The Skeletal System: Structure, Function, and Disorders

Chapter 5: The Skeletal System

5.1 Overview of the Skeletal System

The skeletal system is a complex framework of bones and connective tissues that provides support, protection, movement, and other vital functions in the human body.

• Connective Tissues: The skeletal system contains three main types of connective tissue: bone, ligament, and cartilage.

• Bone: A hard, mineralized tissue primarily composed of calcium salts, but also containing living cells involved in bone formation and remodeling.

• Ligaments: Dense fibrous connective tissues that bind bones together, providing joint stability.

• Cartilage: Composed of collagen and elastin fibers in a ground substance; reduces friction at joints and provides flexible support.

Functions of Bones:

• Support for the body

• Protection of internal organs (e.g., skull protects the brain)

• Facilitation of movement (as levers for muscles)

• Blood cell formation (hematopoiesis in bone marrow)

• Mineral storage (mainly calcium and phosphate)

Bone Structure:

• Long bones have a shaft (diaphysis) and enlarged ends (epiphyses).

• Compact bone forms the shaft and outer layer; spongy bone fills the inner regions of epiphyses.

• Red bone marrow (in spongy bone) produces blood cells; yellow bone marrow (in diaphysis) stores fat.

• Periosteum: A connective tissue membrane covering the bone, containing osteoblasts (bone-forming cells), nerves, blood vessels, and lymphatics.

• Hyaline cartilage covers the ends of long bones, reducing friction and absorbing shock.

• Osteocytes (mature bone cells) are arranged in rings (osteons) and communicate via canaliculi (tiny canals).

Ligaments and Cartilage:

• Ligaments: Composed of parallel collagen fibers, allowing movement while strengthening joints.

• Fibrocartilage: Thick bundles of collagen, withstands pressure (e.g., intervertebral disks, menisci in knees).

• Hyaline cartilage: Smooth, forms embryonic structures, covers ends of mature bones.

• Elastic cartilage: Contains elastin fibers, provides flexibility (e.g., ear).

5.2 Bone Development (Ossification)

Bones develop from cartilage through a process called ossification, involving specialized cells and regulated by hormones.

• Chondroblasts: Cells that form cartilage templates during early development.

• Osteoblasts: Bone-forming cells that secrete osteoid (protein matrix) and enzymes for mineralization.

• Osteocytes: Mature bone cells derived from osteoblasts, maintain bone tissue.

• Growth Plate (Epiphyseal Plate): Region between diaphysis and epiphysis where bone lengthening occurs; contains chondroblasts (cartilage formation) on the outside and osteoblasts (bone formation) on the inside.

• Hormonal Regulation: Bone development and growth are controlled by hormones such as growth hormone, sex hormones, and thyroid hormone.

Key Steps in Bone Formation:

1. Chondroblasts produce cartilage model.

2. Periosteum forms around the cartilage.

3. Osteoblasts invade, secrete osteoid, and facilitate mineralization with calcium phosphate.

4. Osteocytes maintain the bone matrix.

5.3 Mature Bone: Remodeling and Repair

Bone is a dynamic tissue that undergoes continuous remodeling and can repair itself after injury.

• Bone Remodeling: The process of resorption (removal of old bone by osteoclasts) and formation of new bone by osteoblasts.

• Osteoclasts: Cells that secrete enzymes to dissolve bone matrix and release minerals.

• Remodeling Stimuli: Physical activity (e.g., jogging) and mechanical stress stimulate bone formation; inactivity leads to bone loss.

• Homeostasis: Balance between osteoclast and osteoblast activity maintains bone mass.

• Osteoporosis: A condition where bone mass is lost due to imbalance, increasing fracture risk.

Hormonal Regulation of Bone:

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts to resorb bone.

• Calcitonin: Lowers blood calcium by stimulating osteoblasts to deposit bone.

Bone Repair Process:

1. Formation of a hematoma (clotted blood) at the fracture site.

2. Fibroblasts and chondroblasts form a callus (cartilage bridge).

3. Osteoclasts remove dead bone fragments and hematoma.

4. Osteoblasts deposit new bone matrix, converting the callus to bone.

5.4 Organization of the Human Skeleton

The human skeleton is divided into the axial and appendicular skeletons, each with specific bones and functions.

• Bone Classification by Shape:

  • Long bones: Limbs, fingers

  • Short bones: Wrist (carpals)

  • Flat bones: Ribs, cranial bones, sternum

  • Irregular bones: Vertebrae, hip bones

• Total Bones: 206 in the adult human skeleton

Axial Skeleton

• Skull: Protects the brain; includes cranial and facial bones (e.g., frontal, temporal, sphenoid, ethmoid, maxilla, palatine, vomer, zygomatic, nasal, lacrimal, mandible, occipital).

• Hyoid Bone: Not directly attached to other bones; supports tongue, larynx, and pharynx.

• Vertebral Column: 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, 4 fused coccygeal); separated by intervertebral disks.

• Ribs and Sternum: 12 pairs of ribs (7 true, 3 false, 2 floating); sternum (breastbone).

Appendicular Skeleton

• Pectoral Girdle: Clavicles and scapulae; supports upper limbs.

• Arm and Hand: Humerus (upper arm), radius and ulna (forearm), carpals (wrist), metacarpals (palm), phalanges (fingers).

• Pelvic Girdle: Coxal bones, sacrum, coccyx; supports lower limbs and protects pelvic organs.

• Legs: Femur (thigh), tibia and fibula (lower leg), patella (kneecap), tarsals (ankle), metatarsals (foot), phalanges (toes).

5.5 Joints, Ligaments, and Tendons

Joints are points of contact between bones, stabilized by ligaments and tendons, and allow for varying degrees of movement.

• Fibrous Joints: Immovable; e.g., sutures in the skull, fontanels in infants.

• Cartilaginous Joints: Slightly movable; connected by hyaline cartilage (e.g., intervertebral disks, rib-sternum connections).

• Synovial Joints: Freely movable; bones separated by a fluid-filled cavity lined with synovial membrane that secretes synovial fluid for lubrication.

• Hyaline Cartilage: Covers articulating surfaces to reduce friction.

• Joint Capsule: Encloses synovial joints, reinforced by ligaments.

• Types of Synovial Joints:

  • Hinge joints: Movement in one plane (e.g., knee, elbow); knee contains menisci (cartilage disks) and bursae (fluid sacs).

  • Ball-and-socket joints: Allow movement in multiple directions (e.g., shoulder, hip).

• Ligaments: Connect bone to bone; provide joint stability.

• Tendons: Connect muscle to bone; stabilize joints during movement.

5.6 Disorders of the Skeletal System

Various disorders can affect bones, joints, and connective tissues, impacting movement and health.

• Sprains: Damage to ligaments; minor sprains may heal, but complete tears often do not heal naturally.

• Bursitis and Tendinitis: Inflammation of bursae or tendons; healing is often slow and incomplete.

• Arthritis: Degeneration of cartilage covering bone ends, leading to pain and reduced mobility.

Summary Table: Types of Connective Tissue in the Skeletal System

Type	Main Components	Function	Location/Example
Bone	Calcium salts, collagen, living cells	Support, protection, movement, mineral storage, blood cell formation	Skeleton (e.g., femur, skull)
Ligament	Dense collagen fibers	Connects bones, stabilizes joints	Knee, elbow, shoulder
Cartilage	Collagen, elastin, ground substance	Reduces friction, absorbs shock, supports structures	Joints, ear, nose, intervertebral disks

Key Equations

• Bone Mineralization: (No specific equation, but the process involves deposition of calcium phosphate in the osteoid matrix.)

• Calcium Homeostasis:

$\text{PTH} \uparrow \Rightarrow \text{Osteoclast Activity} \uparrow \Rightarrow \text{Blood Calcium} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Osteoblast Activity} \uparrow \Rightarrow \text{Blood Calcium} \downarrow$

Example: Bone Repair After Fracture

• A person fractures their femur. Blood vessels at the site rupture, forming a hematoma.

• Fibroblasts and chondroblasts form a soft callus of cartilage between bone ends.

• Osteoclasts remove dead bone; osteoblasts deposit new bone, converting the callus to hard bone.

• Remodeling restores the bone's original shape and strength.

Additional info: Bone health is influenced by nutrition (calcium, vitamin D), physical activity, and hormonal balance. Disorders such as osteoporosis are more common in postmenopausal women due to decreased estrogen levels.