The Skeletal System Quiz

The Skeletal System

• The skeletal system is composed of skeletal bones, cartilage, ligaments, and connective tissue, which stabilizes the skeleton.
• Bones are dynamic organs, made of several tissue types.

Functions of the Skeletal System

• Support: Provides a framework for other organs.
• Protection: Ribs protect the heart and lungs, the skull protects the brain, the vertebrae protect the spinal cord, and the pelvic bones protect the reproductive organs.
• Assistance in Movement: Muscles pull on bones to cause movement.
• Mineral Storage and Release: Bones store calcium ions (98% of the body's calcium), and phosphate ions.
• Blood Cell Production: Bone marrow produces red blood cells, white blood cells, and platelets.
• Triglyceride Storage: Yellow marrow stores (fat) cells.

Anatomy of a Bone

• Diaphysis: The shaft of a long bone.
• Epiphyses: The ends of a long bone.
• Metaphysis: The growth zone between the diaphysis and epiphyses.
• Articular surface cartilage: Covers the surfaces of bones where they meet to form joints.
• Periosteum: The outer lining of bone.
• Medullary cavity: The hollow cavity within the diaphysis.
• Endosteum: The inner lining of bone.

Anatomy of a Bone - Epiphysis

• Contains spongy bone
• Red marrow, which contains blood progenitor cells

Anatomy of a Bone - Diaphysis

• The medullary cavity consists of yellow marrow.
• Yellow marrow consists of fat cells.

Anatomy of a Bone - Metaphysis

• Narrow growth zone between the epiphysis and diaphysis.

Anatomy of a Bone - Histological Organization of Mature Bone

• The matrix of bone: Calcium phosphate converts to hydroxyapatite crystals, make up 2/3 of the bone mass.
• Hydroxyapatite crystals resist compression.
• Collagen fibers: Make up 1/3 of the bone matrix, contribute to tensile strength, and make the bone tissue strong.
• Bone cells (osteocytes): contribute only 2% to the bone mass.

Anatomy of a Bone - Cells of Mature Bone - Osteocytes

• Mature bone cells that maintain the protein and mineral content of the matrix.
• Located in depressions called lacunae.
• Matrix layer associated with osteocytes is called lamellae.
• Small channels extending from osteocytes to bone capillaries (canaliculi).

Anatomy of a Bone - Cells of Mature Bone - Osteoblasts

• Immature bone cells
• On the inner and outer surfaces of bones.
• Produce osteoid (which is a part of the matrix of bone)
• Involved in making new bone in a process called osteogenesis.

Anatomy of a Bone - Cells of Mature Bone - Osteoprogenitor cells

• Bone stem cells
• Found on the inner layer of the periosteum and inner lining of endosteum.
• Differentiate into new osteoblasts
• Heavily involved in bone repair.

Anatomy of a Bone - Cells of Mature Bone - Osteoclasts

• Multinucleated cells
• Secrete acids, dissolving bone, releasing stored calcium and phosphate into the blood.
• This process is called osteolysis.
• Derived from bone marrow.

Anatomy of a Bone - Two Types of Osseous Tissue

• Compact bone: Dense and solid, forms bone walls.
• Spongy bone: (aka trabecular or cancellous bone) open network of plates. Surrounds the medullary cavity. Spongy bone contains bone marrow.

Anatomy of a Bone - Compact Bone (Consists of Osteons)

• Osteons: Basic functional unit of bone.
• Consists of central canals, canaliculi, osteocytes, lacunae, and lamellae (layers).

Anatomy of a Bone - Functional Differences Between Compact and Spongy Bone

• Compact bone: Conducts stress from one end of a long bone to the other. Its tremendous strength is from end to end. Less effective when stress is applied to the side.
• Spongy bone: Arranged in parallel struts, called trabeculae. Forms branching plates; these plates form an open network. Creates the lightweight nature of bones.

Cartilage

• Not as strong as bone, but more flexible.
• Matrix contains collagen and elastic fibers.
• Chondrocytes: cartilage cells, lie within lacunae.
• Has no nerves or blood vessels, relying on nearby tissue for nutrient and waste exchange; this makes it slow to heal.
• Hyaline cartilage: found at the ends of long bones, nose, ribs, larynx, and trachea.
• Fibrocartilage: found in intervertebral disks and the knee.
• Elastic cartilage: found in ear flaps and epiglottis.

Fibrous Connective Tissue

• Made of rows of fibroblasts separated by bundles of collagenous fibers.
• Makes up ligaments and tendons.
• Ligaments connect bone to bone, tendons connect muscle to bone.

Bone Development and Growth

• Osteogenesis: Bone formation.
• Calcification: The deposition of calcium ions into bone tissue.
• Endochondral ossification: Chondrocytes are replaced with bone cells, occurs in long bones.
• Intramembranous ossification: Bone forms directly from mesenchymal cells. Occurs in flat bones like skull bones.

Bone Development and Growth - Epiphyseal Plate

• Cartilage area in the metaphysis.
• During bone growth this cartilage converts to bone near the diaphysis.
• The width of this zone gets narrower as we age and eventually the area is devoid of cartilage, turning into the epiphyseal line.

Bone Development and Growth Factors

• Hormones:
  • Parathyroid hormone (PTH): Stimulates osteoclasts, increases calcium levels, maintains calcium homeostasis, and increases calcium absorption. Reduces calcium loss from the kidneys.
  • Calcitonin: inhibits osteoclasts, Decreases calcium levels, maintains calcium homeostasis, and increases the rate of calcium loss in the urine and removes calcium from the blood to be used by the bone.
  • Growth hormone (somatotropin): Stimulates bone growth, maintains normal activity of the epiphyseal cartilage, dwarfism (too little GH), gigantism, and acromegaly (excess GH).
• Estrogen and Testosterone: Stimulate osteoblast activity. Faster bone growth in young people than epiphyseal cartilage expansion, until bone growth ceases.
• Vitamin D: Formed in the skin by sunlight, converted to a hormone needed for calcium absorption from the digestive system. Rickets are caused with low levels of vitamin D, resulting in bone deformities.

Bone Maintenance, Remodeling, and Repair

• Remodeling of Bone: Inactivity causes degeneration. After several weeks, inactivity causes bones to lose about 1/3 of their mass. Stress/weight stimulates osteoblasts.
• Aging and the Skeletal System: When young, osteoblast activity balances with osteoclast activity. Older people have declining osteoblast activity. Osteoclast activity remains high, causing porous bones. This can lead to osteoporosis.

Articulations (joints)

• Where bones meet.
• Classified as fibrous, cartilaginous, or synovial.
• Fibrous: Immovable (sutures between cranial bones).
• Cartilaginous: Slightly movable (costal cartilages, intervertebral disks).
• Synovial: Freely movable.

Articulations (Synovial joints)

• Ligaments: Support and strengthen joints.
• Joint capsule: Formed by ligaments, surrounds the joint.
• Synovial membrane: Lines the joint capsule, secretes synovial fluid (for lubrication).
• Bursae: Fluid-filled sacs that lessen friction.
• Menisci: C-shaped fibrocartilage, give stability and absorb shock.
• Types of Synovial Joints:
  • Ball-and-socket joints: Allow movement in all planes, rotational movements (e.g., hips and shoulders).
  • Hinge joints: Movement in a single direction (e.g., elbows and knees).