Bone

What is the makeup of bone by weight?

60% inorganic

30% organic

10% water

What type of tissue is bone?

Connective

What are the 3 bone structures?

1. trabecular (cancellous) bone

2. compact (cortical) bone

3. bone marrow cavity

What type of collagen is associated with bone formation?

Type I collagen

How is collagen type I organized?

Triple helix formed by three alpha chains, made of one alpha2 chain and two alpha1 chains

The triple helix is processed and assembled into a larger structure

Formula for organic bone

Type I collagen + ground substance

Osteogenesis imperfecta

Defect in assembly or production of type I collagen characterized by low bone mass and bone fragility

Can also be due to an error in bone mineralization

Dominantly inherited

What makes up the inorganic component of bone?

Hydroxyapatite, made of calcium and phosphorous

Biological apatite

Human bone mineral

Not the exact pure form due to substitutions of atoms or ions which in turn affect the strength and modeling of bone

Most notable sub is carbonate instead of phosphate

What lines the bone marrow cavity in bone?

Periosteum (2 layers) and endosteum

What are the sources of mesenchymal stem cells (MSCs) in bone?

Periosteum, bone marrow, endostem

Osteoblasts

Cells lining the surface of existing bone that secrete type I collagen and mineralize bone

Where do osteoblasts originate from?

MSCs in the bone marrow, periosteum, and endosteum

Osteoid

Unmineralized ECM (collagen)

How does mineralization work?

Hydroxyapatite crystals precipitate onto collagen fibers, mineralizing the osteoid into bone

Facilitated by osteoblasts, which form the osteoid

Osteocytes

Differentiated form of osteoblasts that have become trapped in the bone they form

How do vesicles play a role in bone mineralization?

Calcium and phosphate are pumped into vesicles in the osteoid bone matrix (made of collagen type I collagen). The vesicle environment is ideal for crystal formation. The crystalline structures grow on type I collagen and eventually rupture to create a calcifying globule (mineralized nodule)

T/F: MSCs differentiate into osteoblasts and osteocytes

True. This requires transcription factors to express the genes of the differentiated cells.

What transcription factors are required for osteoblast differentiation?

1. Runx2

2. Osterix (Osx, Sp7)

How is Sp7 (osterix) related to osteogenesis imperfecta?

Without the conversion from osteoblast progenitors down the lineage to osteoblasts, there is low bone density and mineral frailty

T/F: Osteocytes have a role in microgravity-induced bone loss?

True. Osteocytes are mechanosensors in bone. Gravity disrupts the cells' abilities to detect mechanical forces on bone, leading to bone resorption and reduced bone formation

How much of the bone cells is constituted by osteocytes?

95%

Canaliculi

Cytoplasmic projections (string-like) that form channels, allowing osteocytes to communicate within bone

Extensions of osteocyte processes

Lacunae

Chambers within bone where osteocytes reside

Gap junctions

Found at the end of canaliculi, these facilitate communication between osteocytes

Why do osteocytes need to communicate?

Controlling bone formation and bone resorption

T/F: Osteocytes only communicate with other osteocytes

False. They communicate both with osteocytes and osteoblasts

What role does extracellular fluid play in bone formation?

It transmits mechanical stress on the bone which influences bone formation and resorption, and it provides nutrition to osteocytes. ECF is found in the canaliculi

What genes are found on pre-osteoclasts and osteoclasts?

RANK receptor and c-FMS

RANK

receptor activator of nuclear factor KB

binds RANKL to induce osteoclastogenesis

c-Fms

M-CSF receptor or colony stimulating factor 1 receptor

Induces hematopoietic stem cells (HSCs) to progress towards osteoclast lineage

What are proteins made by osteoblasts?

RANKL and M-CSF

RANKL

ligand for RANK receptor (receptor found on pre-osteoclasts)

M-CSF

Macrophage colony stimulating factor

binds to c-Fms receptor on monocytes to induce expression of RANK, needed for osteoclast formation

T/F: There must be osteoblasts present in order for osteoclasts to form?

True. This process is facilitated by RANKL and M-CSF, made by osteoblasts. These bind to receptors on pre-osteoclasts

What is the sequence of cell differentiation to make osteoclasts?

Hematopoietic stem cells (HSCs) -> monocytes (pre-osteoclasts) -> osteoclasts

Where are HSCs made and found?

Bone marrow

T/F: RANKL can exist as either a cell membrane receptor or a soluble ligand

True. RANKL is produced as a cell membrane receptor on the cell surface, and as a soluble protein form after cleavage, can extend outward further to bind to RANK receptor

What induces pre-osteoclasts to express RANK?

M-CSF from osteoblasts binding to c-Fms on pre-osteoclasts

What induces the expression of genes required to convert pre-osteoclasts to osteoclasts?

M-CSF and RANKL binding to receptors on pre-osteoclasts

Osteoprotegerin (OPG)

Decoy receptor for RANKL that when bound to RANKL, inhibits RANKL from binding to RANK, essentially outcompeting it. This results in no osteoclast formation.

T/F: OPG can be made as a receptor or soluble form

True

What is the sequence of osteoblast activation?

Commitment -> Proliferation -> Fusion -> Activation

As more cells are made, they communicate and migrate towards each other to fuse. This allows them to become multinucleated and larger in size. This increases surface area and increases efficiency in remodeling bone. Involves M-CSF and RANKL

What are the parts of a bound osteoclast?

1. Sealing Zone (SZ)

2. Membrane ruffled border (RB)

3. Functional Secretory Domain (FSD)

Sealing Zone

Superstructure made of podosomes that separate what is underneath the osteoclast from the surrounding environment

Podosomes

Dot-like actin containing units that bind to form a sealing zone

T/F: The sealing zone is static

False. Efficiency of remodeling actually depends on podosome assembly and disassembly. This allows osteoclasts to move around and degrade bone.

Ruffled border

Extension of plasma membrane into resorption lacunae

Resorption lacunae

aka Howship lacunae

space between RB and bone ECM

What keeps the material found in the resorption lacunae inside?

Sealing zone

How are hydroxyapatite crystals dissolved in bone resportion?

Mature osteoclasts secrete HCl into the resorption lacuna to dissolve the hydroxyapatite crystals (or biological apatite). This is necessary to remove the collagen from the bone.

Key enzymes and transporters involved in bone remodeling

Carbonic anhydrase II

H+-ATPase proton pump

Bicarbonate/chloride exchanger

These together create H+ molecules and Cl- molecules available to make HCl, the dissolving agent used by osteoclasts, and maintain a balance of charges in the osteoclast

How can the collagen network be removed?

After inorganic hydroxyapatite is dissolved, the collagen network is exposed. Collagenases degrade the type I collagen.

Collagenases are released by osteoclasts

Functional Secretory Domain (FSD)

Facilitates the transport of large vesicles housing degraded products from below the sealing zone to the ECM space

Degradation products are endocytosed by the RB membrane and transported via FSD

What are the two types of ossification?

1. Intramembranous

2. Endochondral

Where is intramembranous ossification found?

flat bones of skull

Where is endochondral ossification found?

Bones of appendicular skeleton

Intramembranous ossification?

MSCs differentiate into osteoblasts and form without cartilaginous base

Endochondral ossifcation

MSCs differentiate into osteoblasts which migrate into a cartilaginous base to form ossification centers to begin bone formation

Perichondrium

Dense irregular tissue surrounding cartilage where osteoblasts and MSCs line and form ossification centers

Periosteum

Connective tissue layer covering the outer surface of bone

What are the two layers of periosteum?

1. Outer fibrous layer

2. Inner osteogenic layer

Outer fibrous layer of periosteum

Made of collagen and fibroblasts that gives strength to the periosteum and provides protection from infection

Inner osteogenic layer of periosteum

Made of MSCs and osteoblasts

Sharpey's fibers

Collagen fibers anchoring periosteum to bone.

Compact and trabecular bone

Compact bone is the shell of bone, and trabecular bone is within that shell

Cortical bone

aka lamellar or compact bone

Arranged in concentric layers that make up an osteon

Osteon

Structural unit of compact bone

Vertebral columns

Lamella

Each layer of the osteon, made of osteocytes

Haversian canals

major passageways running in the direction of the length of long bones, providing paths for blood vessels that give nutrients to the bone

How are collagen fibers arranged in compact bone?

Alternating arrangements of collagen in layers of osteon. Within one layer (lamella), the collagen fibers are in the same direction. Each next layer is the opposite direction. This gives strength to the bone.