Bones and Skeletal Tissues

Overview

This chapter introduces the structure, function, and development of bones and skeletal tissues, focusing on their anatomy, physiology, and clinical relevance. Understanding these concepts is essential for comprehending the human skeletal system's role in support, movement, and homeostasis.

Learning Objectives

• Describe the structure, types, and locations of bone and cartilage tissues.

• Explain bone growth (appositional and interstitial) and the classification of bones.

• Identify the gross and microscopic anatomy of bone, including compact and spongy bone.

• Discuss bone markings and their significance.

• Summarize the process of bone development (ossification/osteogenesis).

• Describe bone growth regulation, remodeling, and repair.

• Classify fractures and outline the stages of bone healing.

• Recognize common bone disorders and their causes.

Skeletal Cartilages

Basic Structure, Types, and Locations

The human skeleton initially consists of cartilage, which is later replaced by bone except in areas requiring flexibility.

• Cartilage: A resilient, avascular connective tissue composed primarily of water and chondrocytes (cartilage cells) embedded in an extracellular matrix.

• Perichondrium: A dense connective tissue layer surrounding cartilage, providing nutrients and resisting outward expansion.

• Cartilage types:

  • Hyaline cartilage: Most abundant; provides support and flexibility. Found in articular surfaces, costal cartilages, respiratory structures, and nasal cartilage.

  • Elastic cartilage: Similar to hyaline but contains more elastic fibers. Found in the external ear and epiglottis.

  • Fibrocartilage: Contains thick collagen fibers; provides great tensile strength. Found in intervertebral discs and menisci of the knee.

Growth of Cartilage occurs by:

• Appositional growth: New matrix is secreted on the surface by cells in the perichondrium.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Cartilage can calcify with age but is not the same as bone.

Functions of Bones

• Support: Framework for the body and soft organs.

• Protection: Shields the brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities.

• Fat storage: Yellow marrow stores triglycerides for energy.

• Hormone production: Osteocalcin regulates insulin secretion and energy metabolism.

Classification of Bones

By Location

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles.

By Shape

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

• Short bones: Cube-shaped (e.g., wrist, ankle); sesamoid bones (e.g., patella) form within tendons.

• Flat bones: Thin, flat, slightly curved (e.g., sternum, scapula, ribs, skull bones).

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

Bone Structure

Gross Anatomy

• Compact bone: Dense outer layer; appears smooth and solid.

• Spongy bone: Honeycomb of trabeculae (small, flat pieces); spaces filled with bone marrow.

Structure of Long Bones

• Diaphysis: Shaft; compact bone surrounding the medullary cavity (contains yellow marrow in adults).

• Epiphyses: Bone ends; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal plate/line: Site of bone growth in length during childhood; becomes the epiphyseal line in adults.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces (except joints); outer fibrous layer and inner osteogenic layer (contains stem cells).

• Endosteum: Connective tissue membrane lining internal bone surfaces and trabeculae.

Bone Marrow

• Red marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone and diploë of flat bones.

• Yellow marrow: Fat storage; can convert to red marrow if needed.

Bone Markings

Bone markings are features on bones that serve as sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves.

• Projections: Outward bulges (e.g., tuberosity, crest, trochanter).

• Depressions: Bowl- or groove-like cutouts (e.g., fossa, sulcus).

• Openings: Holes or canals (e.g., foramen, canal).

Microscopic Anatomy of Bone

Bone Cells

• Osteogenic cells: Stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells; secrete osteoid (unmineralized bone matrix rich in collagen).

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as mechanosensors.

• Bone-lining cells: Flat cells on bone surfaces; help maintain matrix.

• Osteoclasts: Multinucleated cells derived from hematopoietic stem cells; responsible for bone resorption.

Compact Bone Structure

• Osteon (Haversian system): Structural unit; elongated cylinder parallel to bone's long axis, consisting of concentric lamellae.

• Lamellae: Rings of bone matrix with alternating collagen fiber orientation for strength.

• Central (Haversian) canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae, allowing communication and nutrient/waste exchange.

• Interstitial and circumferential lamellae: Fill gaps between osteons and encircle the bone's circumference.

Spongy Bone

• Trabeculae align along lines of stress; contain irregular lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

• Organic components: Cells (osteogenic, osteoblasts, osteocytes, bone-lining, osteoclasts) and osteoid (collagen fibers and ground substance).

• Inorganic components: Hydroxyapatite (mineral salts, mainly calcium phosphate) responsible for bone hardness and resistance to compression.

Bone Development (Ossification/Osteogenesis)

• Bone tissue formation begins in the embryo (week 8) and continues through life.

• Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most bones below the skull (except clavicles).

• Intramembranous ossification: Bone develops from fibrous membranes; forms flat bones of the skull and clavicles.

Endochondral Ossification Steps

1. Bone collar forms around diaphysis of cartilage model.

2. Central cartilage in diaphysis calcifies, forming cavities.

3. Periosteal bud invades cavities, forming spongy bone.

4. Diaphysis elongates, medullary cavity forms.

5. Secondary ossification centers appear in epiphyses.

6. Epiphyses ossify; hyaline cartilage remains at epiphyseal plates and articular surfaces.

Intramembranous Ossification Steps

1. Ossification centers form in fibrous connective tissue.

2. Osteoid is secreted and calcifies.

3. Woven bone and periosteum form.

4. Lamellar bone replaces woven bone; red marrow appears.

Postnatal Bone Growth

Growth in Length (Interstitial Growth)

• Occurs at the epiphyseal plate; involves five zones: resting, proliferation, hypertrophic, calcification, and ossification.

• Chondrocytes divide, enlarge, and are replaced by bone tissue, lengthening the bone.

• Epiphyseal plate closure marks the end of growth in length (around age 18 in females, 21 in males).

Growth in Width (Appositional Growth)

• Bones thicken in response to stress; osteoblasts add bone matrix to the periosteum, while osteoclasts remove bone from the endosteal surface.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity.

• Thyroid hormone: Modulates growth hormone effects.

• Sex hormones: Promote growth spurts and epiphyseal plate closure at puberty.

Bone Remodeling

• Bone is continuously renewed through deposition (by osteoblasts) and resorption (by osteoclasts).

• Remodeling is regulated by hormonal (e.g., parathyroid hormone, calcitonin) and mechanical factors (e.g., Wolff's law: bone adapts to stress).

Key Equation:

Calcium Homeostasis

• Blood calcium levels are tightly regulated (9–11 mg/100 mL).

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts.

• Calcitonin: Lowers blood calcium (minor effect in humans).

• Vitamin D is required for intestinal calcium absorption.

Fractures and Bone Repair

Classification of Fractures

• Position: Nondisplaced (normal alignment) vs. displaced (misaligned).

• Completeness: Complete (all the way through) vs. incomplete.

• Skin involvement: Open (compound) vs. closed (simple).

Common Types of Fractures

• Comminuted

• Compression

• Spiral

• Epiphyseal

• Depressed

• Greenstick

Stages of Bone Repair

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

Bone Disorders

• Osteomalacia and rickets: Poor mineralization; soft, weak bones (often due to vitamin D or calcium deficiency).

• Osteoporosis: Bone resorption exceeds deposition; decreased bone mass and increased fracture risk (common in postmenopausal women).

Treatments include calcium/vitamin D supplements, weight-bearing exercise, and medications that inhibit osteoclasts.

Summary Table: Types of Cartilage

Summary Table: Bone Cells