Bones and Bone Tissue: Structure, Function, Growth, and Repair

Bones and Bone Tissue

The Skeletal System: Overview

The skeletal system is a complex organ system that includes bones, joints, and supporting tissues. Bones are the main organs, with adults typically having 206 bones. Each bone contains osseous tissue, dense regular and irregular connective tissue, and bone marrow.

• Bones: Provide structure and support.

• Joints: Allow movement and flexibility.

• Bone Marrow: Site of blood cell formation and fat storage.

Functions of the Skeletal System

Bones serve several essential functions for the human body:

• Protection: Bones such as the skull, sternum, and ribs protect vital organs.

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals (calcium, phosphorus, magnesium) critical for electrolyte and acid-base balance.

• Blood Cell Formation: Red bone marrow is the site of hematopoiesis (blood cell production).

• Fat Storage: Yellow bone marrow stores triglycerides in adipocytes.

• Movement: Muscles attach to bones, and contraction generates movement at joints.

• Support: The skeleton supports body weight and provides a structural framework.

Bone Structure

Classification of Bones by Shape

Bones are classified based on their shape, which relates to their function and location:

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

• Short Bones: About as long as wide, cube-shaped (e.g., wrist, ankle bones).

• Flat Bones: Thin and broad (e.g., skull, pelvis).

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

• Sesamoid Bones: Small, flat, oval-shaped bones within tendons (e.g., patella).

Structure of a Long Bone

Long bones have a distinct anatomy that supports their function:

• Periosteum: Outer dense irregular connective tissue membrane with blood vessels and nerves.

• Perforating Fibers: Collagen anchors that attach periosteum to bone matrix.

• Diaphysis: Shaft of the bone, containing the medullary (marrow) cavity lined by endosteum.

• Epiphyses: Ends of the bone, filled with red marrow and covered with articular cartilage (hyaline cartilage).

• Compact Bone: Dense outer layer resisting compression and twisting.

• Spongy (Cancellous) Bone: Inner honeycomb-like structure providing space for marrow.

• Epiphyseal Lines: Remnants of growth plates in mature bones.

Structure of Short, Flat, Irregular, and Sesamoid Bones

These bones share similarities with long bones but have fewer structures. In flat bones, the spongy bone is called diploë, and some skull bones contain sinuses to reduce weight.

Blood and Nerve Supply to Bone

Bones are highly vascularized and innervated:

• Short, Flat, Irregular, Sesamoid Bones: Blood supply mainly from periosteum.

• Long Bones: Blood supply from periosteum and nutrient arteries entering through the nutrient foramen.

Red and Yellow Marrow

Bone marrow changes with age:

• Red Bone Marrow: Hematopoietic tissue, abundant in children.

• Yellow Bone Marrow: Contains adipocytes and blood vessels, predominant in adults.

• Adult Red Marrow Locations: Pelvic bones, proximal femur/humerus, vertebrae, ribs, sternum, clavicles, scapulae.

Bone Marrow Transplantation

Bone Marrow Harvest and Transplant

Bone marrow transplantation is used to treat diseases like leukemia and sickle-cell anemia. Donor marrow is harvested from the pelvic bone and transplanted after recipient marrow is destroyed.

Peripheral Blood Stem Cell Donation

An alternative involves stimulating hematopoietic cells to enter the blood, filtering them, and returning blood to the donor.

The Extracellular Matrix of Bone

Inorganic Matrix

The inorganic matrix makes up about 65% of bone weight and consists mainly of hydroxyapatite crystals (calcium and phosphorus), providing strength and resistance to compression.

• Other Components: Bicarbonate, potassium, magnesium, sodium salts.

Organic Matrix (Osteoid)

The organic matrix comprises about 35% of bone weight and includes collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and osteocalcin.

• Collagen: Resists torsion and tensile forces.

• Osteocalcin: Binds calcium and hydroxyapatite, organizing the matrix.

• Glycosaminoglycans/Proteoglycans: Draw water to resist compression.

• Glycoproteins: Bind crystals and cells together.

Bone Cells

Types of Bone Cells

Bone is dynamic, constantly remodeled by three main cell types:

• Osteoblasts: Build bone, derived from osteogenic cells, perform bone deposition.

• Osteocytes: Mature osteoblasts trapped in lacunae, maintain ECM.

• Osteoclasts: Large, multinucleated cells that break down bone (bone resorption).

Osteoblasts and Osteocytes

Osteoblasts secrete organic matrix and assist in forming inorganic matrix. When trapped, they become osteocytes, which maintain ECM and recruit osteoblasts for repair.

Osteoclasts

Osteoclasts resorb bone by secreting hydrogen ions and enzymes, releasing minerals and organic components into the blood.

Osteopetrosis

Osteopetrosis is a disease caused by defective osteoclasts, leading to increased bone mass but weak, brittle bones. Infantile and adult forms differ in severity and symptoms.

Histology of Bone

Compact Bone

Compact bone is organized into osteons (Haversian systems):

• Lamellae: Concentric rings of bone matrix.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small cavities housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae.

• Interstitial and Circumferential Lamellae: Connect and strengthen bone.

• Perforating (Volkmann) Canals: Connect central canals and carry blood vessels.

Spongy Bone

Spongy bone consists of trabeculae, covered with endosteum, containing lamellae, lacunae, and canaliculi, but lacking central canals. Blood supply comes from bone marrow vessels.

Bone Formation: Ossification

Ossification (Osteogenesis)

Ossification is the process of bone formation, continuing through childhood. Two types of bone are formed:

• Primary (Woven) Bone: Immature, irregular collagen, abundant osteocytes, little inorganic matrix.

• Secondary (Lamellar) Bone: Mature, organized lamellae, parallel collagen, more inorganic matrix.

Types of Ossification

• Intramembranous Ossification: Forms flat bones (skull, clavicles) from mesenchymal membrane; spongy bone forms first.

• Endochondral Ossification: Forms most bones below the head from hyaline cartilage; compact bone forms first.

Steps of Intramembranous Ossification

• Osteoblasts develop in the primary ossification center from mesenchymal cells.

• Osteoblasts secrete organic matrix, which calcifies; trapped osteoblasts become osteocytes.

• Osteoblasts lay down trabeculae of early spongy bone; some mesenchyme becomes periosteum.

• Osteoblasts in periosteum lay down early compact bone; matrix is remodeled.

• Fontanels in newborn skulls represent incomplete fusion.

Steps of Endochondral Ossification

• Chondroblasts in perichondrium differentiate into osteoblasts.

• Osteoblasts build bone collar on external surface; internal cartilage calcifies and chondrocytes die.

• Osteoblasts replace calcified cartilage with early spongy bone; secondary ossification centers and medullary cavity develop.

• Epiphyses ossify; cartilage remains in epiphyseal plates and articular cartilage.

Comparison: Intramembranous vs. Endochondral Ossification

• Intramembranous: Flat bones, spongy bone forms first.

• Endochondral: Long/short bones, compact bone forms first.

Bone Disorders

Osteoporosis

Osteoporosis is caused by inadequate inorganic matrix, making bones brittle and prone to fractures. Risk factors include diet, age, sex, exercise, hormones, genetics, and certain diseases. Prevention includes adequate calcium/vitamin D, exercise, and medications.

Achondroplasia

Achondroplasia is the most common cause of dwarfism, resulting from abnormal growth factor receptors on cartilage. It leads to decreased endochondral ossification and abnormal bone development.

Gigantism and Acromegaly

Both are caused by excess growth hormone. Gigantism occurs before epiphyseal plate closure, causing excessive growth. Acromegaly occurs after closure, causing enlarged bones and soft tissues.

Bone Growth

Longitudinal Growth

Long bones grow in length at the epiphyseal plate, which has five zones:

• Zone of Reserve Cartilage: Cells not directly involved in growth.

• Zone of Proliferation: Actively dividing chondrocytes.

• Zone of Hypertrophy and Maturation: Mature chondrocytes.

• Zone of Calcification: Dead, calcified chondrocytes.

• Zone of Ossification: Calcified chondrocytes and osteoblasts build bone.

Appositional Growth

Bones grow in width by appositional growth, where osteoblasts lay down new bone between periosteum and bone surface, thickening the compact bone.

The Role of Hormones in Bone Growth

• Growth Hormone: Increases mitosis of chondrocytes, activity of osteogenic cells, and stimulates osteoblasts.

• Testosterone: Increases appositional growth and mitosis, accelerates epiphyseal plate closure.

• Estrogen: Similar effects, but less pronounced; epiphyseal plates close earlier in females.

Bone Remodeling

Bone Remodeling Process

Bone remodeling is a continual process involving bone deposition (by osteoblasts) and bone resorption (by osteoclasts). It maintains calcium homeostasis, repairs bone, replaces old bone, and adapts to stress.

Bone Deposition

Osteoblasts secrete organic matrix and facilitate inorganic matrix formation, leading to calcification.

Bone Resorption

Osteoclasts secrete hydrogen ions and enzymes to break down bone matrix, releasing minerals and organic components.

Remodeling in Response to Tension and Stress

• Compression and Tension: Stimulate bone deposition.

• Pressure: Stimulates bone resorption.

Other Factors Influencing Bone Remodeling

• Hormones: Testosterone promotes deposition; estrogen inhibits osteoclasts.

• Age: Hormone levels decline, reducing protein synthesis and protective effects.

• Nutrient Intake: Calcium, vitamin D, K, C, and protein are essential for bone health.

Calcium Ion Homeostasis

Calcium ions are vital for muscle contraction, nerve transmission, and blood clotting. Blood calcium is regulated by hormones:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Decreases blood calcium by stimulating bone deposition.

Bone Repair

Steps of Fracture Healing

• Hematoma Formation: Blood fills the gap between bone fragments.

• Soft Callus Formation: Fibroblasts and chondroblasts produce connective tissue and cartilage.

• Bone Callus Formation: Osteoblasts lay down primary bone.

• Remodeling: Primary bone is replaced with secondary bone.

Classes and Types of Fractures

• Simple (Closed) Fractures: Skin and tissue remain intact.

• Compound (Open) Fractures: Damage to surrounding tissue.

• Treatment: Stabilization and immobilization; closed or open reduction.

Table: Types of Fractures

Type	Description
Spiral	Twisting force causes spiral-shaped break
Compression	Bone is crushed under weight
Comminuted	Bone fragments into several pieces
Greenstick	Bone breaks on one side, bends on the other (common in children)
Epiphyseal	Fracture at epiphyseal plate
Other	Additional info: Other fracture types may include transverse, oblique, and impacted fractures

Summary

This guide covers the structure, function, growth, remodeling, and repair of bones and bone tissue, providing a comprehensive overview for college-level anatomy and physiology students.