SKELETAL SYSTEM

Collagen fibers

rope-like, strong; give bones tensile strength (resists pulling/twisting).

Mineral salts (calcium phosphate → hydroxyapatite)

provide hardness & compressive strength (resists crushing).

Proteoglycans

trap water, make tissues more resilient & flexible.

Long bones

longer than wide; e.g., femur, humerus.

Diaphysis

shaft, contains medullary cavity (yellow marrow).

Epiphysis

ends, spongy bone (red marrow).

Epiphyseal plate

growth region (cartilage → bone).

Periosteum

dense outer covering.

Endosteum

thin lining inside.

Articular cartilage

hyaline cartilage at joints.

Short bones

cube-like; e.g., carpals, tarsals.

Flat bones

thin; protect organs, muscle attachment; e.g., skull, sternum, ribs.

Irregular bones

complex shapes; e.g., vertebrae, facial bones.

Osteoblasts

build new bone.

Osteocytes

mature cells in lacunae, maintain matrix.

Osteoclasts

break down bone (resorption).

Lamellae

concentric matrix rings

Canaliculi

tiny canals that allow nutrient/waste exchange between osteocytes.

Compact bone (cortical)

dense, strong; made of osteons with central (Haversian) canal.

Spongy bone (cancellous)

lattice-like trabeculae; spaces filled with red marrow; lighter but weaker.

Length (Longitudinal Growth)

• Happens at the epiphyseal plate (growth plate).

• Chondrocytes make new cartilage on the epiphyseal side.

• Cartilage on the diaphyseal side is replaced by bone.

• Bone lengthens while plate thickness stays the same.

• Stops at adulthood → cartilage replaced by bone →Stops at adulthood → cartilage replaced by bone → epiphyseal line forms.

Thickness (Appositional Growth)

• Osteoblasts in periosteum add new bone to the surface.

• Osteoclasts widen the medullary cavity inside.

• Bone becomes thicker, stronger, but not too heavy.

• Continues throughout life, unlike length growth.

Bone Remodeling

• Balance of osteoblast (build) & osteoclast (break down) activity.

• Childhood: formation > destruction (growth).

• Adulthood: balanced.
  

Aging: destruction > formation (bone loss).

Parathyroid Hormone (PTH)

• Activates osteoclasts → breaks down bone.

• Raises blood calcium.

Calcitonin

• Activates osteoblasts → builds bone.

• Lowers blood calcium.

Human Growth Hormone (HGH) & Insulin-like Growth Factor (IGF)

• Stimulate cartilage and bone growth.

• Drive childhood growth and lengthening of bones.

Sex Hormones (Estrogen & Testosterone)

• Cause growth spurts at puberty.

• Close epiphyseal plates, ending growth in length.

Minerals

Calcium, phosphorus, and magnesium → strengthen bone matrix.

Vitamins

• Vitamin A: regulates bone cell activity.

• Vitamin C: needed for collagen.

• Vitamin D: helps absorb calcium.

Lifestyle Factors

• Exercise → strengthens bones.

• Balanced diet → supplies minerals and vitamins.