The Skeletal System: Structure, Function, and Classification

The Skeletal System

Introduction

The skeletal system forms the structural framework of the human body, providing support, protection, and enabling movement. It is composed of bones, cartilage, ligaments, and joints, and is essential for various physiological functions including mineral storage and blood cell production.

Functions of Bone

Main Functions

• Support: Bones provide a rigid framework that supports the body and maintains its shape.

• Protection: Bones protect vital organs such as the brain (skull), heart and lungs (rib cage), and spinal cord (vertebral column).

• Movement: Bones serve as attachment points for muscles. When muscles contract, they pull on bones, producing movement at joints.

• Mineral Storage and Homeostasis: Bones store minerals, primarily calcium and phosphorus, which can be released into the bloodstream to maintain mineral balance (homeostasis).

• Blood Cell Production (Hematopoiesis): Red bone marrow within certain bones produces red blood cells, white blood cells, and platelets.

• Triglyceride Storage: Yellow bone marrow stores fat, serving as an energy reserve.

Classification of Bones

By Shape

• Long Bones: Longer than they are wide (e.g., femur, humerus). Characterized by a shaft (diaphysis) and two ends (epiphyses).

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and often curved (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, some facial bones).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

By Density

• Compact Bone: Dense and solid, forms the outer layer of all bones and the bulk of long bones. Provides strength for weight bearing.

• Spongy (Cancellous) Bone: Less dense, contains many spaces. Found mainly at the ends of long bones and inside flat bones. Contains red bone marrow.

Structure of a Typical Long Bone

Main Features

• Diaphysis: The shaft or central part of a long bone.

• Epiphyses: The proximal and distal ends of the bone.

• Metaphysis: The region between the diaphysis and epiphyses; in growing bones, contains the epiphyseal (growth) plate.

• Articular Cartilage: Thin layer of hyaline cartilage covering the epiphyses where bones form joints. Reduces friction and absorbs shock.

• Periosteum: Tough, dense irregular connective tissue covering the bone except at joint surfaces. Contains bone-forming cells (osteoblasts), aids in bone growth, repair, and serves as an attachment point for tendons and ligaments.

• Medullary Cavity: Hollow, cylindrical space within the diaphysis. Contains yellow bone marrow in adults (fat storage) and red bone marrow in newborns (blood cell production).

• Endosteum: Thin membrane lining the medullary cavity, containing bone-forming cells.

Bone Cells

Types and Functions

• Osteoprogenitor Cells: Stem cells found in the periosteum and endosteum. Only bone cells that divide; differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete the bone matrix (collagen and other proteins). Once trapped in their own matrix, they become osteocytes.

• Osteocytes: Mature bone cells that maintain bone tissue. Reside in small spaces called lacunae.

• Osteoclasts: Large cells formed from the fusion of white blood cells. Responsible for bone resorption (breaking down bone matrix), important for bone remodeling and calcium release.

Histology of Bone Tissue

Compact Bone

• Osteons (Haversian Systems): Structural units of compact bone, consisting of concentric rings (lamellae) around a central canal (Haversian canal) containing blood vessels and nerves.

• Lacunae: Small spaces between lamellae housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing for nutrient and waste exchange between osteocytes.

• Interstitial and Circumferential Lamellae: Fill spaces between osteons and encircle the bone, respectively.

Spongy Bone

• Trabeculae: Lattice-like network of thin columns of bone. Spaces between trabeculae are filled with red bone marrow.

• Osteocytes: Located within lacunae in the trabeculae, connected by canaliculi.

• Location: Found mainly in the epiphyses of long bones, inside flat bones (e.g., hip, skull), and vertebrae.

Organization of the Skeleton

Axial Skeleton

• Definition: Bones along the central axis of the body.

• Main Components:

  • Skull (cranial and facial bones)

  • Vertebral column (cervical, thoracic, lumbar vertebrae, sacrum, coccyx)

  • Thoracic cage (ribs and sternum)

Appendicular Skeleton

• Definition: Bones of the limbs and girdles that attach them to the axial skeleton.

• Main Components:

  • Pectoral (shoulder) girdle (clavicle, scapula)

  • Upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges)

  • Pelvic girdle (hip bones)

  • Lower limbs (femur, tibia, fibula, tarsals, metatarsals, phalanges)

Joints and Associated Structures

Classification of Joints

• Fibrous Joints: Bones joined by fibrous tissue; little to no movement (e.g., sutures of the skull).

• Cartilaginous Joints: Bones joined by cartilage; allow more movement than fibrous joints but less than synovial joints (e.g., intervertebral discs).

• Synovial Joints: Most movable type; bones separated by a joint cavity filled with synovial fluid, surrounded by a joint capsule (e.g., knee, shoulder).

Structure of a Synovial Joint

• Articular Cartilage: Covers the ends of bones at the joint, reducing friction and absorbing shock.

• Joint Capsule: Encloses the joint cavity; consists of an outer fibrous layer and an inner synovial membrane.

• Synovial Fluid: Lubricates the joint, nourishes cartilage, and reduces friction.

• Ligaments: Dense connective tissue that stabilizes joints by connecting bone to bone.

• Menisci: Pads of fibrocartilage that improve fit between bones, absorb shock, and distribute load (e.g., knee joint).

Summary Table: Types of Bone Cells

Cell Type	Location	Function
Osteoprogenitor	Periosteum, endosteum	Stem cells; differentiate into osteoblasts
Osteoblast	Bone surface	Form new bone matrix
Osteocyte	Lacunae within bone	Maintain bone tissue
Osteoclast	Bone surface	Resorb (break down) bone matrix

Key Equations

• Calcium Homeostasis: The balance of calcium between bone and blood is regulated by hormones such as parathyroid hormone (PTH) and calcitonin.

• Bone Remodeling: $\text{Bone Mass} = \text{Bone Formation} - \text{Bone Resorption}$

Additional Info

• Osteology is the scientific study of bones, including their structure, function, and diseases.

• Bone health is influenced by nutrition (especially calcium and vitamin D), physical activity, and hormonal balance.