Bone basics

Bones

Bones are the hardest structure in the body, made up by 10% cells and 90% matrix

Growth Plates

These are usually at the end of long bones, where cartilage (hyaline) is found, and osteoblasts (converts) and chondroblasts (deposits) help build bone.

Fontanels

These are creases found in the skulls of newborns to allow for the growth of the brain

Ossification

This is the process of bone formation (6th to 7th week of embryonic development)

Functions of Skeletal Systems

• Structural support

• Movement + muscle

• Protection

• Mineral storage of calcium and phosphate to form hydroxyapatite for bone density

• Blood cell formation (marrow)

Ligaments

Dense fibrous connective tissue (type 1 collagen)

• bone to bone

• provides strength and support for joints to enable movement

Tendon

Connects muscle to bone

Cartilage

• fibres of collagen (type 2) and elastin in a gel matrix (proteoglycans that are high in h20 to provide fluid distribution)

• Cushions the vertebrae (spine bones)

• Reduces friction on joints

Hyaline

Located in:

• Embryonic structures (cartilage is the building block for bones)

• Protects the end of long bones (ie. humerus)

Elastic cartilage

The most flexible

Located in:

• Outer ear

• tip of the nose

Fibrocartilage

Toughest form of cartilage

Located in:

• Intervertebral disks (vertebrae)

• Menisci (knee)

• Pubic region

Perichondrium

Surrounds cartilage, making it a resilient "tissue" (springs back)

Extracellular matrix in bones

• Calcium and phosphate (hydroxyapatite to make bones strong)

• Collagen

• Proteins

• Water

This matrix is hard, unlike the ECM in cells, which is gelatinous

Osteoblasts

These are the builders. They produce the ECM in the form of osteoids, which later turns into the bones.

Osteogenic cells

These are the stem cells for bones, which later becomes osteoblasts

Osteocytes

These are the maintainers who help support and maintain the bone matrix. They stem from osteoblasts

Osteoclasts

These are the destroyers; they break down bone, usually during maintenance, fractures, or when calcium levels are low

Compact bone

Made up of osteons (haversian system)

• Forms the shaft (diaphysis) and ends

Yellow bone marrow

Found in the shaft, it contains fat for readily usable ATP and decreases the weight of the bone

Spongey Bone

These are weaker and are generally in a trabecular form lattice structure to decrease the weight of the bone and allow it to store blood.

• Trabeculae contain lamellae, lacunae, and osteocytes, but are randomly places in these branches as they are too small to contain osteons

Haversian System

Found only in compact bones

• Lamellae: The exterior of the bone contains collagen to allow the bone to “give” a little

• Lacunae: Contains the osteocytes, canaliculi (branches that give nutrients to the osteocytes)

• Central canal: Contains the vessels and nerves

These osteons compile together to form the compact bone

Data on bone remodeling

• 500 mg of calcium leaves/enters the adult skeleton

• Spongey tissue is replaced 3 to 4 years

• Compact tissue is replaced every 10 years

Bone Remodeling

1. Osteoclasts come and break down the bone area (Ca in blood goes up)

2. Osteoblasts repair the bone (Ca in blood goes down)

3. Osteocytes maintain the bone

This will occur only in the periosteal and endosteal surfaces

Stresses on the body that affect how the bone grows and is shaped.

Ie, compressions, tension. bending, shear, torsion, or breakage.

Intramembranous Ossification

This occurs only in flat bones, originating from the bases of mesenchyme (a type of tissue that does not need to differentiate into cartilage).

1) Cells cluster together to form a center. Here, they’ll differentiate into osteoblasts

2) Osteoblasts secrete osteoid, which is calcified into bone

3) Spongy tissue forms and the periosteum (outer vessel for bones)

4) Trabeculae and periosteum thicken to become compact

Endochondral Ossification

This occurs for all other bones except flat ones from the hayline cartilage

Epiphysis

Rounded ends of bones

Diaphysis

Longer ends of bones

Epiphyseal plates

These are also known as growth plates. Cartilage cells are found here and push the epiphysis away from the diaphysis.

• Chondroblasts divide and mature into chondrocytes, which then calcifies into bone

Organization of Cartilage

1. Rest Zone

2. Proliferation (mitosis)

3. Hypertrophic (older cartilage enlarges)

4. Calcification (hardens)

5. Ossification (bone formation)

Postnatal growth

• Bones lengthen (interstitial) and widen (appositional)

• Plates thin

• Diaphysis and the epiphysis fuse

Hormone regulation

• hGH: Stimulates the growth plates

• Thyroid hormone: ensures that the bones are at the proper size

• Sex hormones (produced by the adrenal glands and reproductive organs): Promotes bone growth and induces the closure of growth plates

Sesamoid bone

There is only one, your knee cap (patella)

Endosteum

Interior part of the bone (wrap around internally)

Periosteum

External wrap around the bone

Medullary Cavity

Contains yellow marrow (fat)

Short, Irrgeular, and Flat bones

These contain bone marrow but no cavity as it is found in the trabeculae cavities

Simple fracture

Bone is broken but skin is not torn

Compound Fractures

Bones are broken and skin is torn

Type of Fractures

• Compression

• Compound

• Transverse

• Comminuted (breakage everywhere 3+)

• Spiral (breakage is in the shape of an S or oblique)

• Greenstick (only one side has broken while the rest is bent)

• Epiphyseal (epiphysis separates from diaphysis)

• Depressed

Healing a Fracture

1) blood pools in (hematoma)

2) fibrocartilaginous callus (temporary fibrous tissue and cartilage) forms

3) Bony callus forms from osteoblasts

4) Bone heals, usually there is a bump around the site as the body overdoes the healing

Bone Growth = Age

• Young, bone formation is more than reabsorption

• In adults, bone formation is equal to reabsorption

• Elderly (35+), bone formation is less than reabsorption (chondrocytes decrease)

Osteoporosis

Due to age, bones become more porous, making them more brittle. This disease is much more prevalent in women because of menopause (50+ = 6.5%)

Risk Factors:

• Diet (low vitamin d and calcium)

• Low exercise

• Hormonal imbalance

Osteomalacia

Adults - bones are poorly mineralized

Rickets

Children - bones are poorly mineralized

Paget’s disease

Excessive bone deposition

Osteosarcoma

Bone cancer as sarcoma, means tissue cancer.