Chapter 5 The Skeletal System: Structure, Function, and Disorders

The Skeletal System

Overview of the Skeletal System

The skeletal system is a fundamental component of human biology, providing structure, protection, and facilitating movement. It consists of various connective tissues that work together to support the body and enable its functions.

• Bone: Hard elements forming the skeleton.

• Ligaments: Dense fibrous connective tissue attaching bones to other bones.

• Cartilage: Specialized connective tissue with collagen and elastin fibers in a gel-like ground substance; reduces friction in joints and cushions vertebrae.

Functions of Bone

Bones serve multiple essential functions in the human body:

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Movement: Acts as levers for muscles.

• Blood Cell Formation: Occurs in red bone marrow.

• Mineral Storage: Stores calcium and phosphate.

Anatomy of Bones

Bones are living tissues composed of several cell types and structures:

• Compact Bone: Forms the shaft (diaphysis) and ends (epiphyses) of long bones; contains marrow space with yellow bone marrow (fat).

• Spongy Bone: Lattice-like support in ends of long bones; spaces may contain red bone marrow (site of blood cell production).

• Periosteum: Connective tissue covering the bone.

• Osteoblasts: Bone-forming cells; generate new bone during growth and repair.

• Osteocytes: Mature bone cells arranged in rings called osteons (Haversian systems).

• Osteons/Haversian System: Cellular arrangement in compact bone; central canal contains blood vessels.

• Lacunae: Spaces housing osteocytes, connected by canaliculi.

Ligaments and Cartilage

• Ligaments: Attach bone to bone, providing strength and enabling movement in joints.

• Cartilage: Provides support under pressure; types include fibrocartilage (intervertebral disks, menisci), hyaline (embryonic structure, joint surfaces), and elastic cartilage (outer ear, nose, epiglottis).

Bone Development and Growth

Bone formation begins in the embryo and continues through childhood and adolescence:

• Early Fetal Development: Cartilage models form via chondroblasts.

• Later Fetal Development: Osteoblasts replace cartilage with bone, secreting osteoid and enzymes to crystalize calcium phosphate.

• Growth Plate (Epiphyseal Plate): Site of bone lengthening; chondroblasts produce cartilage, osteoblasts replace it with bone.

• Bone Diameter Growth: Osteoblasts deposit bone beneath periosteum.

Bone Remodeling and Repair

Mature bone undergoes continuous remodeling and repair, influenced by diet, exercise, age, and hormones:

• Remodeling: Changes in shape, size, and strength; weight-bearing exercise increases bone mass.

• Osteoclasts: Cells that remove bone (resorption).

• Osteoblasts: Deposit new bone.

• Bone Homeostasis: Balance between osteoblast and osteoclast activity.

• Osteoporosis: Loss of bone mass due to imbalance.

Hormonal Regulation of Bone Cells

• Parathyroid Hormone (PTH): Increases osteoclast activity when blood calcium is low.

• Calcitonin: Increases osteoblast activity when blood calcium is high.

Bone Repair Process

• Fracture: Hematoma forms, followed by inflammation.

• Fibroblasts: Migrate to area, some become chondroblasts.

• Fibrocartilage Callus: Formed at fracture site.

• Osteoclasts: Remove dead bone and hematoma remnants.

• Osteoblasts: Deposit new bone; repair takes weeks to months.

Classification and Organization of Bones

• Skeleton: 206 bones and connective tissues.

• Bone Types: Long (limbs, fingers), short (wrist), flat (cranial, sternum, ribs), irregular (coxal, vertebrae).

Axial and Appendicular Skeleton

• Axial Skeleton: Skull, sternum, ribs, vertebral column.

• Appendicular Skeleton: Pectoral girdle, arms, pelvic girdle, legs.

Skull and Facial Bones

• Cranial Bones: Frontal, parietal, temporal, sphenoid, ethmoid, occipital.

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

• Sinuses: Air spaces within bones.

Hyoid Bone

• Located in upper neck, under chin; does not articulate with other bones.

• Attachment for muscles of tongue, larynx, pharynx.

Vertebral Column

• Regions: Cervical (7), thoracic (12), lumbar (5), sacral (5 fused), coccygeal (4 fused).

• Intervertebral Disks: Cushion vertebrae, assist movement and flexibility.

Ribs and Sternum

• Ribs: 12 pairs; protect lungs and heart.

• Sternum: Protects heart; breastbone.

Appendicular Skeleton: Upper and Lower Limbs

• Pectoral Girdle: Clavicles, scapulas.

• Arms: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Coxal bones (ilium, ischium, pubis), sacrum.

• Legs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Comparison of Male and Female Pelvic Girdle

• Female pelvis is broader, shallower, and has a wider opening, adapted for pregnancy and birth.

Joints (Articulations)

• Fibrous Joints: Immovable; e.g., skull sutures.

• Cartilaginous Joints: Slightly movable; e.g., intervertebral disks.

• Synovial Joints: Freely movable; bones separated by fluid-filled cavity.

Synovial Joints and Types of Movement

• Hinge Joint: Movement in one plane; e.g., knee.

• Ball and Socket Joint: Wide range of movement; e.g., hip, shoulder.

• Menisci: Disks of cartilage to reduce friction.

• Bursae: Small sacs of fluid for cushioning.

• Ligaments: Attach bone to bone; e.g., ACL.

• Tendons: Attach muscle to bone; e.g., Achilles tendon.

Diseases and Disorders of the Skeletal System

• Sprains: Over-stretched or torn ligaments; slow healing.

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

• Osteoporosis: Excessive bone loss; increased fracture risk.

• Arthritis: Inflammation of joints; includes osteoarthritis (wear-and-tear) and rheumatoid arthritis (autoimmune).

Osteoporosis Prevention and Treatment

• Prevention: Adequate calcium and vitamin D, consistent exercise, weight-bearing activities.

• Treatment: Bisphosphonates inhibit osteoclasts; drugs like Forteo® stimulate osteoblasts.

Arthritis

• Osteoarthritis: Cartilage wears out, increased friction, pain, loss of mobility.

• Rheumatoid Arthritis: Autoimmune inflammation of joints.

Table: Cells Involved in Bone Development and Maintenance

Cell Type	Function
Osteoblast	Bone formation
Osteocyte	Maintains bone matrix
Osteoclast	Bone resorption
Chondroblast	Cartilage formation
Fibroblast	Connective tissue formation and repair

Table: Types of Joints

Joint Type	Degree of Movement	Example
Fibrous	Immovable	Skull sutures
Cartilaginous	Slightly movable	Intervertebral disks
Synovial	Freely movable	Knee, shoulder, hip

Key Equations

• Bone Remodeling Balance:

$\text{Bone Mass Change} = \text{Osteoblast Activity} - \text{Osteoclast Activity}$

• Calcium Homeostasis:

$\text{Blood Calcium Level} \xrightarrow{\text{PTH}} \text{Increase Osteoclast Activity}$

$\text{Blood Calcium Level} \xrightarrow{\text{Calcitonin}} \text{Increase Osteoblast Activity}$

Example: Osteoporosis Risk Factors

• Post-menopausal women lose bone mass faster than men.

• Risk factors include smoking, sedentary lifestyle, inadequate calcium, being underweight.

Example: Joint Injuries

• ACL tears in the knee often require surgical repair.

• Carpal tunnel syndrome results from repetitive stress in the wrist.