The Skeletal System: Structure, Function, and Disorders

Chapter 5: The Skeletal System

5.1 The Skeletal System: Structure and Components

The skeletal system is a complex framework that provides support, protection, movement, blood cell formation, and mineral storage. It is composed of various connective tissues, each with distinct functions and properties.

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

• Bone: Hard, mineralized tissue primarily made of calcium, but also contains living cells involved in formation and remodeling. Bones bleed when cut, indicating their vascular nature.

• Ligaments: Dense fibrous connective tissues that bind bones together, composed of closely packed collagen fibers.

• Cartilage: Flexible tissue with collagen and elastin, reduces friction in joints. Types include fibrocartilage (withstands pressure), hyaline cartilage (smooth, covers bone ends), and elastic cartilage (flexible).

• Functions of Bones:

  • Support

  • Protection

  • Movement

  • Blood cell formation (hematopoiesis)

  • Mineral storage (calcium and phosphate)

• Bone Structure:

  • Diaphysis: Cylindrical shaft of a long bone.

  • Epiphysis: Enlarged knob at the end of a long bone.

  • Compact Bone: Dense, forms shafts and covers ends.

  • Spongy Bone: Less dense, fills inner sides of epiphyses, contains red bone marrow.

  • Bone Marrow: Red marrow (produces blood cells) in spongy bone; yellow marrow (fat storage) in diaphysis.

  • Periosteum: Outer connective tissue layer with osteoblasts, nerves, blood vessels, and lymphatic vessels.

  • Hyaline Cartilage: Smooth layer on epiphyses, reduces friction.

• Microscopic Structure:

  • Osteons: Cylindrical structures in compact bone, with osteocytes arranged in rings.

  • Osteocytes: Mature bone cells located in lacunae, connected by canaliculi (thin canals with cytoplasmic extensions and gap junctions for nutrient exchange).

  • In spongy bone, osteocytes receive nutrients directly without central canals.

5.2 Bone Development from Cartilage

Bone formation begins with cartilage and involves specialized cells and hormonal regulation.

• Chondroblasts: Cells that form cartilage; die quickly as bone formation begins.

• Osteoblasts: Bone-forming cells carried into developing periosteum by blood vessels.

• Osteoid: Protein matrix secreted by osteoblasts, which mineralizes to form hard bone.

• Enzymes: Osteoblasts secrete enzymes to facilitate crystallization of calcium phosphate.

• Growth Plate: Region between long bones containing chondroblasts (outside) and osteoblasts (inside), responsible for bone growth.

• Hormonal Control: Bone development is regulated by hormones.

5.3 Mature Bones: Remodeling and Repair

Mature bones are dynamic, constantly undergoing remodeling and repair to maintain strength and function.

• Bone Remodeling: Removal (resorption) and formation of bone tissue.

  • Osteoclasts: Cells that secrete enzymes to dissolve bone matrix and calcium phosphate.

  • Osteoblasts: Form new bone tissue.

• Bone Shape Changes: Remodeling can alter bone shape in response to mechanical stress (e.g., exercise, weight).

• Homeostasis: Balance between osteoclast and osteoblast activity is essential. Imbalance can lead to osteoporosis (loss of bone mass).

• Hormonal Regulation:

  • Parathyroid Hormone (PTH): Stimulates osteoclasts when calcium is low.

  • Calcitonin: Stimulates osteoblasts when calcium is high.

• Bone Repair:

  1. Fracture causes blood vessels to form a hematoma (clotted blood).

  2. Fibroblasts become chondroblasts, forming a callus between broken bone ends.

  3. Osteoclasts remove dead bone fragments and hematoma.

  4. Osteoblasts deposit osteoid matrix and encourage calcium phosphate crystallization, turning callus into bone.

5.4 Bones Fit Together: Classification and Skeleton Structure

Bones are classified by shape and organized into the axial and appendicular skeletons, forming the structural framework of the human body.

• Bone Classification:

  • Long Bones: Limbs and fingers.

  • Short Bones: Wrist bones, as wide as they are long.

  • Flat Bones: Ribs, cranial bones, sternum; thin, flat, sometimes curved.

  • Irregular Bones: Vertebrae, hip bones; varied shapes.

• Human Skeleton: 206 bones; provides framework, protection, and movement.

• Axial Skeleton: Forms midline of body; includes skull, sternum, ribs, vertebral column.

• Skull:

  • Cranial Bones: Protect brain (frontal, temporal, sphenoid, ethmoid, occipital).

  • Facial Bones: Maxilla, palatine, vomer, zygomatic, nasal, lacrimal, mandible.

  • Sinuses: Air spaces in skull bones lighten skull and affect voice.

  • Foramen Magnum: Large opening at base for vertebral column connection.

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

• Vertebral Column: Supports head, protects spinal cord, attachment for limbs.

  • 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), 4 coccygeal (fused).

  • Separated by intervertebral disks (fibrocartilage).

• Ribs and Sternum:

  • 12 pairs of ribs: upper 7 attached to sternum by hyaline cartilage; 8-10 attached indirectly; bottom pairs are floating ribs.

• Appendicular Skeleton: Includes limbs and girdles (pectoral and pelvic).

• Pectoral Girdle: Right/left clavicles and scapulas; supports upper limbs.

• Arm and Hand: 30 bones; humerus, ulna, radius, carpals (wrist), metacarpals (palm), phalanges (fingers).

• Pelvic Girdle: Coxal bones, sacrum, coccyx; attaches to vertebral column and forms pubic symphysis.

• Legs: Femur (thigh bone), tibia, fibula, patella (kneecap), tarsals (ankle/heel), metatarsals (foot), phalanges (toes).

Table: Classification of Bones

Type	Examples	Characteristics
Long Bones	Femur, humerus, fingers	Longer than wide, shaft and ends
Short Bones	Wrist (carpals)	As wide as long
Flat Bones	Ribs, cranial, sternum	Thin, flat, sometimes curved
Irregular Bones	Vertebrae, hip bone	Varied shapes

5.5 Joint Connections: Types and Functions

Joints, ligaments, and tendons connect bones and allow for varying degrees of movement and stability.

• Joints: Points of contact between bones; classified by mobility.

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

• Cartilaginous Joints: Slightly movable; connected by hyaline cartilage (e.g., vertebrae, lower ribs).

• Synovial Joints: Freely movable; bones separated by fluid-filled cavity lined with synovial membrane.

  • Synovial fluid lubricates joint.

  • Articulating surfaces covered with hyaline cartilage.

  • Joint capsule encloses joint.

  • Types: Hinge joint (knee, elbow), Ball and socket joint (shoulder, hip).

  • Knee joint has menisci (cartilage disks) and bursae (fluid sacs).

• Ligaments: Strengthen and stabilize joints; composed of parallel collagen fibers.

• Tendons: Connect muscle to bone; also stabilize joints.

Table: Types of Joints

Type	Mobility	Examples
Fibrous	Immovable	Skull sutures, fontanels
Cartilaginous	Slightly movable	Vertebrae, rib-sternum joints
Synovial	Freely movable	Knee, elbow, shoulder, hip

5.6 Disorders of the Skeletal System

The skeletal system is susceptible to various disorders affecting bones, joints, ligaments, and tendons.

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

• Bursitis and Tendinitis: Inflammation of bursae or tendons; healing is difficult.

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

• Osteoporosis: Loss of bone mass due to imbalance in bone remodeling.

Additional info:

• Bone remodeling is essential for adapting to mechanical stress and maintaining calcium homeostasis.

• Hormonal regulation ensures proper bone growth and repair throughout life.

• Synovial joints are the most common and allow for a wide range of movements.