Comprehensive Study Notes on Bone Tissue: Structure, Classification, Growth, and Repair

Bones: Functions, Classification, and Structure

Functions of Bone Tissue

Bones serve multiple essential functions in the human body, providing structural support, protection, movement, mineral storage, and blood cell formation.

• Support: Framework that supports and anchors all soft organs. Leg bones act as pillars for the body trunk, and ribs support the thorax.

• Protection: Skull and vertebrae surround and protect the nervous system; rib cage protects vital organs like the heart and lungs.

• Movement: Skeletal muscles use bones as levers to move the body.

• Mineral Storage: Bones store minerals (calcium, phosphate) and release them into the bloodstream as needed.

• Blood Cell Formation: Hematopoiesis occurs within the marrow cavities of bones.

Bone Classification by Shape and Composition

Bones are classified by their shape and the proportion of compact and spongy bone tissue.

• Compact Bone: Dense and smooth, forms the outer layer of bones.

• Spongy Bone: Composed of small needlelike or flat pieces called trabeculae; spaces are filled with marrow.

Bones are also classified by shape:

• Long Bones: Longer than wide, mostly compact bone. Examples: limbs (except wrists and ankles).

• Short Bones: Cube-like, mostly spongy bone. Examples: wrist and ankle bones.

• Flat Bones: Thin, flattened, usually curved. Two parallel compact bone surfaces with spongy bone in between. Examples: sternum, ribs, skull bones.

• Irregular Bones: Complicated shapes, mainly spongy bone enclosed by thin layers of compact bone. Examples: vertebrae, hip bones.

Gross Anatomy and Microscopic Structure of Bone

Gross Anatomy of a Long Bone

Long bones share a common structure, including the diaphysis, epiphyses, membranes, and articular cartilage.

• Diaphysis: Shaft, composed of a thick collar of compact bone surrounding a medullary cavity (contains yellow marrow).

• Epiphyses: Ends of the bone, usually expanded, with a thin layer of compact bone and spongy bone interior.

• Epiphyseal Plate: Hyaline cartilage in growing bones, site of bone lengthening.

• Periosteum: Double-layered membrane covering the diaphysis; outer fibrous layer and inner osteogenic layer (osteoblasts, osteoclasts).

• Endosteum: Delicate membrane lining internal bone surfaces, contains bone-forming cells.

• Articular Cartilage: Hyaline cartilage covering epiphyses at joint surfaces, reduces friction and absorbs shock.

Microscopic Anatomy of Bone

Bones are composed of compact and spongy bone, each with distinct microscopic features.

• Compact Bone: Contains osteons (Haversian systems), lamellae, central (Haversian) canals, perforating (Volkmann's) canals, and canaliculi.

• Spongy Bone: Consists of trabeculae arranged along lines of stress, with irregular lamellae and osteocytes interconnected by canaliculi.

Table: Comparison of Compact and Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Outer layer of bones	Interior of bones, ends of long bones
Structure	Osteons, lamellae, central canals	Trabeculae, irregular lamellae
Function	Strength, protection	Lightweight, supports marrow

Chemical Composition and Bone Remodeling

Chemical Composition of Bone

Bones are made of organic and inorganic components:

• Organic: Cells (osteoblasts, osteocytes, osteoclasts) and osteoid (collagen fibers, ground substance).

• Inorganic: Mineral salts, mainly hydroxyapatite (calcium phosphate crystals).

These components provide bones with strength, flexibility, and hardness.

Bone Remodeling

Bones are dynamic tissues that continually change. About 5-7% of bone is remodeled each week by deposition (osteoblasts) and resorption (osteoclasts).

• Deposition: Occurs when bone is injured or added strength is required.

• Resorption: Osteoclasts digest bone matrix and release calcium into the blood.

• Hormonal Control: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts.

Mechanical stress also influences bone remodeling.

Bone Development and Growth

Ossification (Bone Formation)

The human skeleton begins as cartilage and is replaced by bone through ossification, which occurs by two main processes:

• Intramembranous Ossification: Bone develops from fibrous membranes, mainly forming cranial bones and clavicles.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage, forming most bones below the skull except clavicles.

Growth of Bones

• Interstitial Growth: Growth in length at the epiphyseal plate.

• Appositional Growth: Growth in thickness by addition of bone matrix to the external surface.

Bone Markings and Fractures

Bone Markings

Bones display bulges, depressions, and holes, which serve as attachment sites for muscles, ligaments, and tendons, or as conduits for blood vessels and nerves.

• Projections: Tuberosity, crest, trochanter, line, tubercle, epicondyle, spine, process.

• Projections for Joint Formation: Head, facet, condyle, ramus.

• Depressions and Openings: Fossa, sinus, foramen, groove, fissure, notch.

Types of Bone Fractures

• Simple: Bone breaks cleanly, does not penetrate skin.

• Compound: Broken ends protrude through skin.

• Comminuted: Bone fragments into many pieces.

• Compression: Bone is crushed.

• Depressed: Broken bone portion is pressed inward.

• Impacted: Broken ends are forced into each other.

• Spiral: Ragged break from excessive twisting forces.

• Greenstick: Bone breaks incompletely, common in children.

Repair of a Simple Fracture

1. Hematoma Formation: Blood vessels break, forming a mass of clotted blood at the fracture site.

2. Fibrocartilaginous Callus Formation: Capillaries grow into the hematoma; fibroblasts and osteoblasts produce collagen fibers and cartilage matrix.

3. Bony Callus Formation: Fibrocartilaginous callus is converted to a hard bony callus of spongy bone by osteoblasts and osteoclasts.

4. Remodeling: Excess material is removed, compact bone is laid down to reconstruct the shaft.

Key Equations and Scientific Terms

• Hydroxyapatite Formula: $Ca_{10}(PO_4)_6(OH)_2$

• Bone Remodeling Rate: $\text{Remodeled Bone} = 0.05 \times \text{Total Bone Mass per Week}$

Summary Table: Types of Bone Fractures

Type	Description
Simple	Clean break, does not penetrate skin
Compound	Broken ends protrude through skin
Comminuted	Bone fragments into many pieces
Compression	Bone is crushed
Depressed	Broken portion pressed inward
Impacted	Broken ends forced into each other
Spiral	Ragged break from twisting
Greenstick	Incomplete break, common in children

Additional info: These notes expand on the original content by providing definitions, examples, and academic context for each topic, ensuring a self-contained study guide suitable for college-level Anatomy & Physiology students.