Classification of Bones

Overview

The human body consists of 206 named bones, each considered an organ. Bones are classified by their location and shape, which helps in understanding their function and structure.

• Axial Skeleton: Bones along the longitudinal axis, such as the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Bones attached to the axial skeleton, including the limbs and girdles.

Classification by Bone Shape

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

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

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Functions of the Skeletal System

• Support: Framework that supports the body and cradles organs.

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

• Movement: Skeletal muscles attach to bones by tendons and use them as levers.

• Mineral Storage: Reservoir for calcium and phosphate.

• Hormone Production: Osteocalcin regulates bone formation and energy metabolism.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Yellow bone marrow stores fat.

Bone Cells and Bone Matrix

Principal Types of Bone Cells

• Osteogenic Cells: Stem cells that differentiate into osteoblasts. Found in periosteum and endosteum.

• Osteoblasts: Bone-building cells that secrete bone matrix. Once surrounded by matrix, they become osteocytes.

• Osteocytes: Mature bone cells in lacunae. Maintain bone matrix and act as stress sensors.

• Osteoclasts: Large, multinucleated cells that resorb (break down) bone. Release lysosomal enzymes and acids to digest bone matrix.

Extracellular Bone Matrix

• Organic Components: Called osteoid, includes collagen fibers and ground substance. Provides tensile strength and flexibility.

• Inorganic Components: Called hydroxyapatite, mainly calcium phosphate. Accounts for about 65% of bone matrix, providing hardness and compression resistance.

Types of Bone Tissue

Spongy Bone

Composes the center of flat bones and epiphyses of long bones.

• Consists of trabeculae (honeycomb-like pieces).

• Trabeculae are oriented along stress lines to resist stress and transfer force.

• Spaces between trabeculae are filled with marrow.

Compact Bone

Composes external layers of all bones and bulk of diaphysis in long bones.

• Arranged in units called osteons (Haversian systems).

• Osteons are composed of concentric lamellae arranged in elongated cylinders parallel to the bone's long axis.

Microscopic Structure of Compact Bone

Lamellae

• Concentric Lamellae: Surround central (Haversian) canal.

• Interstitial Lamellae: Fill spaces between osteons.

• Circumferential Lamellae: Extend around the entire circumference of the diaphysis.

Canals

• Central (Haversian) Canal: Runs through the core of each osteon; contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Connect blood vessels and nerves of periosteum to those in central canals.

• Canaliculi: Tiny canals that connect lacunae to each other and to the central canal, allowing osteocytes to communicate.

Gross Anatomy of Long Bone

Long Bone Regions

• Diaphysis: Tubular shaft composed of compact bone surrounding the medullary cavity, which contains yellow marrow in adults.

• Epiphyses: Ends of long bones, composed of compact bone externally and spongy bone internally, covered by articular cartilage.

• Epiphyseal Line: Remnant of the epiphyseal plate, the area between diaphysis and epiphysis.

Long Bone Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except joint surfaces. Outer layer is dense irregular connective tissue; inner layer contains osteogenic cells.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces.

Bone Formation (Ossification)

Overview

Bone formation occurs in embryos (about 8 weeks) from mesenchymal cells via two processes: intramembranous and endochondral ossification.

Intramembranous Ossification

• Formation of bone directly from fibrous membrane.

• Produces cranial bones and clavicles.

• Steps:

  1. Development of ossification center: Mesenchymal cells cluster and differentiate into osteoblasts.

  2. Formation and calcification of bone matrix: Osteoblasts secrete osteoid, which calcifies.

  3. Formation of trabeculae and periosteum: Osteoid accumulates, forming trabeculae; periosteum develops from mesenchyme.

  4. Formation of lamellar bone: Trabeculae thicken, forming compact bone; red marrow appears.

Endochondral Ossification

• Formation of bone from hyaline cartilage.

• Produces most bones of the body.

• Steps:

  1. Development of cartilage model and bone collar: Mesenchymal cells differentiate into chondroblasts, forming cartilage model; perichondrium becomes periosteum.

  2. Cavitation of cartilage model: Chondrocytes enlarge and die, matrix calcifies, cavities form.

  3. Invasion of internal cavities by periosteal bud: Brings blood vessels, nerves, osteoblasts, and osteoclasts.

  4. Diaphysis elongation and formation of medullary cavity: Osteoclasts break down spongy bone to form cavity.

  5. Development of secondary ossification centers in epiphyses: Occurs after birth; epiphyses ossify, leaving articular cartilage and epiphyseal plate.

Postnatal Bone Growth and Bone Repair

Growth in Length

Occurs at the epiphyseal plate, which consists of five zones:

1. Resting Zone: Inactive cartilage cells near epiphysis.

2. Proliferation Zone: Rapidly dividing chondrocytes, pushing epiphysis away from diaphysis.

3. Hypertrophic Zone: Older chondrocytes enlarge, matrix begins to deteriorate.

4. Calcification Zone: Matrix becomes calcified, chondrocytes die.

5. Ossification Zone: New bone forms as osteoblasts deposit bone matrix.

Epiphyseal plate closure occurs when epiphysis and diaphysis fuse, ending longitudinal growth.

Bone Remodeling

Overview

Bone remodeling is the ongoing replacement of old bone tissue by new bone tissue. Osteoclasts resorb old bone, and osteoblasts form new bone, resulting in complete replacement of the compact bone every 10 years.

• Remodeling: Bone is remodeled based on use, diet, and force applied to bones.

Factors Affecting Bone Growth and Repair

• Dietary Nutrients: Calcium, phosphorus, magnesium, vitamin C (collagen synthesis), vitamin A (osteoblast activity), vitamin D (calcium absorption).

• Hormones: Growth hormone (stimulates growth), thyroid hormone (modulates growth), sex hormones (promote growth spurt and epiphyseal plate closure), leptin (inhibits osteoblast activity).

Homeostatic Imbalances

• Osteomalacia: Adult disorder with poorly mineralized bone, resulting in soft, weak bones.

• Rickets: Osteomalacia in children, caused by vitamin D deficiency or insufficient dietary calcium.

• Osteoporosis: Group of disorders with excessive bone resorption, leading to porous and fragile bones. Common in older women due to estrogen deficiency.

Calcium Homeostasis

Bone is the major reservoir for calcium ions in the body. Blood calcium levels are tightly regulated due to their importance in muscle contraction, nerve function, blood clotting, and other processes. About 99% of the body's calcium is found in bone matrix, with 1% in blood. Hormonal regulation keeps blood calcium levels constant.