The skeletal system consists of bones and various tissues:
Bone tissue
Cartilage
Dense connective tissues (tendons, ligaments)
Blood vessels and nerves
Multifunctional roles:
Support and protect softer tissues
Enable movement
Facilitate blood cell formation
Store minerals
Unique adaptations:
Human skeleton adapted for bipedalism (walking on two legs)
Grasping ability in hands
Divisions of the skeletal system:
Axial skeleton
Appendicular skeleton
Bones differ in:
Size
Shape
Bones share several structural, developmental, and functional characteristics:
Structure includes outer compact bone and inner spongy bone
Development processes such as ossification and remodeling
Functional roles include supporting weight and facilitating movement.
Long Bones
Characteristics: Long, narrow shape with expanded ends
Example: Femur
Short Bones
Characteristics: Cube-like, equal length and width
Example: Carpals
Special type: Sesamoid bones (e.g., patella) embedded in tendons
Flat Bones
Characteristics: Plate-like with broad surfaces
Example: Scapula
Irregular Bones
Characteristics: Varied shape, complex structure
Example: Vertebrae
Key components of a long bone:
Epiphysis: Expanded end of the bone
Diaphysis: Long shaft of the bone
Metaphysis: Region between diaphysis and epiphysis
Articular Cartilage: Covers the epiphysis
Periosteum: Dense connective tissue surrounding the bone
Medullary Cavity: Hollow chamber within the diaphysis containing marrow
Endosteum: Lines the internal spaces of the bone
Bone Marrow: Found in medullary cavity (red or yellow)
Osteocytes: Mature bone cells located in lacunae
Canaliculi: Tiny passageways connecting osteocytes for nutrient and waste exchange
The extracellular matrix consists of:
Collagen fibers providing resilience
Inorganic salts giving hardness to the bone
Compact Bone:
Structure: Composed of cylindrical units called osteons
Characteristics: Solid, strong, and weight-bearing
Spongy Bone:
Structure: Made up of trabeculae (branching plates)
Characteristics: Lighter, somewhat flexible, with spaces reducing weight
Major functions of bones include:
Providing shape and support to the body
Protecting vital organs (e.g., skull protects the brain)
Aiding in body movement (joint functionality)
Blood cell production (hematopoiesis in red bone marrow)
Storing minerals (inorganic salts)
Blood cell formation occurs in red bone marrow:
Produces red blood cells, white blood cells, and platelets
Age Variation: Red bone marrow is gradually replaced by yellow bone marrow (fat storage)
Adult red marrow is located in specific bones: skull, ribs, vertebrae, and pelvis
Around 70% of bone matrix is made of inorganic mineral salts, primarily hydroxyapatite (calcium phosphate)
Other minerals include magnesium, sodium, potassium, carbonate ions
Osteoporosis: Loss of bone density due to mineralization loss
Parathyroid glands regulate blood calcium levels by releasing parathyroid hormone (PTH) or calcitonin based on the levels.
Hormonal response involves:
Osteoblasts depositing calcium when levels are high (calcitonin)
Osteoclasts breaking down bone to release calcium when levels are low (PTH)
Bone development begins during early fetal development and proceeds through adulthood.
Two types of ossification:
Intramembranous Ossification: Flat bones form from connective tissue (e.g., skull bones)
Endochondral Ossification: Bones replace hyaline cartilage (e.g., long bones)
Intramembranous:
Begins with connective tissue
Mesenchymal cells differentiate into osteoblasts
Endochondral:
Initial cartilage model forms
Chondrocytes enlarge, and the matrix breaks down
Osteoblasts invade to form bone
Stages of healing:
Hematoma formation (blood clot)
Soft callus (fibrocartilage formation)
Hard callus (bone replacement)
Bone remodeling (restoration to original shape)
Osteoporosis and fragility fractures:
Common in older adults, especially postmenopausal women
Prevention through exercise, calcium, and vitamin D intake
Number of Bones: Approximately 206 in adult skeleton
Bone types in the skeleton include:
Axial Skeleton
Skull: 22 bones
Vertebral column: 26 bones
Thoracic cage: 25 bones
Appendicular Skeleton
Upper limbs: 60 bones
Lower limbs: 60 bones
Pectoral girdle: 4 bones
Pelvic girdle: 2 bones
Female pelvis: wider for childbirth, with larger pelvic inlet and pubic arch angle
Male pelvis: narrower structure, typically heavier and less flared.