Chapter #6: Bone & Skeletal Tissues

What does the skeleton consist of?

Bones and resilient cartilages.

What surrounds cartilage and supplies nutrients?

Perichondrium (dense irregular CT, vascularized).

What is the human skeleton initially made from?

Cartilages and fibrous membranes.

What are the main components of cartilage tissue?

1) Chondrocytes in lacunae

2) Extracellular matrix with (ground substance & fibers).

What are the three types of cartilage?

1) Hyaline

2) Elastic

3) Fibrocartilage.

Where is hyaline cartilage found?

1) Articular cartilage (ends of long bone)

2) Costal cartilages (ribs to sternum)

3) Respiratory cartilages (larynx, trachea, bronchi) 

4) Nasal cartilage (tip of nose).

Where is elastic cartilage found?

External ear and epiglottis.

What makes elastic cartilage unique?

It is the most flexible cartilage type.

Where is fibrocartilage found?

Menisci of knee and intervertebral discs (discs between vertebrae).

What are fibrocartilage’s properties?

Highly compressible with great tensile strength.

What is appositional growth of cartilage?

Growth from outer perichondrium where cells secrete new matrix externally.

*Adds from (outside)

What is interstitial growth of cartilage?

Chondrocytes divide in lacunae and secrete new matrix from within.

*Expands from (within)

List the seven functions of bone.

1) Support/framework

2) Protection of organs

3) Anchorage for muscles (movement)

4) Mineral & growth factor storage (calcium & phosphate)

5) Blood cell formation (hematopoiesis in red marrow)

6) Triglyceride storage (yellow marrow)

7) Hormone production (osteocalcin for insulin & glucose regulation)

How are bones classified?

By location (axial vs appendicular) and by shape (long, short, flat, or irregular).

What is the axial skeleton?

1) Skull

2) vertebral column

3) Ribs.

What is the appendicular skeleton?

1) Upper and lower limbs

2) Shoulder bones

3) Hip bones.

Define long bones and give examples.

Longer than wide; all limb bones.

Define short bones and give examples.

Cube-like; wrist & ankle bones.

Patella = sesamoid bone (a type of short bone).

Define flat bones and give examples.

Thin and flattened;

1) Sternum

2) Scapula

3) Ribs

4) Most skull bones.

Define irregular bones and give examples.

Complicated shape; vertebrae and pelvic bones.

What are the two types of bone texture?

1) Compact bone (dense outer layer)

2) Spongy bone (looks like honeycomb) small pieces = trabeculae

*inside of compact bone

What fills spaces in spongy bone?

Red or yellow bone marrow.

What are trabeculae?

Small pieces in spongy bone = create honeycomb structure.

What are the ends of a long bone called?

Epiphyses (covered by articular cartilage).

What is the shaft of a long bone called?

Diaphysis (contains medullary cavity filled with yellow marrow in adults).

What separates epiphysis and diaphysis?

Epiphyseal line (remnant of growth plate).

What is periosteum?

Double-layered membrane covering long bone (except articulating surfaces/ not at joints)

* Outer fibrous layer = dense irregular CT.

* Inner osteogenic layer = osteogenic stem cells.

What anchors periosteum to the bone?

Sharpey’s / perforating fibers = made of collagen fibers.

What does periosteum contain?

1) Nerves

2) Blood vessels

3) Anchoring points for tendons/ligaments.

What is endosteum?

Covers internal bone surfaces/canals and contains (osteogenic cells).

Where is red bone marrow found in adults?

1) Heads of femur & humerus

2) Sternum

3) Hip bones.

Where is yellow bone marrow found?

Medullary cavities of long bones in adults.

What are bone markings used for?

Attachment sites for…

1) Muscles

2) Ligaments

3) Tendons

4) Joint surfaces.

What are the 5 major cell types in bone?

1) Osteogenic cells

2) Osteoblasts

3) Osteocytes

4) Osteoclasts

5) Bone lining cells.

Which bone cell type has a different origin than the others?

Osteoclasts (derived from blood stem cells, not embryonic CT).

What are osteogenic cells also called?

Osteoprogenitor cells.

Where are osteogenic cells found?

Periosteum and endosteum.

What do osteogenic cells give rise to?

Osteoblasts or bone lining cells.

What is the function of osteoblasts?

(Immature) bone-forming cells that secrete bone matrix.

What makes up the organic part of the matrix secreted by osteoblasts?

90% collagen protein and 10% calcium-binding proteins.

What else do osteoblasts do besides forming bone?

Help with bone calcification.

How are osteocytes formed?

Osteoblasts surrounded by matrix = become osteocytes.

What are osteocytes?

Mature bone cells that sit in lacunae.

What is the role of osteocytes?

Monitor and maintain bone matrix.

Where are bone lining cells found?

On bone surfaces (periosteal cells externally and endosteal cells internally).

What is the function of bone lining cells?

Help maintain the bone matrix.

What is the origin of osteoclasts?

Derived from blood stem cells (same lineage as macrophages).

What is the structure of osteoclasts?

Giant, multinucleated cells.

Where are osteoclasts located?

Sites of bone resorption.

What is the main function of osteoclasts?

Bone resorption (breaking down bone)

What is the structural unit of compact bone?

Osteon (Haversian system).

What does an osteon look like?

An elongated, weight-bearing cylinder parallel to the long axis of the bone.

What do osteons contain?

Concentric lamellae (rings) with a central Haversian canal for blood vessels & nerves.

How are collagen fibers arranged in lamellae?

Fibers run in one direction in a lamella, and in the opposite direction in the next lamella.

Why are collagen fibers arranged this way?

To resist torsional (twisting) stress.

How do crystals of bone salts align?

In alternating directions, similar to collagen, for added strength.

What runs through the central canal (Haversian canal)?

Blood vessels and nerves.

What connects central canals?

Volkmann’s (perforating) canals (lined with endosteum).

Where do osteocytes reside?

In lacunae at the junctions of lamellae.

How are osteocytes connected?

By canaliculi (tiny hairlike canals) and gap junctions, allowing nutrient and waste transfer.

What are interstitial lamellae?

Incomplete lamellae filling gaps between osteons.

What are circumferential lamellae?

Lamellae that encircle the entire diaphysis, located just inside periosteum and superficial to endosteum.

How does spongy bone appear under the microscope?

Looks disorganized but trabeculae align along stress lines.

What are trabeculae made of?

Few-cells-thick with irregular lamellae and osteocytes connected by canaliculi.

What are the organic components of bone?

Cells and osteoid (matrix) consisting of ground substance (proteoglycans & glycoproteins) and collagen fibers, produced by osteoblasts.

What are the inorganic components of bone?

Hydroxyapatite (mineral salts, mostly calcium phosphates), forming ~2/3 of bone.

What do inorganic crystals form around?

Collagen fibers in the matrix.

Which component of bone lasts long after death?

Inorganic mineral salts.

What is ossification?

The process of bone formation.

What occurs in embryonic ossification?

Formation of the bony skeleton.

What occurs in postnatal ossification?

Bone growth.

What occurs in adult ossification?

Remodeling and repair.

What materials form the early "skeleton" before bone develops?

Fibrous membranes and hyaline cartilage.

What are the two main types of ossification?

1. Endochondral ossification: Cartilage is replaced by bone.

2. Intramembranous ossification: Bone develops directly from fibious membrane.

What type of model is used in endochondral ossification?

Hyaline cartilage “bones.”

Where does bone formation begin in endochondral ossification?

At the primary ossification center in the middle of cartilage.

What happens when blood vessels enter the perichondrium?

It becomes vascularized periosteum, and stem cells turn into osteoblasts.

What forms around the diaphysis during ossification?

A bone collar (osteoblasts secrete bone matrix).

Why does the cartilage in the diaphysis center calcify and form cavities?

Chondrocytes enlarge, signaling matrix calcification → nutrients can’t diffuse → chondrocytes die → cavities form.

What is the periosteal bud and when does it invade the cavity?

At month 3: artery, vein, nerve fibers, red marrow elements, osteogenic cells, and osteoclasts enter.

What happens when periosteal bud enters?

Osteoclasts break down calcified cartilage; osteogenic cells → osteoblasts that form trabeculae.

How does the diaphysis elongate and form the medullary cavity?

Osteoclasts break down spongy bone in the center, forming the medullary cavity.

By birth, what does the bone look like?

Ossified diaphysis with two cartilage epiphyses.

When and where do secondary ossification centers form?

Around birth, in each epiphysis.

How is secondary ossification similar to primary?

Uses periosteal bud elements to convert cartilage to bone.

How does secondary ossification differ from primary ossification?

No medullary cavity forms in the epiphyses.

After secondary ossification, where does hyaline cartilage remain?

Articular cartilage (on epiphysis surfaces) and epiphyseal plate (between epiphysis & diaphysis).

Which bones are formed by intramembranous ossification?

Skull bones and clavicles.

When does intramembranous ossification begin?

~8 weeks after fertilization.

What is the process of intramembranous ossification?

Mesenchymal cells in fibrous CT → osteoblasts form ossification center → osteoid secreted → first trabeculae form.

What happens to osteoblasts during calcification?

They get trapped and become osteocytes.

How does periosteum form?

Vascularized mesenchyme condenses on outer trabeculae.

What do trabeculae beneath the periosteum become?

Thicken → lamellar bone → compact bone on surfaces.

What persists in the middle after intramembranous ossification?

Spongy bone with vascular tissue forming red marrow.

How do long bones grow in length after birth?

Interstitial growth of epiphyseal plate cartilage, replaced by bone.

How do bones grow in thickness?

Appositional growth (adding new bone to outer surface).

After adolescence, which bones grow slowly?

Nose bones and lower jaw.

How is the epiphyseal plate organized?

Epiphyseal side: inactive cartilage; Diaphyseal side: growth columns with 4 zones.

What are the 4 zones of the epiphyseal plate?

1. Proliferation zone: chondrocytes divide, forming new cartilage.

2. Hypertrophic zone: older cartilage cells enlarge; lacunae enlarge.

3. Calcification zone: cartilage matrix calcifies; chondrocytes die and deteriorate.

4. Ossification zone: new bone forms

What forms at the epiphysis–diaphysis border?

Spicules of calcified cartilage.