Chapter 7: Skeletal System – Bone Tissue, Structure, Development, and Homeostasis

Bone as a Tissue

Overview of Skeletal System Components

The skeletal system provides the structural framework for the body and is composed of bones, cartilages, and ligaments. It is essential for movement, protection, and physiological regulation.

• Osteology: The scientific study of bone structure and function.

• Skeletal system: Includes bones, cartilages (precursors and joint surfaces), and ligaments (connect bones at joints).

• Tendons: Connect muscles to bones, enabling movement.

Functions of the Skeleton

Major Roles of Bone Tissue

Bones serve multiple vital functions beyond structural support.

• Support: Maintains body shape, supports muscles, and anchors teeth.

• Protection: Shields vital organs such as the brain, spinal cord, heart, and lungs.

• Movement: Acts as levers for muscles, enabling locomotion and breathing.

• Electrolyte balance: Stores and releases calcium and phosphate ions.

• Acid-base balance: Buffers blood pH by absorbing or releasing alkaline salts.

• Blood formation: Red bone marrow produces blood cells (hematopoiesis).

Bones and Osseous Tissue

Composition and Properties

Bone is a dynamic connective tissue with a mineralized matrix, constantly remodeling and interacting with other body systems.

• Bone (osseous tissue): Matrix hardened by calcium phosphate and other minerals.

• Mineralization (calcification): Process of hardening bone tissue.

• Individual bones contain bone tissue, marrow, cartilage, adipose, nervous, and fibrous connective tissues.

• Richly supplied with nerves and blood vessels, reflecting metabolic activity.

Shapes of Bones

Classification by Shape

Bones are classified based on their shapes, which relate to their functions.

• Long bones: Longer than wide; function as rigid levers (e.g., femur, humerus).

• Short bones: Equal in length and width; allow multidirectional movement (e.g., carpals).

• Flat bones: Thin, often curved; protect soft organs (e.g., skull, ribs).

• Irregular bones: Complex shapes not fitting other categories (e.g., vertebrae).

General Features of Bones

Structural Components

Bones have distinct regions and tissues that contribute to their function and growth.

• Compact (dense) bone: Outer shell providing strength.

• Diaphysis: Shaft of long bone, provides leverage.

• Medullary cavity: Central space containing bone marrow.

• Epiphyses: Enlarged ends, strengthen joints and anchor ligaments/tendons.

• Spongy (cancellous) bone: Lightweight, found at bone ends and inside flat bones.

• Articular cartilage: Hyaline cartilage covering joint surfaces, reduces friction.

• Nutrient foramina: Small holes for blood vessel entry.

Membranes and Growth Regions

• Periosteum: External sheath; outer fibrous layer (collagen) and inner osteogenic layer (bone-forming cells).

• Endosteum: Thin reticular connective tissue lining marrow cavity.

• Epiphyseal plate (growth plate): Hyaline cartilage enabling lengthwise growth; becomes epiphyseal line in adults.

Structure of Bones

Long Bones

Long bones have a central shaft (diaphysis) and two ends (epiphyses), with compact and spongy bone, marrow cavity, articular cartilage, and periosteum.

Flat Bones

Flat bones have a sandwich-like structure: two layers of compact bone enclosing a middle layer of spongy bone (diploe), which absorbs shock.

Histology of Osseous Tissue

Bone Cell Types

Bone tissue consists of cells, fibers, and ground substance. Four principal cell types are involved in bone formation and maintenance.

• Osteogenic (osteoprogenitor) cells: Stem cells in endosteum, periosteum, and central canals; produce new osteoblasts.

• Osteoblasts: Bone-forming cells; synthesize organic matrix and promote mineral deposition.

• Osteocytes: Former osteoblasts trapped in matrix; reside in lacunae, connected by canaliculi; regulate bone density and mineral homeostasis.

• Osteoclasts: Large, multinucleated cells that dissolve bone (bone resorption); formed by fusion of stem cells.

Blood Vessels of Bone

Vascular Supply

Bones are highly vascularized, with specialized channels for blood flow.

• Nutrient foramina: Entry points for blood vessels.

• Perforating (Volkmann's) canals: Transverse/diagonal channels connecting blood supply.

• Central canals: Vertical channels within osteons.

• Circumferential and interstitial lamellae: Layers of bone matrix.

Spongy Bone

Structure and Function

Spongy bone has a porous, lattice-like structure, providing strength with minimal weight.

• Consists of spicules (slivers) and trabeculae (thin plates).

• Spaces filled with red bone marrow.

• Few osteons, no central canals; osteocytes close to marrow.

• Trabeculae align along lines of stress for optimal strength.

Bone Marrow

Types and Functions

Bone marrow is the soft tissue in bone cavities, crucial for blood cell production.

• Red marrow (myeloid tissue): Hemopoietic tissue producing blood cells; found in most bones of children and select adult bones (skull, vertebrae, ribs, sternum, pelvic girdle, proximal humerus/femur).

• Yellow marrow: Fatty tissue in adults; no longer produces blood cells.

Bone Development

Ossification Processes

Bone formation (ossification/osteogenesis) occurs by two main methods during fetal and infant development.

• Intramembranous ossification: Produces flat bones of skull and clavicle; develops within fibrous sheets.

• Endochondral ossification: Most bones form from a pre-existing cartilage model.

Intramembranous Ossification

• Mesenchyme condenses into soft tissue with blood capillaries.

• Mesenchymal cells differentiate into osteogenic cells, then osteoblasts.

• Osteoblasts deposit calcium phosphate in matrix; trabeculae form and mature into compact bone.

• Results in sandwich-like arrangement of flat bones.

Endochondral Ossification

• Mesenchyme forms hyaline cartilage model, covered by perichondrium.

• Perichondrium produces chondrocytes, then osteoblasts; forms bony collar.

• Primary ossification center: Chondrocytes enlarge, matrix calcifies, blood vessels invade.

• Primary marrow cavity: Forms as stem cells fill hollow cavity.

• Secondary ossification center: Forms in epiphyses, creating secondary marrow cavity.

Bone Growth and Remodeling

Growth Mechanisms

• Interstitial growth: Lengthwise growth via cartilage in epiphyseal plate; ends at epiphyseal line.

• Appositional growth: Widthwise growth by deposition of new bone at surface.

• Bone remodeling: Continuous process (10% per year); repairs microfractures, releases minerals, adapts to stress.

Dwarfism

Types and Causes

• Achondroplastic dwarfism: Long bones stop growing in childhood due to cartilage growth failure; normal torso, short limbs.

• Pituitary dwarfism: Lack of growth hormone; normal proportions, short stature.

Mineral Deposition

Process and Disorders

• Mineral deposition (mineralization): Calcium phosphate and other ions crystallize in bone tissue.

• Osteoblasts produce collagen fibers, which become encrusted with minerals.

• Abnormal calcification (ectopic ossification): Occurs in soft tissues (lungs, brain, arteries); calculus is a calcified mass.

Mineral Resorption

Release of Minerals

• Mineral resorption: Osteoclasts dissolve bone, releasing minerals into blood.

• Hydrochloric acid dissolves minerals; acid phosphatase digests collagen.

• Orthodontic appliances (braces) reposition teeth by stimulating bone resorption and deposition.

Calcium Homeostasis

Regulation and Importance

Calcium and phosphate are essential for bone structure and many physiological processes. Blood calcium is tightly regulated.

• Phosphate is a component of DNA, RNA, ATP, phospholipids, and pH buffers.

• Calcium is needed for neuron communication, muscle contraction, blood clotting, and exocytosis.

• Normal blood calcium: 9.2 to 10.4 mg/dl

• Hypocalcemia: Deficiency causes muscle spasms, tetany.

• Hypercalcemia: Excess causes sluggish reflexes, depression.

Bone Cell Type	Function	Location
Osteogenic cells	Stem cells, produce osteoblasts	Endosteum, periosteum, central canals
Osteoblasts	Form bone matrix	Bone surface, under periosteum/endosteum
Osteocytes	Maintain bone tissue, regulate minerals	Lacunae within bone matrix
Osteoclasts	Resorb/dissolve bone	Bone surface

Key Equation:

Normal blood calcium concentration:

$9.2 \text{ to } 10.4 \; \mathrm{mg/dl}$

Additional info: Hormonal regulation of calcium (calcitriol, calcitonin, PTH) is covered in further detail in subsequent slides/sections.