Bones and Skeletal Tissues

Overview of Bone Tissue Topics

This chapter introduces the foundational concepts of bones and skeletal tissues, focusing on their structure, function, classification, and growth. Understanding these topics is essential for comprehending the anatomy and physiology of the human skeletal system.

• Cartilage structure, types, locations, and growth

• Bone functions

• Classifying bone by location and shape

• Compact and spongy bones

• Gross anatomy of long bone

• Microscopic anatomy of bone tissue

• Chemical composition of bone

• Bone formation: endochondral and intramembranous ossification

• Postnatal bone growth and remodeling—Wolff’s law & hormonal influence

• Fracture healing

• Osteoporosis

Cartilage Structure, Types, and Locations

Skeletal Cartilage

Skeletal cartilage is a specialized connective tissue that provides support and flexibility to various parts of the skeleton. It is primarily composed of water, which contributes to its resilience.

• Contains no blood vessels or nerves: Cartilage is avascular and aneural, relying on diffusion for nutrient and waste exchange.

• Perichondrium: A dense connective tissue layer that surrounds cartilage, acting as a girdle to resist outward expansion and providing blood vessels for nutrient delivery.

• Chondrocytes: The primary cells of cartilage, located in small cavities called lacunae within a jelly-like extracellular matrix.

Types of Cartilage

There are three main types of cartilage, each with distinct properties and locations:

• Hyaline cartilage: Most abundant type; provides support, flexibility, and resilience. Contains only collagen fibers. Found in articular joints, costal cartilage (ribs), respiratory structures (larynx), and nasal cartilage (nose tip).

• Elastic cartilage: Similar to hyaline but contains elastic fibers, allowing greater flexibility. Located in the external ear and epiglottis.

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

Growth of Cartilage

Cartilage grows by two mechanisms:

• Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the surface of existing cartilage.

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

Calcification of cartilage can occur during normal bone growth in youth or in old age, but hardened cartilage is not the same as bone.

Bone Functions

Major Functions of Bones

Bones serve multiple critical functions in the human body:

• Support: Provide structural framework for the body and support soft organs.

• Protection: Protect vital organs such as the brain, lungs, and heart.

• Movement: Serve as levers for muscles, enabling movement.

• Mineral Storage: Store minerals, especially calcium and phosphate; bones contain about 98% of the body's calcium.

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

• Fat Storage: Yellow bone marrow stores fat.

• Hormone Production: Osteocalcin is produced by bone to regulate insulin and glucose.

Classification of Bones by Shape

Bone Shapes and Examples

Bones are classified by their shapes, which relate to their functions and locations:

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

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

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull).

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

• Sesamoid bones: Form within tendons (e.g., patella).

Humans have approximately 206 bones, with variable numbers of sesamoid bones.

Gross Anatomy of Bone

Compact and Spongy Bone

Bones consist of two types of tissue:

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

• Spongy bone: Honeycomb-like structure of small, needle-like or flat pieces called trabeculae, with spaces filled by red or yellow bone marrow.

Structure of Long Bones

Long bones have a distinct anatomy:

• Diaphysis: Tubular shaft forming the long axis, composed of compact bone surrounding the medullary cavity (yellow marrow in adults).

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

• Epiphyseal line: Remnant of the childhood epiphyseal plate where bone growth occurs.

Bone Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except joints; contains fibrous (outer) and osteogenic (inner) layers, nerve fibers, blood vessels, and anchoring points for tendons and ligaments.

• Endosteum: Delicate membrane covering internal bone surfaces, including trabeculae and canals; contains osteogenic cells.

Blood Vessels and Nerves

• Bones are highly vascularized, with nutrient arteries and veins entering through nutrient foramina to supply bone marrow and compact bone.

• Nerves accompany blood vessels and pass through nutrient foramina.

Hematopoietic Tissue in Bones

• Red marrow: Found in trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum); in newborns, all spongy bone and medullary cavities contain red marrow.

• Yellow marrow: Can convert to red marrow if needed (e.g., in anemia).

Microscopic Anatomy of Bone Tissue

Bone Cells

• Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete osteoid (collagen and ground substance).

• Osteocytes: Mature bone cells that maintain the matrix and communicate mechanical stress.

• Osteoclasts: Multinucleated cells that resorb bone matrix; derived from macrophage lineage.

Bone Matrix

• Collagen fibers: Provide tensile strength and flexibility.

• Ground substance: Contains hydroxyapatite (calcium phosphate and calcium hydroxide crystals) for hardness and resistance to compression.

Osteon (Haversian System)

The osteon is the structural unit of compact bone, consisting of concentric rings (lamellae) of bone matrix surrounding a central canal.

• Lamellae: Collagen fibers run in different directions in adjacent rings, providing strength and resistance to twisting.

• 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: Hairlike canals connecting lacunae and central canal, allowing communication and nutrient/waste exchange.

Other Lamellae

• Interstitial lamellae: Fill gaps between forming osteons or are remnants of old osteons.

• Circumferential lamellae: Extend around the entire diaphysis, just deep to the periosteum, helping resist twisting.

Microscopic Anatomy of Spongy Bone

• Spongy bone lacks osteons but contains trabeculae with lamellae and osteocytes interconnected by canaliculi.

• Capillaries in the endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Cells and osteoid: Osteoid is secreted by osteoblasts and consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Sacrificial bonds: Collagen molecules contain bonds that stretch and break to dissipate energy and prevent fractures; these bonds re-form if no additional trauma occurs.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone mass, mainly tiny calcium phosphate crystals around collagen fibers, responsible for hardness and resistance to compression.

When organic and inorganic components are balanced, bone is half as strong as steel in resisting compression and as strong as steel in resisting tension.

Bone Formation and Growth

Ossification (Osteogenesis)

Bone formation occurs by two processes:

• Endochondral ossification: Forms most bones below the skull (except clavicles) from hyaline cartilage models. Begins in the second month of development and requires breakdown of cartilage prior to ossification.

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicles) directly from mesenchymal tissue.

Steps in Endochondral Ossification

1. Bone collar forms around diaphysis of cartilage model.

2. Central cartilage in diaphysis calcifies and develops cavities.

3. Periosteal bud invades cavities, forming spongy bone.

4. Diaphysis elongates and medullary cavity forms; secondary ossification centers appear in epiphyses.

5. Epiphyses ossify; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Postnatal Bone Growth

Growth in Length of Long Bones

Occurs at the epiphyseal plate by interstitial growth:

• Epiphyseal plate maintains constant thickness; rate of cartilage growth is balanced by bone replacement.

• Four zones: proliferation (growth), hypertrophic, calcification, and ossification (osteogenic).

• Bone lengthening ceases at epiphyseal plate closure (around age 18 in females, 21 in males).

Growth in Width (Thickness)

• Appositional growth: Osteoblasts beneath periosteum secrete bone matrix on external surface; osteoclasts remove bone on endosteal surface.

• Results in thicker, stronger bones.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity in infancy and childhood.

• Thyroid hormone: Modulates growth hormone activity for proper proportions.

• Sex hormones (estrogen and testosterone): Promote growth spurts and induce epiphyseal plate closure.

Imbalances in hormones can cause abnormal skeletal growth.

Bone Remodeling

Remodeling Process

• About 5-10% of bone mass is recycled each year.

• Spongy bone replaced every 3-4 years; compact bone every 10 years.

• Bone resorption: Osteoclasts break down bone matrix, releasing minerals into the blood.

• Bone deposition: Osteoblasts lay down new bone matrix.

Control of Remodeling

• Hormonal controls: Negative feedback loop regulates blood calcium levels.

• Parathyroid hormone (PTH): Stimulates osteoclasts to resorb bone when blood calcium is low.

• Calcitonin: Lowers blood calcium levels at high doses.

• Mechanical stress: Bones remodel in response to mechanical and gravitational forces (Wolff’s law).

Fracture Healing

Stages of Fracture Repair

1. Hematoma formation: Blood vessels rupture, forming a hematoma at the fracture site.

2. Fibrocartilaginous callus formation: Capillaries grow into hematoma; fibroblasts, cartilage, and osteogenic cells reconstruct bone.

3. Bony callus formation: New trabeculae form, converting callus to spongy bone.

4. Bone remodeling: Excess material is removed, and compact bone reconstructs shaft walls.

Osteoporosis

Definition and Risk Factors

Osteoporosis is a group of diseases where bone resorption exceeds deposition, leading to decreased bone mass and increased fracture risk.

• Most common in aged, postmenopausal women due to decreased estrogen.

• Other risk factors: insufficient exercise, poor diet (low calcium/protein), smoking, genetics, hormone-related conditions, hyperthyroidism, diabetes mellitus, alcohol or certain medications.

Spongy bone of the spine and neck of femur are most susceptible, with vertebral and hip fractures being common.