lecture 4

Study Guide — Skeletal System

1.15 Major Functions of the Skeletal System

• Function: Support

  • Description: Provides structural framework for the body

  • Example: Bones support muscles and organs

• Function: Protection

  • Description: Shields vital organs

  • Example: Skull protects brain; ribs protect heart/lungs

• Function: Movement

  • Description: Acts as levers for muscles

  • Example: Muscles pull on bones to produce movement

• Function: Mineral Storage

  • Description: Stores calcium and phosphorus

  • Example: Bone matrix holds minerals for release into blood

• Function: Blood Cell Production (Hematopoiesis)

  • Description: Occurs in red bone marrow

  • Example: Produces red & white blood cells, platelets

• Function: Fat Storage

  • Description: Yellow marrow stores triglycerides

  • Location: Found in the medullary cavity of long bones

• Function: Hormone Production

  • Description: Osteocalcin helps regulate blood sugar and fat metabolism

  • Role: Skeletal endocrine role

• Key Idea: The skeletal system is not just structure — it’s vital for movement, storage, and metabolic regulation.

1.16 Types of Bones (by Shape)

• Bone Type: Long bones

  • Description: Longer than wide; mostly compact bone

  • Example(s): Femur, humerus, tibia, ulna

• Bone Type: Short bones

  • Description: Cube-shaped; mostly spongy bone

  • Example(s): Carpals (wrist), tarsals (ankle)

• Bone Type: Flat bones

  • Description: Thin, often curved; protect organs

  • Example(s): Skull, ribs, sternum

• Bone Type: Irregular bones

  • Description: Complex shapes

  • Example(s): Vertebrae, pelvis

• Bone Type: Sesamoid bones

  • Description: Embedded in tendons; reduce friction

  • Example(s): Patella (kneecap)

1.17 Major Structures of a Long Bone

Example: The femur

• Structure: Diaphysis

  • Description / Function: Shaft; composed of compact bone surrounding medullary cavity that stores yellow marrow

• Structure: Epiphyses

  • Description / Function: Ends of the bone; spongy bone filled with red marrow and covered by articular cartilage

• Structure: Metaphysis

  • Description / Function: Region where diaphysis and epiphysis meet; includes growth plate (epiphyseal plate/line)

• Structure: Articular Cartilage

  • Description / Function: Hyaline cartilage covering joint surfaces; reduces friction

• Structure: Periosteum

  • Description / Function: Outer fibrous membrane covering bone; contains osteoblasts for growth and repair

• Structure: Endosteum

  • Description / Function: Inner lining of bone cavities; contains bone-forming cells

• Structure: Medullary Cavity

  • Description / Function: Central cavity containing bone marrow (yellow in adults, red in infants)

• Structure: Nutrient Foramina

  • Description / Function: Small openings for blood vessels and nerves

• Mnemonic: “PEMDAN” → Periosteum, Epiphysis, Metaphysis, Diaphysis, Articular cartilage, Nutrient foramen.

1.18 Microscopic Anatomy of Bone (Osteon Composition)

• Two Types of Bone Tissue

  1. Compact Bone (Dense):

  • Location: Found in outer layer; strong and rigid.

  • Structural unit: Osteon (Haversian system).

  1. Parts of an Osteon:

  • Central (Haversian) canal: carries blood vessels and nerves.

  • Lamellae: concentric rings of calcified matrix.

  • Lacunae: small spaces containing osteocytes.

  • Canaliculi: tiny channels connecting lacunae; allow nutrient/waste exchange.

  • Perforating (Volkmann’s) canals: connect osteons transversely.

• Spongy Bone (Cancellous):

  • Location: Found at epiphyses; lighter and porous.

  • Description: Made of trabeculae — thin bone plates with red marrow between them.

  • Feature: No osteons, but still nourished by diffusion through canaliculi.

Bone Cells

• Cell Type: Osteoprogenitor cells

  • Function: Stem cells that form osteoblasts

• Cell Type: Osteoblasts

  • Function: Build bone matrix (bone formation)

• Cell Type: Osteocytes

  • Function: Mature bone cells; maintain bone tissue

• Cell Type: Osteoclasts

  • Function: Break down bone (resorption)

• Key Concept: Bone is a living tissue constantly being remodeled — broken down by osteoclasts and rebuilt by osteoblasts.

1.19 Bone Formation (Ossification)

• Two Main Processes

  1. Intramembranous Ossification:

     • Description: Bone forms directly from mesenchymal (embryonic) tissue

     • Steps:

     1. Mesenchymal cells → osteoblasts

     2. Osteoblasts secrete matrix

     3. Matrix hardens

     4. Formation of compact & spongy bone

     • Examples: Flat bones such as skull, clavicle

  2. Endochondral Ossification:

     • Description: Bone replaces hyaline cartilage model

     • Steps:

     1. Cartilage model forms

     2. Bone collar develops

     3. Cartilage calcifies and dies

     4. Blood vessels & osteoblasts invade

     5. Primary ossification in diaphysis; secondary in epiphyses

     • Examples: Most bones such as femur, humerus, vertebrae

• Comparison Summary:

  • Intramembranous: direct bone formation (no cartilage)

  • Endochondral: bone replaces cartilage model

1.20 Fibrodysplasia Ossificans Progressiva (FOP)

• Definition: A rare genetic disorder where soft connective tissues (muscles, tendons, ligaments) gradually turn into bone — forming a “second skeleton.”

• Cause:

  • Mutation in ACVR1 gene, which controls bone growth signaling (BMP pathway).

  • This mutation causes osteoblast activity to occur abnormally in soft tissues.

• Symptoms:

  • Episodes of painful soft tissue swelling (flare-ups)

  • Progressive immobility as joints fuse

  • Bone formation in muscles, tendons, ligaments

  • Malformed big toes (early sign)

  • No known cure; trauma or surgery can worsen condition

• Relationship to Connective Tissue:

  • Normally, connective tissues support, bind, and cushion organs.

  • In FOP, these tissues abnormally ossify, losing flexibility and function.

  • Disrupts homeostasis, mobility, and respiration as bone replaces soft tissue.

• Key Insight: FOP demonstrates what happens when bone formation regulation fails, showing how vital controlled connective tissue function is to normal skeletal development.

Quick Review Summary

• Skeletal functions: support, protection, movement, storage, blood production.

• Bone types: long, short, flat, irregular, sesamoid.

• Long bone structures: diaphysis, epiphysis, periosteum, marrow cavity, etc.

• Microscopic anatomy: osteons (compact bone), trabeculae (spongy bone).

• Bone formation: intramembranous (flat bones) vs. endochondral (most bones).

• FOP: genetic condition affecting connective tissue leading to abnormal bone formation; caused by ACVR1 mutation.

• Personal Note: I'm not totally confident about remembering all the structures in a long bone and their specific roles, like the difference between the periosteum and endosteum.