Bones and Skeletal Tissues: Structure, Function, and Development

Bones and Skeletal Tissues

Introduction to the Skeletal System

The skeletal system forms the structural framework of the human body, providing support, protection, and facilitating movement. It is composed of bones and associated skeletal tissues, including cartilage, which play critical roles in growth, repair, and mineral storage.

6.1 Skeletal Cartilages

Structure and Types of Cartilage

• Skeletal cartilage is a resilient tissue primarily composed of water, lacking blood vessels and nerves.

• Perichondrium is a dense connective tissue layer that surrounds cartilage, providing nutrients and resisting outward expansion.

• Cartilage cells, called chondrocytes, are found in lacunae within a jelly-like extracellular matrix.

Types of Cartilage

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

• Elastic cartilage: Contains elastic fibers; found in the external ear and epiglottis.

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

6.2 Functions of Bones

Major Functions

• 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.

• Triglyceride (fat) storage: Stored in bone cavities for energy.

• Hormone production: Osteocalcin regulates insulin secretion, glucose levels, and metabolism.

6.3 Classification of Bones

Axial and Appendicular Skeleton

• Axial skeleton: Skull, vertebral column, rib cage (long axis of the body).

• Appendicular skeleton: Limbs and girdles attaching limbs to the axial skeleton.

Bone Shapes

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

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

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

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

6.4 Bone Structure

Gross Anatomy of Bone

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

• Spongy bone: Honeycomb structure of trabeculae; spaces filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploë) covered by compact bone.

• Periosteum covers outside; endosteum covers inside.

• Bone marrow scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers areas involved in movable joints.

Structure of a Typical Long Bone

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

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

• Epiphyseal line: Remnant of growth plate between diaphysis and epiphysis.

Bone Membranes

• Periosteum: Double-layered membrane covering external surfaces (except joints); outer fibrous layer (dense irregular connective tissue), inner osteogenic layer (bone-forming cells).

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

Bone Marrow

• Red marrow: Hematopoietic tissue found in trabecular cavities of spongy bone and diploë of flat bones; active in newborns and certain adult bones (e.g., femur, hip bone).

• Yellow marrow: Fat storage; can convert to red marrow if needed (e.g., anemia).

Bone Markings

Bone markings serve as sites for muscle, ligament, and tendon attachment, joint formation, and passageways for blood vessels and nerves. They are classified as projections, depressions, or openings.

Name of Bone Marking	Description	Illustration
Crest	Narrow ridge of bone; usually prominent	Hip bone crest
Spine	Sharp, slender, often pointed projection	Vertebral spine
Foramen	Round or oval opening through a bone	Skull foramen
Fossa	Shallow, basinlike depression	Mandibular fossa
Condyle	Rounded articular projection	Mandibular condyle
Meatus	Canal-like passageway	Ear canal
Others	Various other projections and depressions	See illustrations

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).

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

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

• Osteoclasts: Multinucleate cells; resorb (break down) bone matrix.

Compact Bone Structure

• Osteon (Haversian system): Structural unit; elongated cylinders with concentric lamellae (rings of bone matrix).

• 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; allow communication and nutrient/waste exchange.

• Interstitial and circumferential lamellae: Fill gaps between osteons and encircle the diaphysis, respectively.

Spongy Bone Structure

• Composed of trabeculae aligned along lines of stress.

• No osteons; trabeculae contain irregularly arranged lamellae and osteocytes connected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes bone cells and osteoid (ground substance and collagen fibers).

• Provides tensile strength and flexibility.

• Sacrificial bonds between collagen molecules dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts), mainly calcium phosphate crystals.

• Responsible for bone hardness and resistance to compression.

• Bones are half as strong as steel in compression, as strong as steel in tension.

6.5 Bone Development (Ossification)

Overview of Ossification

• Ossification (osteogenesis) is the process of bone tissue formation.

• Begins in the embryo (month 2), continues postnatally until early adulthood, and persists as remodeling throughout life.

Types of Bone Formation

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

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

Growth in Length of Long Bones

• Occurs at the epiphyseal plate via interstitial growth.

• Plate consists of five zones: resting, proliferation, hypertrophic, calcification, and ossification.

• Chondrocytes proliferate, enlarge, die, and are replaced by bone tissue, resulting in bone lengthening.

Growth in Width (Appositional Growth)

• Bones thicken in response to stress or added weight.

• Osteoblasts add bone matrix to external surface; osteoclasts remove bone from internal surface.

• Results in thicker, stronger bones without excessive weight.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates epiphyseal plate activity in children.

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

• Sex hormones (testosterone, estrogens): Promote growth spurts and induce epiphyseal plate closure at puberty.

6.6 Bone Remodeling

Bone Remodeling Process

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

• Remodeling occurs at periosteal and endosteal surfaces; about 5–7% of bone mass is recycled weekly.

• Remodeling units consist of adjacent osteoblasts and osteoclasts.

Bone Resorption

• Osteoclasts digest bone matrix, releasing calcium and phosphate into the blood.

• Enzymes and acids break down matrix; digested products are released into interstitial fluid and blood.

• Osteoclast activity is regulated by parathyroid hormone (PTH) and immune proteins.

Bone Deposit

• Osteoblasts secrete new bone matrix (osteoid seam).

• Calcification occurs when sufficient calcium and phosphate are present, and alkaline phosphatase activity is high.

Control of Bone Remodeling

• Hormonal controls: PTH increases blood calcium by stimulating osteoclasts; calcitonin (from thyroid) lowers blood calcium at high doses.

• Mechanical stress: Bone adapts to stress by thickening or remodeling along lines of force.

Summary Table: Bone Cells and Functions

Cell Type	Origin	Function
Osteogenic cell	Mesenchymal stem cell	Bone stem cell; differentiates into osteoblasts
Osteoblast	Osteogenic cell	Bone-forming cell; secretes osteoid
Osteocyte	Osteoblast	Mature bone cell; maintains bone matrix
Bone-lining cell	Osteoblast	Maintains bone matrix on surfaces
Osteoclast	Hematopoietic stem cell	Bone-resorbing cell

Additional info: Understanding bone structure and remodeling is essential for diagnosing and treating bone diseases such as osteoporosis, and for appreciating the dynamic nature of the skeletal system throughout life.