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