Bones and Bone Structure

Introduction to the Structure and Growth of Bones

The skeletal system is a fundamental component of human anatomy, providing structure, protection, and support for movement. This section explores the organization, classification, and cellular composition of bones, as well as their growth and physiological roles.

Main Divisions of the Skeleton

Axial and Appendicular Skeleton

• Axial Skeleton (80 bones): Includes the bones of the skull, thorax, and vertebral column. It forms the longitudinal axis of the body and provides protection for vital organs such as the brain, heart, and lungs.

• Appendicular Skeleton (126 bones): Comprises the bones of the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton. It facilitates movement and interaction with the environment.

Functions of the Skeletal System:

• Provides a rigid framework that supports the body and anchors muscles.

• Acts as a reservoir for minerals (calcium, phosphate) and fats (yellow marrow).

• Red bone marrow produces blood cells (erythrocytes, leukocytes, platelets).

• Shields vital organs (e.g., skull protects brain, ribs protect heart and lungs).

• Acts as levers for muscles, enabling movement.

Classification of Bones by Shape

Bone Categories

• Flat Bones: Thin, parallel surfaces. Examples: cranial bones, sternum, ribs, scapulae. Function: Protect underlying soft tissues and provide surface area for muscle attachment.

• Sutural (Wormian) Bones: Small, irregular bones found between cranial bones. Number and shape vary among individuals.

• Long Bones: Relatively long and slender. Examples: humerus, femur, radius, ulna, tibia, fibula. Function: Support weight and facilitate movement.

• Irregular Bones: Complex shapes with short, flat, notched, or ridged surfaces. Examples: vertebrae, pelvic bones, facial bones.

• Sesamoid Bones: Small, flat, and shaped like a sesame seed. Develop inside tendons. Example: patella. Function: Protect tendons from stress and wear.

• Short Bones: Small and boxy. Examples: carpals (wrist), tarsals (ankle).

Bone Markings

Bones display various markings that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels.

• Depressions and Openings: Foramen (small hole for passage of nerves/vessels), fossa (shallow depression), groove (narrow channel).

• Elevations and Projections: Crest (prominent ridge), spine (sharp, slender projection), tubercle/tuberosity (rounded projection), condyle (rounded articular area).

Example: The foramen magnum in the skull allows passage of the spinal cord.

Structure of a Typical Long Bone

Parts and Internal Features

• Epiphysis: Expanded area at each end, consisting largely of spongy bone covered by compact bone. Articular cartilage covers portions involved in joint formation.

• Metaphysis: Connects epiphysis to diaphysis; contains the growth plate (epiphyseal plate).

• Diaphysis (Shaft): Contains the medullary (marrow) cavity, filled with red marrow (hematopoiesis) and yellow marrow (adipose tissue for energy storage).

Blood Supply and Innervation

• Nutrient artery and vein: Enter through the nutrient foramen to supply the marrow cavity.

• Metaphyseal arteries/veins: Supply the metaphysis and connect to epiphyseal vessels.

• Periosteum: Outer layer with blood vessels and nerves supplying superficial osteons.

Bone Cells and Their Functions

Types of Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts. Located in the inner periosteum, endosteum, and passageways containing blood vessels. Essential for fracture repair.

• Osteoblasts: Produce new bone matrix (osteogenesis/ossification). Secrete osteoid (unmineralized matrix) and assist in mineralization. Become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells that maintain protein and mineral content of matrix. Reside in lacunae, interconnected by canaliculi. Cannot divide.

• Osteoclasts: Large, multinucleated cells that remove and remodel bone matrix by secreting acids and enzymes (osteolysis). Release stored minerals.

Table: Bone Cell Types and Functions

Bone Matrix Composition

Organic and Inorganic Components

• Collagen fibers: Account for ~1/3 of bone weight; provide flexibility.

• Calcium phosphate (Ca3(PO4)2): ~2/3 of bone weight; interacts with calcium hydroxide to form hydroxyapatite crystals, providing strength.

Compact vs. Spongy Bone

Structural and Functional Differences

• Compact Bone: Dense outer layer; functional unit is the osteon (concentric lamellae around a central canal). Strong along its length; contains blood vessels in central canals.

• Spongy Bone: Inner layer; lamellae form trabeculae (struts and plates) creating an open network. No blood vessels in matrix; nutrients reach osteocytes via canaliculi. Red marrow found between trabeculae.

Why the Difference Matters: Compact bone provides strength for weight-bearing, while spongy bone reduces weight and supports marrow functions.

Appositional Bone Growth

Mechanism and Importance

• Definition: Increase in bone diameter by adding new layers of bone matrix under the periosteum.

• Process: Osteogenic cells differentiate into osteoblasts, which add circumferential lamellae. Osteoclasts remove matrix at the inner surface to enlarge the medullary cavity.

• Role: Allows bones to grow in thickness and adapt to stress.

Bone Formation: Ossification Processes

Endochondral Ossification

Most bones, especially long bones, form by replacing a cartilage model with bone.

• Embryonic skeleton starts as hyaline cartilage.

• Cartilage is gradually replaced by bone through a series of steps:

  1. Cartilage model enlarges; chondrocytes die, leaving cavities.

  2. Blood vessels grow around cartilage; perichondrium cells become osteoblasts.

  3. Blood vessels penetrate cartilage; fibroblasts become osteoblasts, forming spongy bone at the primary ossification center.

  4. Medullary cavity forms; bone grows in length and diameter.

  5. Secondary ossification centers form in epiphyses.

  6. Epiphyseal cartilage (growth plate) separates epiphysis from diaphysis.

  7. Bone grows in length at the growth plate until puberty, when epiphyseal closure occurs.

Intramembranous Ossification

Forms bone directly from mesenchymal tissue without a prior cartilage model. Produces flat bones (e.g., skull, clavicle).

• Mesenchymal cells differentiate into osteoblasts, secrete osteoid matrix.

• Osteoid becomes mineralized; bone matrix forms at ossification centers.

• Bone grows outward in spicules; osteoblasts become osteocytes.

• Blood vessels enter, spicules fuse, forming spongy bone.

• Remodeling produces compact bone and periosteum.

Bone Abnormalities

Growth Disorders

• Achondroplasia: Epiphyseal cartilage grows slowly and is replaced by bone early, resulting in short, stocky limbs.

• Marfan Syndrome: Excessive cartilage formation at growth plates; results in tall, slender limbs and may affect other connective tissues, including the heart.

• Clubfoot (Talipes Equinovarus): Inherited abnormality causing feet to turn medially and invert; treated with casts or supports.

• Gigantism: Overproduction of growth hormone before puberty; results in abnormally long bones.

• Acromegaly: Overproduction of growth hormone after epiphyseal plates close; bones thicken, especially in face, jaw, and hands.

• Fibrodysplasia Ossificans Progressiva (FOP): Genetic mutation causing bone formation in soft tissues; no effective treatment.

Bones as Mineral Reservoirs

Mineral Storage and Homeostasis

• Calcium: Most abundant mineral in the body; 99% stored in bones. Essential for muscle contraction, blood coagulation, and nerve impulse transmission.

• Phosphate: Important for bone strength and cellular functions.

• Other minerals: Magnesium, sodium, carbonate.

Organs Regulating Calcium: Intestines (absorption), bones (storage/release), kidneys (excretion/reabsorption).

Hormonal Regulation of Calcium

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, enhancing intestinal absorption (via calcitriol), and reducing urinary loss.

• Calcitriol: Active form of vitamin D; increases intestinal absorption of calcium.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts, decreasing intestinal absorption, and increasing urinary excretion.

Table: Hormonal Effects on Blood Calcium

Bone Fractures and Healing

Types of Fractures

• Closed (Simple) Fracture: Internal break; skin not penetrated.

• Open (Compound) Fracture: Bone projects through skin; risk of infection and bleeding.

Fracture Healing Process

1. Fracture Hematoma Formation: Large clot closes injured vessels within hours.

2. Callus Formation: Internal callus (spongy bone) unites inner edges; external callus (cartilage and bone) stabilizes outer edges.

3. Spongy Bone Formation: Cartilage of external callus replaced by spongy bone; dead bone removed.

4. Compact Bone Formation: Spongy bone replaced by compact bone; remodeling eliminates evidence of fracture.

Example: After a fracture, the first temporary structure to form is the fracture hematoma.

Summary Table: Key Concepts

Additional info: These notes synthesize textbook and lecture content for comprehensive exam preparation in Anatomy & Physiology, focusing on bone structure, function, growth, and clinical relevance.