3 - Structures and Development of Cartilage and Bone

What are the five types of bones by shape and their key characteristics?

• Long – Length > width; has shaft + ends (diaphysis + epiphyses)

• Short – Length ≈ width; spongy inside

• Flat – Thin; compact–spongy–compact sandwich

• Irregular – Complex shape; many ridges/holes

• Sesamoid – Small, round; in tendons under stress

What’s the difference between the cranium and postcranium classification of bones, and how is the cranium subdivided?

• Cranium = skull + hyoid
  → Split into:

  • Neurocranium (protects brain)

  • Viscerocranium (forms face)

• Postcranium = all other bones below the skull

What’s the difference between axial and appendicular classification?

• Axial = bones along the midline, including:

  • Skull, vertebral column, ribs, sternum

  • Covers both thorax (chest) and trunk (entire torso)

• Appendicular = limbs + girdles (shoulder & pelvic)

  • Upper limb, lower limb, scapula, clavicle, hip bones

What is a girdle in the skeletal system?

A girdle is a bony structure that connects a limb to the axial skeleton.

• Shoulder girdle = scapula + clavicle

• Pelvic girdle = hip bones (ilium, ischium, pubis)

What is the basic lineage path for bone and cartilage cell development?

• Mesenchymal stem cells (MSCs)
  ↓
  → (optionally: osteochondroprogenitor)
  ↓

• Osteoprogenitor cells → Osteoblasts → Osteocytes

• Chondroprogenitor cells → Chondroblasts → Chondrocytes

What is cartilage, and what is it made of?

Cartilage is a flexible, avascular connective tissue made of:

• Chondrocytes in lacunae

• ECM with Type II collagen (mainly), proteoglycans, and water

What are the two types of cartilage growth and how do they differ?A

• Interstitial (endogenous):
  Chondrocytes divide, secrete matrix, and get pushed into separate lacunae → growth from within

• Appositional (exogenous):
  Chondroprogenitor cells in perichondrium divide → form new chondroblasts that add matrix to the outside

What is the perichondrium and what are its layers?

The perichondrium is a dense connective tissue layer that surrounds most cartilage (except articular and fibrocartilage). It has:

• Outer fibrous layer → fibroblasts + Type I collagen, vascular

• Inner chondrogenic layer → chondroprogenitor cells for appositional growth

What are the three ECM zones of cartilage and their key features?

• Pericellular matrix – Closest to chondrocyte, collagen type VI, fine fibrils, signal transduction

• Territorial matrix – Surrounds pericellular, collagen type II, high proteoglycan content, basophilic, cushions cell

• Interterritorial matrix – Majority of ECM, collagen type II dominant, lower PGs, structural support

What are the three types of cartilage?

Hyaline, fibrocartilage, and elastic cartilage

What are the 4 key features of hyaline cartilage?

• Type II collagen

• Rich in proteoglycans and water

• Chondrocytes alone or in isogenous groups

• Has perichondrium (except at articular surfaces)

What is an articular surface?

The smooth surface at the end of bones where they meet to form a joint.

What are the zones of articular cartilage and their roles?

1. Superficial zone – Flat chondrocytes, parallel fibers(to surface), resists shear

2. Transitional zone – Rounder cells, angled fibers

3. Deep zone – Vertical fibers, aligned with compression

4. Tidemark – Boundary between calcified and uncalcified cartilage

What are the 5 key features of fibrocartilage?

• Mainly type I collagen

• Few chondrocytes, arranged in rows or columns

• No perichondrium

• Found in areas of high compression (e.g. intervertebral discs, pubic symphysis)

• Present in fibrocartilaginous entheses (tendon/ligament insertion into bone)

What are the 4 key features of elastic cartilage?

• Rich in elastic fibers, some type II collagen

• Chondrocytes within dense elastic fiber networks

• Has perichondrium

• Provides flexibility with shape retention

What is bone?

Bone is a connective tissue with a calcified extracellular matrix, composed of type I collagen, ground substance, and minerals, giving it strength and rigidity. It supports the body and stores minerals like calcium.

What are osteoblasts and what do they do?

• Origin too

• Derived from mesenchymal stem cells via osteoprogenitor cells

• Found in a single layer on the bone surface

• Secrete osteoid: an unmineralized matrix rich in collagen type I and ground substance

What are osteocytes and how do they form?

• An a unique feature they have

• Form when an osteoblast gets trapped in the matrix it secreted

• Reside in lacunae within the mineralized matrix

• Extend thin processes through canaliculi to connect with other osteocytes

• Help with mechanosensation and regulating bone remodeling

What are osteoclasts and what is their origin and function?

• Origin too

• Cell characteristic too

• Derived from hematopoietic stem cells (not mesenchymal)

• Large, multinucleated cells

• Resorb bone by secreting digestive enzymes and acid

• Essential for bone remodeling and calcium regulation

What is intramembranous ossification?

Bone forms directly from mesenchymal stem cells, without a cartilage model.
– MSCs become osteoblasts, which secrete osteoid
– Osteoid mineralizes to form bone
– Trapped osteoblasts become osteocytes

What is endochondral ossification?

Bone forms by replacing a cartilage model, usually hyaline cartilage.

What happens to chondrocytes during endochondral ossification?

• Resting – inactive reserve chondrocytes

• Proliferation – chondrocytes divide and stack into vertical columns

• Hypertrophy – chondrocytes enlarge, preparing for apoptosis

• Calcification – matrix becomes mineralized; chondrocytes die

• Ossification – empty spaces left by dead chondrocytes are invaded by vessels and osteoblasts → bone replaces cartilage

How does the perichondrium turn into periosteum in endochondral ossification?

– Perichondrium becomes periosteum as it starts producing osteoblasts
– These osteoblasts make a thin layer of bone called the bone collar around the diaphysis
– Blood vessels penetrate through the collar, bringing in:
 • Osteoprogenitor cells from MSCs → become osteoblasts
 • Hematopoietic stem cells (HSCs) → become osteoclasts
– This invasion is called the periosteal bud
– It starts the primary ossification center inside the diaphysis

What are Primary and Secondary Ossification Centers, and what’s the main difference?

• POC (Primary Ossification Center): First site of bone formation in the diaphysis during fetal development.

• SOC (Secondary Ossification Center): Forms after birth in the epiphyses.

• No bone collar usage in secondary

Where does hyaline cartilage remain after ossification?

• At articular cartilage (joint surface).

• At epiphyseal plates (growth plates) until after puberty, when they close and are replaced by bone.

: What is the difference between immature (woven) and mature (lamellar) bone?

• Woven bone: Random collagen, fast growth, seen in fetus or fracture healing.

• Lamellar bone: Organized layers, stronger, replaces woven bone in adults.

What are the two types of mature bone?

• Compact bone: Dense, outer layer, has osteons.

• Spongy (cancellous) bone: Inner, porous, has trabeculae and bone marrow.

What is a lamella in bone?

Thin sheet of mineralized matrix arranged in layers.

What are the features of compact bone?

• Lamellae types

• Osteons

• Canals

• Lamellae types:

  • Circumferential (outer/inner surface)

  • Concentric (around osteons)

  • Interstitial (between osteons)

• Osteons: Circular units with central Haversian canal.

• Canals:

  • Volkmann’s (perforating): Bring vessels from periosteum.

  • Haversian (central): Run inside osteons.

  • Anastomosing/communicating: Connect canals.

What are the 4 features of spongy bone?

• Has trabeculae (thin branches of bone).

• No osteons, but has lamellae.

• Contains bone marrow (red or yellow).

• Osteocytes in lacunae with canaliculi still present.

What’s the difference between red and yellow bone marrow?

• Red marrow: Makes blood cells, found in flat bones and ends of long bones.

• Yellow marrow: Fat storage, more common in adults, found in long bone shafts.

What is the endosteum?

• A thin connective tissue layer lining inner bone surfaces (e.g., medullary cavity, trabeculae, Haversian and Volkmann canals).

• Contains osteoblasts, bone lining cells, and sometimes osteoprogenitors.

• Plays a key role in bone growth, repair, and remodeling.

What are the functions of cartilage and bone?

• 3 for cartilage

• 4 for bone

Cartilage:

• Serves as a template for ossification during development

• Covers articular surfaces to reduce friction

• Provides flexible support for structures like the ear, nose, and airway

Bone:

• Protects and supports soft tissues and organs

• Acts as levers for muscles to enable movement

• Stores and releases minerals (e.g., calcium, phosphate)

• Contains bone marrow for blood cell production and fat storage