Skeletal_System

Overview of the Skeletal System

• 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

Bone Shape and Structure

• Bones differ in:

  • Size

  • Shape

Similar Features

• 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.

Bone Classification by Shape

1. Long Bones

   • Characteristics: Long, narrow shape with expanded ends

   • Example: Femur

2. Short Bones

   • Characteristics: Cube-like, equal length and width

   • Example: Carpals

   • Special type: Sesamoid bones (e.g., patella) embedded in tendons

3. Flat Bones

   • Characteristics: Plate-like with broad surfaces

   • Example: Scapula

4. Irregular Bones

   • Characteristics: Varied shape, complex structure

   • Example: Vertebrae

Structure of a Long Bone

• 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)

Microscopic Structure of Bone

• 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

Types of Bone: Compact vs. Spongy 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

Bone Functions

• 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)

Hematopoiesis and Bone Marrow

• 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

Inorganic Salt Storage

• 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

Hormonal Control of Blood Calcium Levels

• 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 and Repair

• 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 vs. Endochondral Ossification

• Intramembranous:

  1. Begins with connective tissue

  2. Mesenchymal cells differentiate into osteoblasts

• Endochondral:

  1. Initial cartilage model forms

  2. Chondrocytes enlarge, and the matrix breaks down

  3. Osteoblasts invade to form bone

Steps of Fracture Repair

• Stages of healing:

  1. Hematoma formation (blood clot)

  2. Soft callus (fibrocartilage formation)

  3. Hard callus (bone replacement)

  4. Bone remodeling (restoration to original shape)

Clinical Applications: Bone Health

• Osteoporosis and fragility fractures:

  • Common in older adults, especially postmenopausal women

  • Prevention through exercise, calcium, and vitamin D intake

Overall Skeletal Organization

• 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

Differences Between Male and Female Pelvis

• Female pelvis: wider for childbirth, with larger pelvic inlet and pubic arch angle

• Male pelvis: narrower structure, typically heavier and less flared.