L10: Bone stem cells

Question 1

bone

Answer 1

Bones vary by: shape, size, symmetry.

Symmetry is established in utero, patterned by HOX genes.

Embryonic development: More bones exist early and fuse over time.

Developmental origins:

Cranial neural crest → bones of the head

Mesoderm → rest of the skeleton

🧬 Bone Cell Types
Osteoblasts: build bone

Osteoclasts: resorb (break down) bone

They crosstalk with osteoblasts for bone remodeling

Osteocytes: mature bone cells derived from osteoblasts

Osteoprogenitor cells: stem cells that give rise to osteoblasts

Lining cells: inactive osteoblasts on bone surfaces

Chondrocytes: form cartilage

🧠 Growth & Regulation
Bone growth is controlled by:

Cells & signalling molecules

Cell-cell interactions

Pathological conditions (e.g. disease states)

🏗️ Bone Functions
Living, dynamic tissue

Constant remodelling

Support & structure

Muscle attachment

Site of haematopoiesis (blood cell production in marrow)

Mineral reservoir: stores calcium, phosphate, etc.

Helps maintain mineral homeostasis

Osteoblasts also help regulate blood–bone interactions

🦷 Special Note
Bone in teeth is mineralised like skeletal bone.

Question 2

bone cells

Answer 2

Osteoblasts
Function: Bone-forming cells

Shape: Cuboidal, polarised (nucleus on one side; RER & Golgi on the other)

Location: Sit on bone surface

Size: Large

Secrete: Mainly Type I collagen (forms osteoid)

Osteoid: Unmineralised bone matrix → later mineralised to form bone

Fate: Become osteocytes once embedded in matrix

🧬 Osteocytes
Former osteoblasts embedded in mineralised matrix

Long-lived

Function: Maintain bone health; communicate with each other via canaliculi

🔄 Osteoclasts
Function: Bone-resorbing cells

Origin: Multinucleated cells (formed by fusion)

Feature: Ruffled border increases surface area

Mechanism: Secretes H⁺ and Cl⁻ ions → forms HCl

HCl dissolves bone mineral (acid demineralisation)

Dysfunction: Poor resorption due to lack of protons, nuclei, or enzymes

🌱 Cell Origins
Osteoblasts & Chondrocytes: Derived from Mesenchymal Stem Cells (MSCs)

Signals (e.g. growth factors) activate lineage-determining genes to guide differentiation

Question 3

osteocyte differentiation pathway

Answer 3

MSC → Osteocyte Differentiation Pathway
1. Mesenchymal Stem Cell (MSC)
⬇️ Signals activate transcription factors:

Runx2

Osterix (Osx)
⬇️

2. Osteoprogenitor Cell

Early committed to bone lineage

⬇️

3. Pre-Osteoblast / Transitory Osteoblast

Expresses Alkaline Phosphatase (ALP)

Preparing bone matrix

⬇️

4. Mature Osteoblast

Secretes Osteocalcin (bone matrix protein)

Lays down osteoid (unmineralised matrix)

⬇️

5. Osteocyte (embedded in bone)

Terminally differentiated

Expresses Smp1 and Sclerostin (Sost)

Maintains bone homeostasis

Question 4

osteoclasts

Answer 4

Osteoclast Lineage & Differentiation
Origin:

Osteoclasts are derived from haematopoietic stem cells (HSCs)

Specifically from the monocyte/macrophage lineage (phagocytic cells)

Key Signals for Differentiation:

Macrophage Colony-Stimulating Factor (M-CSF)
⤷ Drives HSCs → macrophage precursors

RANKL (Receptor Activator of Nuclear Factor κB Ligand)
⤷ Stimulates precursors to become osteoclasts

Osteoclast Features:

Multinucleated

Phagocytic (related to macrophages)

Responsible for bone resorption

Question 5

ossification

Answer 5

Endochondral Ossification
➡️ Bones formed via a cartilage intermediate
➡️ Needed for long bones (e.g. arms, legs) and vertebrae
➡️ No cartilage = no bone in this process

🔑 Steps (in order):
Formation of cartilage model

Cartilage takes the shape of future bone

Cartilage matures and mineralises

Formation of bony collar

Around midshaft (diaphysis), cartilage starts to be replaced

Periosteum forms

Outer covering of developing bone

Capillary invasion

Blood vessels grow into the cartilage

Osteoclast migration and invasion

Haematopoietic cells (from blood) enter → form osteoclasts

Middle part is resorbed to make room for bone marrow

Primary ossification center forms

In the diaphysis (shaft)

Secondary ossification centers form

In the epiphyses (ends of bone)

🔶 Intramembranous Ossification
➡️ Bones formed directly from stem cells (no cartilage intermediate)
➡️ Used for flat bones of the skull, calvaria, clavicle

🔑 Steps (in order):
Bone forms in a ‘membrane’ or cell condensation

Calvaria in embryo starts as islands of bone

These islands later remodel to form skull cap

Direct formation of osteoblast precursors

Periosteum forms (outer layer)

Differentiation to osteoblasts

Matrix deposition and mineralisation

Osteoblasts lay down osteoid → later mineralised

Vascularisation

Blood vessels invade the new bone

Continued growth and remodelling

Question 6

secondary ossification

Answer 6

After birth, secondary ossification centers form at the ends of long bones, called the epiphyses.

This is separate from the primary ossification center, which is in the diaphysis (shaft).

These centers allow the ends of bones to grow and develop properly.

Growth plates (epiphyseal plates) remain between the epiphysis and diaphysis to allow longitudinal growth during childhood and adolescence.

Eventually, these plates fuse, ending bone growth.

📌 In short:
Secondary ossification = bone formation at epiphyses (bone ends) after birth
Epiphysis = end part of a long bone, initially growing separately from the shaft

Question 7

why does remodelling occur

Answer 7

Bone remodelling
Why remodel?
Calcium homeostasis. 3 organs mediating blood calcium level: bone, gut, kidney.
Hypocalcemic? (not enough ca in blood) osteoclasts will be activated, resorb bone, mineral calcium go back into bloodstream and restore levels.
Skeletal homeostaiss/ bone mass.
Steady- state: a balance of resoprtion and formation
Unbalanced: r>f
Increased bone mass e.g: osteosclerosis
f>r bone loss diseases e.g: osteoporosis

Adaptation- mechanical forces. Bones constantly under mechanical forces. Limbs, lower limbs more mechanically loaded than skull bone i.e some under more force than others?
Compression (pressing together)
Tension (pulling apart)
Torsion (twisting)
Shear (tearing across)

Question 8

ARF

Answer 8

Occurs in cycles: Activation → Resorption → Formation (ARF)
Involves the Basic Multicellular Unit (BMU) — coordinated group of cells

🔹 1. Activation
Quiescent (inactive) osteocytes/lining cells sense damage or signals.

They send signals to recruit osteoclast precursors.

🔹 2. Resorption
Osteoclasts differentiate and activate.

Form ruffled border and sealing zone.

Secrete acid (H⁺, Cl⁻) to dissolve bone mineral.

Osteoclasts undergo apoptosis after resorption.

🔹 3. Reversal Phase
Area is cleaned and prepared for new bone.

Signals (e.g. from dying osteoclasts) recruit osteoblast precursors.

🔹 4. Formation
Osteoblasts proliferate, then lay down osteoid (new bone matrix).

Matrix mineralises to form mature bone.

Some osteoblasts become embedded → become osteocytes.

Surface returns to lining/quiescent osteocytes until next cycle.

🔄 Coupling
Osteoblasts and osteoclasts regulate each other through feedback signals.

This ensures balanced bone loss and formation.

Question 9

regulating remodelling

Answer 9

Regulation of Remodelling
Hormones (e.g. PTH, calcitonin, oestrogen)

Cytokines & growth factors

Local signalling molecules (e.g. RANKL, OPG)

Transcription factors (e.g. Runx2)

egulation of Bone Remodelling: Mechanical Loading
📌 Mechanical Stress & Bone
Bone architecture adapts to mechanical demands.

Stress-bearing determines bone mass and 3D structure.

Different bones experience different loading forces, so are regulated differently.

⚙️ How It Works
More mechanical loading → more bone formation

Less mechanical loading (e.g. bed rest, microgravity) → bone resorption

🧠 Osteocytes = Mechanosensors
Osteocytes detect mechanical strain in bone.

Send signals to stimulate or suppress osteoblasts/osteoclasts.

Bone is laid down along lines of greatest compressive or tensile stress (Wolff’s Law).

🔁 Key Concept
External forces determine internal bone remodelling.

Question 10

osteoporosis

Answer 10

Osteoporosis
Bone Fragility: Bones break easily due to weakness and loss of bone mass.

Mechanism:

Increased osteoclast activity → Excessive bone resorption.

Disruption of bone remodelling balance → More resorption than formation.

Result: Weak bones that are prone to fractures.

⚖️ Key Factor: Estrogen Deficiency
Menopause → Estrogen deficiency → Increased osteoclast activity → Accelerated bone loss.

💊 Therapy for Osteoporosis
Antiresorptive Therapy:

Goal: Reduce bone resorption.

Examples: Bisphosphonates, Denosumab.

Anabolic Therapy:

Goal: Promote bone formation.

Examples: Teriparatide (parathyroid hormone).

🧬 Targeting Cell Differentiation
Decrease HSC differentiation → Reduces osteoclast (OC) activity.

Increase MSC differentiation → Promotes osteoblast (OB) formation.

Question 11

sclerosteosis

Answer 11

High Bone Mass (HBM)
Sclerosteosis: A rare genetic disorder characterized by high bone mass.

Cause: Mutation in the sclerostin (SOST) gene → Loss of function of sclerostin.

Effect: Unregulated bone formation leads to increased bone mass.

🔬 Sclerostin
Gene: SOST (osteocyte-specific gene)

Role: Secreted by osteocytes, regulates bone formation.

Inhibits bone formation by downregulating Wnt signalling (a pathway that promotes bone growth).

🏋️ Mechanical Loading & Sclerostin
Mechanical loading (e.g., weight-bearing exercise) decreases sclerostin secretion.

Effect: Reduced sclerostin → Increased bone formation.

How: Increased Wnt signalling promotes osteoblast activity, leading to more bone formation.

💉 Therapeutic Target: Anti-sclerostin Antibody
Romosozumab: An anti-sclerostin antibody.

Action: Inhibits sclerostin → Bone anabolic effect → Increases bone formation