3. The Skeletal System

Axial skeleton

Forms the long axis of the body (ex. skull, vertebrae, ribs)

Appendicular skeleton

Forms the limbs & respective girdles

*Scapula, clavicle, & os coxa are appendicular!

What are the 4 main classifications of bones according to their shape?

1. Long

2. Short

3. Flat

4. Irregular

1. What is a long bone?

2. Provide an example

1. Bones that are longer than they are wide

2. Femur

*Phalanges are a long bone

1. What is a short bone?

2. Provide an example

1. Small, cube-shaped bones

   1. Carpals

1. What is a flat bone?

2. Provide an example

1. Bones that look really smooth

2. Sternum

1. What is an irregular bone?

2. Provide an example

1. Any bone that doesn’t fit into any of the other main categories

2. Ox coxa

What are the 5 specifications/additional categories of bones based on structure?

1. Wormian

2. Pneumatic

3. Sesamoid

4. Accessory

5. Heterotrophic

1. What is a Wormian bone?

2. Provide an example

1. Bones within the skull’s sutures

2. Bones on the lambdoid suture

1. What is a pneumatic bone?

2. Provide an example

3. What are 2 functions of the pneumatic bone?

1. Bones that contain air (sinuses)

2. Frontal bone

3. Makes your head lighter & amplifies sound when you’re talking or listening

What is an accessory bone?

An extra bone that forms because it failed to fuse during development

1. What is a sesamoid bone?

2. Provide an example

1. A type of short bone that is within a tendon

2. Patellar

1. What is a heterotrophic bone?

2. What causes it?

1. Bones that form where they shouldn’t (within soft tissue)

2. Disease or genetic mutation

What are the 3 types of bone cells?

1. Osteoblasts

2. Osteocytes

3. Osteoclasts

What is an osteoblast?

Builder cells that secrete the matrix of the cell; bone forming cell

What is an osteocyte?

Cells that maintain the matrix; mature bone cell

What is an osteoclast?

Cells that break down the bone matrix & releases calcium; demolition crew

What are the 2 main components of the extracellular matrix of bone?

1. Osteoid (organic)

2. Hydroxyapatite (inorganic)

1. What bone cell is the source of the osteoid?

2. What is an osteoid made up of (3)?

3. What is its function in the bone?

1. Osteoblasts

2. Collagen with some proteoglycans & glycoproteins

3. Provides tensile strength (allows bone to be flexible)

1. What is the source of hydroxyapatite?

2. What is hydroxyapatite made up of (2)?

3. What is its function in the bone?

1. Blood plasma

2. Calcium & phosphate

3. Provides compressional strength (hardness)

What is an osteon?

Basic structural unit of lamellar bone

What is concentric lamellae?

The individual layers of the osteon

1. What is the central canal of an osteon?

2. What does it contain (4)?

1. The middle hole of the osteon

2. Nerves, arteries, veins, lymphatics (NAVaLs)

What is interstitial lamellae in an osteon?

The osteons not in circles in between other formed osteons

Label the parts of the microscopic structure of compact bone

1. Circumferential lamellae

2. Periosteum

3. Perforating fibers/Sharpey’s fibers

4. Perforating canal

5. Spongy bone (trabeculae)

What are canaliculi?

Eyelash-looking structures on an osteon that allow osteocytes to communicate

What are the 2 types of bone tissue?

1. Spongy (cancellous) bone

2. Compact bone

1. What are the plates of lamellar bone in spongy bone called?

2. What are those spaces filled with?

1. Trabeculae

2. Red bone marrow

What are the 2 functions of red bone marrow in spongy bone?

1. Produces blood cells (hematopoiesis)

2. Nourishes the bone cells of the trabeculae

1. What is the name of the membrane that lines trabeculae?

2. What are 2 bone cells it contains?

1. Endosteum

2. Osteoblasts & osteoclasts

What are 2 characteristics of compact bone that makes it different than spongy bone?

1. It’s dense, heavier, & more solid

2. It contains yellow bone marrow (fat)

In a flat bone, what is it called when thin layers of compact bone surround a spongy bone (like a sandwich)?

Diploe

Label the gross anatomy of a long bone

1. Diaphysis

2. Epiphysis

3. Epiphyseal line

4. Articular cartilage

5. Medullary cavity

6. Yellow bone marrow

7. Endosteum

8. Periosteum

What are the 2 sublayers of the periosteum?

1. Outer fibrous layer

2. Inner osteogenic layer

1. What does the outer fibrous layer of the periosteum contain (2)?

2. What is its function?

1. Dense irregular CT & sensory nerves

2. Holding & protecting the bone

What does the inner osteogenic layer of the periosteum contain (2)?

Osteoblasts & osteoclasts

What is osteogenesis/ossification?

The process of bone formation that occurs before birth

What are the 2 processes of osteogenesis?

1. Intramembranous ossification

2. Endochondral ossification

1. What is the model used in intramembranous ossification?

2. What bones are formed from intramembranous ossification?

1. Fibrous membrane

2. Flat bones of the skull & clavicles

1. What is the model used in endochondral ossification?

2. What bones are formed from endochondral ossification?

1. Hyaline cartilage

2. All bones except for flat bones of the skull & clavicles

What is alkaline phosphatase?

Enzyme made by osteoblasts & chondroblasts that makes calcium available to go into the matrix of the bone from the blood

What is woven bone?

• Early, disorganized, freshly made, new bone

• Similar to spongy bone, but the trabeculae aren’t as organized

• Trabeculae are thinner and not as strong

Which one of the 3 primary germ layers does bone come from?

Mesoderm

What are 4 steps in intramembranous ossification?

1. Membrane model formation

2. Bone matrix formation

3. Periosteum develops

4. Compact bone formation

What happens during Step 1: Membrane model formation in intramembranous ossification?

• Begin in fibrous membrane, made up of mesenchyme

• Mesenchymal cells cluster & change into osteoblasts

• This is now called the ossification center

What happens during Step 2: Bone matrix formation in intramembranous ossification?

• Osteoblasts secrete osteoid (thin-layer of matrix)

• Osteoid hardens (calcifies)

• Osteoblasts become osteocytes

What happens during Step 3: Periosteum develops in intramembranous ossification?

• Trabeculae form as more osteoids fuse together

• Mesenchyme flattens to form the periosteum, which contains osteoblasts

What happens during Step 4: Compact bone formation in intramembranous ossification?

• Osteoblasts in the periosteum create compact bone for the external surface

• Spongy bone is filled with red bone marrow on the inside

What are the 5 steps in endochondral ossification?

1. Cartilage model formation

2. Formation of bone collar & loss of cartilage matrix

3. Periosteal bud begins ossification inside the developing bone

4. Formation of spongy bone & medullary cavity

5. Ossification os the epiphyses

What happens during Step 1: Cartilage model formation in endochondral ossification?

• Starting from hyaline cartilage

• Has the shape of the future bone & will enlarge via interstitial growth

What happens during Step 2: Formation of bone collar & loss of cartilage in matrix in endochondral ossification?

• Osteoprogenitor cells (precursor to osteoblasts) form in the diaphysis & turn into osteoblasts

• These osteoblasts deposit a thin layer of bone on the outside of the cartilage. This is called the bone collar

• Chondrocytes inside diaphysis get bigger (hypertrophy)

• Big chondrocytes release alkaline phosphatase to calcify cartilage in the bone

• Chondrocytes die

What happens during Step 3: Periosteal bug begins ossification inside the developing bone in endochondral ossification?

• Blood vessel enters (periosteal bud)

• Periosteal bud contains NAVaLs, osteoclasts, osteoblasts, & red bone marrow elements

• Osteoblasts from periosteal bud begin to build spongy bone over dead cartilage cells (primary center of ossification)

What happens during Step 4: Formation of spongy bone and medullary cavity in endochondral ossification?

• Spongy bone forms in center of diaphysis

• Osteoclasts eat away at the spongy bone to make the medullary cavity

• 2 more periosteal buds come in at the epiphyses to form them (secondary ossification center)

What happens during Step 5: Ossification of the epiphyses in endochondral ossification?

• Lots of spongy bones form in epiphyses and it stays spongy bone

• There’s a small stripe of cartilage left between the epiphysis and the diaphysis. This is the epiphyseal plate

What are 2 disorders related to osteogenesis?

1. Osteogenesis imperfecta

2. Hypophosphatasia

What is osteogenesis imperfecta?

Production of defective or inadequate collagen (no osteoid)

What is hypophosphatasia?

Low levels of alkaline phosphatase, so less calcium into bone (no hydroxyapatite)

Where does post-natal longitudinal bone growth occur?

At the epiphyseal plate (before it turns into epiphyseal line)

What are the 4 zones in the epiphyseal plate that deal with post-natal bone growth?

1. Growth (proliferation) zone

2. Hypertrophic zone

3. Calcification zone

4. Ossification zone

What happens at the growth (proliferation) zone of the epiphyseal plate?

Mitosis of cartilage cells (growing more cells)

What happens at the hypertrophic zone of the epiphyseal plate?

Cartilage cells are going through hypertrophy (getting big) & are starting to secret alkaline phosphatase

What happens at the calcification zone of the epiphyseal plate?

Matrix is becoming calcified so cartilage cells are dying

What happens at the ossification zone of the epiphyseal plate?

Bone formation

What direction does longitudinal bone growth at the epiphyseal plate happen?

Bone grows outwards from the diaphysis

What is achondroplastic dwarfism?

• When the cartilage cells in the growth zone of the epiphyseal plate don’t undergo mitosis

• The long bones don’t grow very long

What is appositional both growth?

Adding tissue to the outside of the bone to make it grow thicker as it grows longer

What 3 hormones regulate post-nasal bone growth?

1. Growth hormone

2. Thyroid hormone

3. Estrogen & testosterone

What’s the growth hormone pathway (3)?

1. Growth hormone is released by the pituitary gland via liver

2. Liver makes insulin-like growth factor (IGF)

3. IFG makes chondroblasts & osteoblasts undergo mitosis (aka bones get longer)

What is the thyroid hormone pathway (3)?

1. Pituitary gland releases thyroid-stimulating hormone (TSH)

2. Thyroid releases thyroid hormone (T3 or T4)

3. Regulates by a negative feedback system

What are the 2 effects thyroid hormone has on growth?

1. Maintains growth hormone activity

2. Stimulates osteoblasts

What is the estrogen/testosterone pathway (3)?

1. Osteoblasts are stimulated during puberty

2. Hormones make the bones grow a lot until you reach the end of puberty

3. Estrogen causes epiphyseal plate to turn into bone (epiphyseal line)

What is Wolff’s law?

Putting more stress on the bones makes them thicker; the more you use the bones with muscle, the stronger they become