarge as osteoblasts lay down more osteoid, until finally enough so that there is solid regions of bone produced. (remember it will initially be woven bone, remodeling will make lamellar bone). As matrix surrounds osteoblasts, they will become osteocytes. Osteoprogenitor cells will continue to differentiate into more osteoblasts at edge of bone, expanding periphery (called appositional growth) Skull, facial bones, clavicle.

Explain intramembranous ossification. Where does it occur? Does it require a cartilage model/template to form the bone? What first differentiations will occur? The final product of these will produce what?

Occurs in mesenchymal fibrous tissue (the membrane), does NOT use cartilage model. Mesenchymal cells will differentiate into osteoprogenitor cells, which will differentiate into osteoblasts. They will produce type 1 collagen and the other bone matrix proteins to produce non mineralized bone matrix called osteoid.

Bony collar then forms - What is the "bony collar"?

Ossification will start as a ring of perichondrium in the diaphysis, but switch to being periosteum. Progenitor cells will turn into osteoblasts instead of chondroblasts and start laying down osteoid (JUST LIKE INTRAMAEMBRANOUS OSSIFICATION).

What are osteoclasts? What are they derived from? What increases differentiation from their derivations? What is required for this? What does osteoprotegerin do to this?

These are giant, multinucleated cells that are derived from granulocyte-monocyte progenitor cells (CFU-GM) And increased need for bone resoprtion/remodeling causes more differentiation. Differentiation requires RANK interaction with RANKL (receptor activator of nuclear factor kB) L=ligand Increased RANKL from stream cells, lymphocytes will drive differentiation. Osteoprotegerin serves as decoy receptor for RANK and inhibits osteoclasts differentiation.

Now bone architecture. What are osteoprogenitor cells? Where are they found?

These are resting cells that can transform into osteoblasts, arising from mesenchymal stem cells, found at periosteal and endosteal surfaces.

Now endochondral ossification. Does it require a cartilage model to form the bone? Mesenchymal cells here will differetiate into what and produce what? Explain the cartilage model

Yes it does - mesenchymal cells will differentiate into chondroblasts, producing type II collagen and laying down cartilage matrix - this produces a "cartilage model" that essentially models the eventual bone. Grows by appositional and interstitial growth as embryo develops.

What two layers make up the synovial membrane? What types of cells are in the first? Are these epithelial?

Synovial cell layer - 1-4 cells thick composed of two types that look similar by light microscopy but are actually functionally different Type A cells - 25% are macrophage like, clean up joint debris Type B cells - 75% are fibroblast like and secrete synovial fluid. NOT EPITHELIAL, no basal lamina Supporting layer: Loose connective tissue with blood vessels, nerves, usual connective tissue cell population

What are the three regions of the osteoclasts when they are fully functional?

Ruffled border (apical), clear zone, and basolateral region.

What is secondary ossification?

SO centers develop in epiphysis, leaving only a plate of cartilage at epiphysis-metaphysis junction (epiphyseal plate) which will continue to produce bone length until hormonal changes at puberty will replace epiphyseal cartilage.

Explain the ruffled border.

"Apical" - These are the cells actually resting on the bone. "Howship's Lacuna" is the dip in bone where osteoclasts rests and resorbs bone. Carbonic Anhydrase will produce carbonic acid inside osteoclast, with subsequent protons, which will transfer to EC region of ruffled border with ATP dependent proton pumps. The organic matrix will be removed by lysosomal enzymes, released into EC region of ruffled border.

Explain the sub components that make up the organic component. There are six major categories. What is osteonectin and osteopontin?

-Type I collagen (90%) -Proteoglycans -MAGs (multiadhesive glycoproteins) Two examples of MAGS - Osteonectin: adheres collagen fibers to hydroxyapatite Osteopontin: adheres cells to bone matrix -Osteocalcin - captures calcium from circulation, attracts, stimulates osteoclasts in bone remodeling -Growth factors and cytokines (TNF-a, TGF-b, IGFs, PDGFs, ILs) Lacunae - contains one osteocyte per lacunae. Canaliculi connect lacunae

Explain basolateral region.

All of the rest of the cell. has excess bicarbonate from acid production released.

What are osteoblasts? What do they secrete? How do they appear on the bone surface? What sort of junctions do they have? What can they differentiate into?

Bone forming cell, secretes bone matrix (type I collagen and other bone matrix proteins found as a single layer of cells lining a bone surface have gap junctions to other osteoblasts and osteocytes Can differentiate into osteocytes if surrounded by bone matrix, or bone lining cells if they remain on a bone surface like endosteum, or isotonic and perforating canals.

What happens to the cartilage in the region of the "bony collar"

Cartilage in region of bony collar becomes calcified, chondrocytes eventually die. Calcified cartilage will be removed, forms spicules of dead cartilage in primitive marrow cavity which will eventually be invaded by blood vessels and osteoprogenitor cells that eventually become osteoblasts in marrow cavity.

Explain synovial fluid. What does it contain? Does it clot? What does it nourish?

Clear, thick, sticky fluid, lubricates joint/bursa. Contains large volumes of hyaluronic acid from type B cells, with high affinity for water. This produces an ultra filtrate of blood that doesnt clot. Also nourishes chondrocytes of the avascular articular cartilage.

What are the two subcategories of lamellar bone?

Compact and spongy bone. Compact (cortical) is highly dense bone, with lamellar structure. Spongy ( trabecular, cancellous) is formed of thin anastomosing bone spicules, spaces between trabeculae are filled with marrow and blood vessels. More cells per unit volume.

Describe the cortex.

Contains mostly compact bone, as opposed to spongy. Outermost aspect has "circumferential lamellae" around entire outer circumference The inner aspect adjacent to medullary cavity has "inner circumferential lamellae"

Explain the process of it. What is a "cutting cone"? What is "closing cone" ? What is going on in the center of the tunnel, and what does the tunnel become? How do we form interstitial lamellae? Does this occur in spongy bone?

Cutting cone is formed as osteoclasts tunnel through existing bone, forming new osteons. Osteoblasts follow in wake, laying done new bone as "closing cone" In the center of tunnel, blood vessels proliferate, tunnel becomes osteonic canal. Remnants of older osteons partially destroyed by these tunnels form interstitial lamellae. Yes, occurs on surfaces of bony trabeculae

woven bone is basically baby lamellar bone. Now lamellar bone. What is an osteon? What is within it? What is interstitial lamellae? What are canaliculi and volkmann canals? What is outer/inner circumferential lamellae? Is this part of the osteon?

Functional unit of lamellar bone. contains concentric lamellae of bone matrix with a central canal (see photo). inside is the osteonic canal/haversian canal that contains blood vessels and nerves. Interstitial lamellae is basically remnants of older osteons that are left over from remodeling. Canaliculi - allow osteocytes (in lacunae) to receive nutrition and signals from isotonic canal, and to connect to neighboring osteocytes with gap junctions Volkmann canals run perpendicular to osteons supplying osteonic canal with blood vessels and nerves. (its basically developed bone) Outer/inner circumferential lamellae - these are lamellar bone that surrounds inner and outer circumference of the whole bone, NOT AN OSTEON

Where is the synovial membrane found? What is its function?

In synovial joints: Ends of articulating bones covered by hyaline cartilage within a joint capsule Also in sheaths around tendons and ligaments (bursae) Secretes synovial fluid, a lubricant for joint/bursa

What are the two processes of bone formation?

Intramembranous ossification, and endochondral ossification. They usually combine to actually make a bone.

What are the categories of bones by shape? What is Diaphysis, Epiphyses, and Metaphysis?

Long bones (longer than they are wide): Femur, tibia, humerus, metacarpals. These can be divided into Diaphysis: Shaft of bone (mid portion) Epiphyses: ends of the bone Metaphysis: Junction of the two Short bones (equal length and diameter): Carpal bones Flat bones (plate like layers of compact bone with spongy bone in between): Calvaria, sternum Irregular bones (what doesnt fit in above categories): Vertebrae, ethmoid)

How is the synovial membrane organized?

Loose folds (villi) and lining cells have microvilli, resulting in significant surface area for production of synovial fluid (knee is thought to have 100 sq meters of synovial membrane!)

Explain the primary ossification center. This will expand to produce which region?

Next, osteoblasts will line residual cartilage spicules, lay down osteoid on them, and this central region is called primary ossification center. Will expand to produce ENTIRE bone diaphysis.

Explain blood supply to the bones. Where do they enter? Where do they go, and how do they get to cortical (compact) bone? Do the outer regions of cortical bone have their own supply? Explain venous drainage and lymph drainage.

Nutrient foramina where feeding artery will enter (long bones found in diaphysis more commonly). Enter marrow cavity and arteries go from marrow, enter cortical bone via perforating (Volkmann) canals. Yes, arteries that directly penetrate cortical bone Veins leave via periosteum and nutrient foramina, and lymphatic drainage found only in periosteum.

Overall structure of lamellar bones includes what three categories? Describe the first.

Periosteum, Cortex, and Medullary (Marrow) Cavity. Periosteum covers the outer surface of bone, and contains two layers. Outer fibrous layer: Made up of dense connective tissue, has attachment sites for ligaments, has collagen fibers anchoring periosteum to bone matrix called sharpeys fibers Inner cellular cellayer: osteogenic layer, has osteoprogenitor cells (stem like). Layer very reduced in cells that are not growing.

Increasing LENGTH of bone is done how? Explain the Zone of reserve Zone of proliferation Zone of hypertrophy Zone of calcified cartilage Zone of resorption

Requires proliferation of cartilage in the epiphysis/metaphysis of bone. Zone of Reserve : Cartilage not doing anything, waiting to be recruited into growth process Zone of proliferation: Proliferating chondrocytes form columns extending toward the diaphysis Zone of hypertrophy: Chondrocytes enlarge; start secreting vascular endothelial growth factor (VEGF) directing vascular invasion of the cartilage (bringing with them osteoprogenitor cells) Zone of calcified cartilage: Hypertrophic chondrocytes degenerate & eventually undergo apoptosis; cartilage matrix becomes calcified Zone of resorption: Osteoprogenitor cells from the vessels that came from the marrow cavity become osteoblasts which lay down osteoid on the calcified cartilage; cartilage is removed and replaced by bone

Explain clear border.

Ring like perimeter of cytoplasm, defining edges of ruffled border. Expresses EC matrix adhesion molecules that will help attachment to bone, seal off region of ruffled border. Will interact with actin.

Explain the process of mineralization. Is this regulated via cells or extracellularly? Whats the role of osteocalcin? What does the increased Ca2+ and PO4- do to osteoblasts? The high concentrations of Ca2+ and PO4- does what?

This is a cell regulated event, even though actual deposition of Ca2+ and PO4- occurs extracellulary. Osteocalcin binds extracellular Ca2+ to create locally high concentrations of EC calcium, which causes osteoblasts to secrete alkaline phosphatase, leading to locally high conc of PO4-, further stimulating Ca2+ increase. Cause osteoblasts to release "matrix vesicles" by exocytosis. Vesicles will contain more alkaline phosphatase which cleaves more PO4 ions from other matrix molecules. Causes crystallization of CaPO4, initiating deposition of hydroxyapatite crystals.

Describe the medullary cavity. What is the inner lining? After the inner lining, what kind of bone is here? What kind of marrow is found? Where is this in adults?

This is the marrow cavity, or the hollow portion in the diaphysis. Inner lining of the compact region is called the endosteum, this contains bone lining cells, osteoprogenitor cells, osteoblasts, osteoclasts. Moving further interior you find spongy bone, and then marrow. The marrow is yellow or red: Yellow: pure adipose Red: Hematopoetic tissue mixed with adipose tissues (50/50). In adults, red marrow limited to proximal long bones and axial skeleton (skull, vertebrae, sternum, pelvis

What are osteocytes? How do they communicate with neighboring osteocytes? What do they respond to in bone tissue? What are the three physiologic states of these cells? Eventually, osteocytes will...

Transformed osteoblasts that are not completely surrounded by matrix (the space is called a lacunae!) They communicate via cytoplasmic processes and gap junctions in canaliculi. Respond to mechanical stress in the bone to regulate bone production or resorption via slight flexibility in bone and changes in micro fluid levels in the lacunae and canaliculi Quiescent - not doing anything Formative - producing bone matrix and inhibiting osteoclasts via osteoprotegerin Resorptive: Degrading matrix via matrix metalloproteinases Die, and become replaced in process of bone remodeling. Live about 10-20 years.

Comparing and contrasting lamellar vs woven bone. Explain woven bone. Which is more cellular, and how are osteocytes and collagen fibers arranged? What about formation/development of woven bone? If we find woven bone in a person, what does it mean?

Typically much more cellular than lamellar, with random organization of osteocytes. Collagen fibers also arranged much more randomly. Forms faster than lamellar bone, eventually remodeled into lamellar bone If we find woven bone, that indicates either immaturity or injury response in healing. (this is the right picture in the diagram)

Osteoclast regulation. What do osteoclasts do when bone resorption is complete? What is the role of parathyroid hormone (PTH) in osteoclast activity? What does estrogen do? What does calcitonin do? We can think of bone as a major reservoir for...

Undergo apoptosis. Interacts with PTH receptors on osteocytes, osteoblasts, and T lymphocytes, causing production of RANKL. (remember job of PTH is to raise blood calcium) Estrogen suppreses RankL production (post menopausal increases) Calcitonin from thyroid cells reduces osteoclast activity. Calcium, under these hormonal controls.