Bones and Bone Structure

Bones and Bone Structure

• Skeletal system includes:

  • Bones of the skeleton (oste/o, osseous, ossi)

  • 206 bones in total

    • Axial: 80 bones

    • Appendicular: 126 bones

  • Cartilages, ligaments, and other connective tissue

    • Calcification: Cartilage becomes hard.

    • Ossification: Cartilage turns into bone.

Primary Functions of the Skeletal System

• Support: Provides body shape, supports body weight, and forms the body structure.

• Storage: Stores minerals (calcium and phosphorus) and lipids.

  • Calcium and Phosphate (PO42-)

• Blood Cell Production: Occurs in red bone marrow.

• Protection: Protects internal organs.

• Leverage: Assists in movement.

Bone Classification by Shape

• Sutural: Found in the cranium (skull).

• Irregular: Vertebrae (26 total).

• Short: Carpals (wrist) and tarsals (ankle).

• Flat: Parietal bone (skull) and scapulae (shoulder blades).

• Long: Humerus, femur, ulna, radius, tibia, and fibula.

• Sesamoid: Patella (kneecap).

Bone Markings

• Foramen: Natural opening in the body.

  • Foramen magnum: Located at the base of the skull.

  • Obturator foramen: Located in the hips (largest foramen).

• Sinuses

Structure of a Long Bone

• Diaphysis (Shaft):

  • Longest part of the long bone.

  • Wall of compact bone.

  • Medullary cavity (marrow cavity) in the center.

• Epiphysis (Wide Part at Each End):

  • Located at each end of the long bone (two epiphyses).

  • Mostly spongy bone (trabecular bone).

• Metaphysis:

  • Located between the epiphysis and diaphysis.

  • Area where the diaphysis and epiphysis meet.

• Periosteum:

  • Outermost layer of the long bone (except at joints).

  • dia = long

  • meta = middle

  • epi = surface

Spongy and Compact Bone

• Spongy Bone:

  • Located at joints (synovial joints).

  • Provides shock absorption.

• Compact Bone:

  • Supports body weight.

  • Enables bipedal motion.

Structure of Flat Bones

• Examples: Parietal bones of the skull, ribs, scapula, and sternum.

• Diploe: Layer of spongy bone within the cranium, filled with trabeculae.

• Spongy bone is also known as trabecular bone.

• Spongy bone is found in the epiphysis.

Specialized Cells

• Osteocytes

• Osteoblasts

• Osteoclasts

• Osteogenic cells (aka osteoprogenitor cells)

Bone Tissue

• Dense, supportive connective tissue (spongy and compact bone).

• Contains specialized cells.

• Solid extracellular matrix with collagen fibers.

  • Organic Matrix (Osteoid):

    • Mainly made of collagen fibers.

  • Inorganic Matrix:

    • Mainly made of hydroxyapatite (gives bones their hardness).

    • Mix of calcium phosphate and calcium carbonate.

Key Bone Structures

• Osteon

• Canaliculi

• Lamellae

• Lacunae

Characteristics of Bone

• Dense matrix due to deposits of calcium salts.

• Hydroxyapatite Ca{10}(PO4)6 (OH)2 gives bone hardness.

• Osteocytes: Mature bone cells within lacunae, organized around blood vessels.

• Lacunae: Small openings containing osteocytes.

• Osteon: Basic functional unit of compact bone.

• Canaliculi: Narrow passageways for nutrient and gas exchange.

• Lamellae: Layers within the osteon.

Bone Cells

• Osteogenic cells (aka osteoprogenitor cells):

  • Stem cells of bone tissue that become osteoblasts.

• Osteoblasts:

  • Build bone tissue.

• Osteocytes:

  • Mature bone cells.

• Osteoclasts:

  • Break down/dissolve bone tissue.

Lamellae

• Concentric lamellae: Surround the central canal.

• Circumferential lamellae: Located at outer and inner bone surfaces.

• Lacunae contain osteocytes within the osteon.

Periosteum

• Covers outer surfaces of bones (2 layers).

• Outer fibrous layer and inner cellular layer.

• Inner layer = endosteum

Bone Matrix

• Major component of compact bone.

• Interacts with calcium hydroxide to form crystals of hydroxyapatite.

• Responsible for bone hardness.

• Matrix Proteins

• Collagen Fibers

  • About one-third of bone mass is collagen fibers.

  • Collagen fibers are type I collagen.

  • Provide flexibility.

Sure! Here's an easy explanation of the bone matrix:

The bone matrix is the stuff between bone cells that makes bones strong and hard.

It has two main parts:

1. Collagen – a soft, stretchy protein that gives bones flexibility (so they don’t break too easily).

2. Minerals (mostly calcium and phosphate) – make bones hard and strong.

🦴 Together, they help bones be tough but not brittle — strong enough to support the body but able to handle pressure without snapping.

Bone Cells

• Make up only 2% of bone mass.

• Four types: osteoblasts, osteoclasts, osteocytes and osteogenic cells

  • Blast = build

  • Clast = break/dissolve

  • Bone tissue = bone matrix

• All found in compact bone

Osteon

• Functional unit of compact bone.

Spongy Bone

• Lacks osteons (also called trabecular bone).

• Matrix forms an open network of trabeculae.

• Lacks capillaries and venules; receives blood supply and nutrients by diffusion.

Bone Marrow

*Red Bone Marrow
* Fills spaces between trabeculae.
* Forms blood cells (WBC, RBC).
* Contains blood vessels that supply nutrients to osteocytes by diffusion.
* RBC = erythrocytes
* WBC = leukocytes

• Yellow Bone Marrow

  • Found in other sites of spongy bone.

  • Stores fat (adipocytes).

Appendicular Skeleton

• Division of the skeleton system.

Weight-Bearing Bones

• Trabeculae in epiphysis of femur transfer forces from pelvis to compact bone of femoral shaft.

• Femur: Longest bone in the body.

• Medial side of shaft compresses (downward force).

  • Towards the midline (medial), aka longitudinal axis.

• Lateral side experiences tension (resists breaking).

Periosteum

• Membrane that covers outside bones (except within joint cavities).

• Outer fibrous layer and inner cellular layers.

Functions of Periosteum

• Isolates bone from surrounding tissues.

• Participates in bone growth and repair.

  • Repair: Osteoblasts.

  • Bone growth: Appositional growth (bones get wider).

Endosteum

• Incomplete cellular layer that lines medullary cavity (inside).

• Found in both spongy and compact bone.

• Contains osteogenic cells (stem cells).

• Lines the medullary cavity of long bones (found in diaphysis of compact bone).

• Medullary cavity: Inside of bone marrow (yellow/red).

Bone Marrow Location

• Trabecular (spongy bone).

• Medullary cavity (compact bone).

Bone Development

• Ossification (osteogenesis): Bone formation.

  • Osteo = bone, genesis = to make or produce.

  • Connective tissue (cartilage) will eventually become bone.

Types of Ossification

• Endochondral ossification: How most of our bones form.

• Intramembranous ossification: How our flat bones form (skull, face, and clavicle).

• Some human bones grow until about age 25.

  • Height (Stop growing) – x-ray.

  • Epiphyseal line: Indicates the end of height growth.

• Calcification: Deposition of calcium salts, making bones harder and stronger. Occurs during ossification.

Cartilage Growth

• Hyaline cartilage (most common).

• Interstitial growth: Growth in length.

• Appositional growth: Growth in width.

• Endochondral ossification is when most of the bone grows.

Endochondral Ossification Steps

• Formation of cartilage (Stem cells connective tissue, messecynal cells will become chondrocytes).

• Interstitial (height) / appositional (width).

• Primary ossification center.

• Formation of the medullary cavity.

• Secondary ossification center.

• Formation of epiphyseal plate.

• End of puberty → epiphyseal line (stop growing in height).

• Epiphyseal closure: Moment when you stop growing in height.

  • Men stop growing at 21.

  • Women stop growing at 19.

• Epiphyseal plate: Still growing.

• Epiphyseal line: Stopped growing.

• The event when the plane becomes a line is called the epiphyseal closure

• You see this change with X-rays (medical imaging).

• Thickens and strengthens long bones.

• Resists bending and stretching.

Bone Parts

• Epiphysis (spongy bone):

  • Upper part.

• Metaphysis:

  • Middle part.

• Diaphysis (compact bone):

  • Actual middle

Intramembranous Ossification

• Produces dermal bones such as the mandible (lower jaw) and clavicles (collarbones).

  • Dermal → skin (dermis).

Precursors

• Endochondral ossification → cartilage.

• Intramembranous ossification → mesenchymal tissue (only present during fetal development)

• Mesenchymal cells (adults) → connective tissue

Vitamins & Hormones effects on bone

• Cholecalciferol becomes calcitriol, which increases calcium levels.

• Calcitonin: Has the opposite (antagonistic) effect to calcitriol; decreases calcium levels.

• Osteopenia: Losing osteocytes (mature bone cells) due to inadequate ossification (due to aging).

Nutritional and Hormonal Effects on Bone

• Minerals:Calcium and phosphorus are required in the diet.

  • Phosphorus + calcium = hydroxyapatite (gives bones its hardness).

• Magnesium: Required for bone mineral density and helps in converting vitamin D to its active form (D3 → cholecalciferol).

• Iron: Used for blood because of hemoglobin, which transports oxygen and carbon dioxide.

Vitamin D3

• Fat-soluble.

• Only vitamin we can make.

• We can also get it from diet

Fractures

• Open: Pierces the skin.

• Close: Does not pierce the skin.

• Greenstick: Common in children.

Types of bone condition

• Osteopenia (osteo = bone) (penia = condition of decreased activity):

  • Women lose 8% of bone mass due to menopause (loss of estrogen), which accelerates bone mass loss.

  • Men lose 3% of bone mass.

• Osteoporosis (osteo = bone, poro = holes, osis = disease):

  • Severe loss of bone mass.

  • Bones become brittle and prone to fracture.

• Common fracture sites: Hips, spine, wrists.

• In women, osteoporosis accelerates after menopause due to a decrease in estrogen.

• In men, testosterone maintains bone health.