Bones and Skeletal Tissues: Structure, Function, and Microscopic Anatomy

Bones and Skeletal Tissues

Introduction

The skeletal system provides the framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals and blood cell production. This section covers the structure, classification, and microscopic anatomy of bones and skeletal tissues.

Skeletal Cartilages

Types and Functions

• Chondrocytes are the primary cells found in cartilage, residing in spaces called lacunae and surrounded by an extracellular matrix.

• Three main types of cartilage:

  • Hyaline cartilage: Provides support, flexibility, and resilience. Found in articular surfaces, costal cartilages, respiratory structures, and the nasal cartilage.

  • Elastic cartilage: Contains more elastic fibers, allowing for flexibility. Located in the external ear and epiglottis.

  • Fibrocartilage: Highly compressible with great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

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

Growth of Cartilage

• Appositional growth: New matrix is secreted on the external surface of cartilage by chondroblasts in the perichondrium.

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

• Calcification: Occurs during normal bone growth and sometimes in old age, where cartilage becomes hardened due to calcium salt deposits.

Classification of Bones

By Location and Shape

• There are 206 named bones in the adult human skeleton.

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

• Appendicular skeleton: Bones of the limbs and girdles.

• Four bone shapes:

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

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

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

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

Functions of Bones

• Support: For body and soft organs.

• Protection: For brain, spinal cord, and vital organs.

• Movement: 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: In bone cavities.

• Hormone production: Osteocalcin regulates bone formation and influences energy metabolism.

Bone Structure

Gross Anatomy

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

• Spongy bone (cancellous or trabecular): Honeycomb of flat pieces called trabeculae, found deep to compact bone.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone covered by compact bone.

• Bone is sandwiched between connective tissue membranes: periosteum (outer) and endosteum (inner).

• No shaft or epiphyses; bone marrow throughout spongy bone but no marrow cavity.

• Hyaline cartilage covers articular surfaces.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis; contains compact bone surrounding the medullary cavity.

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

• Epiphyseal line: Remnant of childhood bone growth at the junction of diaphysis and epiphysis.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except at joints. Outer fibrous layer of dense irregular connective tissue; inner osteogenic layer contains osteogenic cells. Rich in nerve fibers and blood vessels; anchoring points for tendons and ligaments.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces; contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red marrow: Found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum). In adults, red marrow is found in heads of femur and humerus, and in flat bones. Yellow marrow can convert to red marrow if necessary.

Bone Markings

• Projections: Sites of muscle and ligament attachment.

• Surfaces: Form joints.

• Depressions and openings: Allow passage of blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Tissue Cell Types

• Osteogenic cells: Also called osteoprogenitor cells; stem cells in periosteum and endosteum that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete unmineralized bone matrix (osteoid) containing collagen and calcium-binding proteins; actively mitotic.

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

• Bone lining cells: Flat cells on bone surfaces; help maintain matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Bone-resorbing cells derived from hematopoietic stem cells; giant, multinucleate; break down bone matrix.

Compact Bone (Microscopic Anatomy)

• Osteon (Haversian system): Structural unit of compact bone; elongated cylinder parallel to long axis of bone.

• Lamellae: Hollow tubes of bone matrix; collagen fibers in adjacent rings run in different directions for strength.

• Central (Haversian) canal: Runs through the core of each osteon; contains blood vessels and nerve fibers.

• Perforating (Volkmann's) canals: Canals at right angles to the central canal; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae to each other and the central canal; allow osteocytes to communicate and exchange nutrients.

Lamellae Types

• Interstitial lamellae: Incomplete lamellae not part of a complete osteon; fill gaps between forming osteons or remnants of bone remodeling.

• Circumferential lamellae: Just deep to periosteum and superficial to endosteum; extend around the entire surface of the diaphysis.

Canaliculi Formation

• Osteoblasts secrete bone matrix and maintain contact with each other and osteocytes via cell projections with gap junctions.

• When matrix hardens and cells are trapped, canaliculi form, allowing communication and nutrient/waste transfer.

Table: Classification of Bones by Shape

Shape	Description	Examples
Long bones	Longer than wide; shaft with two ends	Femur, humerus
Short bones	Cube-shaped; mostly spongy bone	Carpals, tarsals, patella (sesamoid)
Flat bones	Thin, flattened, usually curved	Sternum, ribs, skull bones
Irregular bones	Complicated shapes	Vertebrae, hip bones

Table: Major Functions of Bones

Function	Description
Support	Framework for body and soft organs
Protection	Protects brain, spinal cord, and vital organs
Movement	Levers for muscle action
Mineral storage	Reservoir for calcium and phosphorus
Blood cell formation	Hematopoiesis in red marrow
Triglyceride storage	Fat storage in bone cavities
Hormone production	Osteocalcin secretion for bone formation and metabolism

Key Equations and Terms

• Osteoid: Unmineralized bone matrix secreted by osteoblasts; contains collagen and calcium-binding proteins.

• Hematopoiesis: Blood cell formation occurring in red marrow.

• Osteon: Fundamental structural unit of compact bone.

Example: The femur is a long bone that supports body weight and enables movement by acting as a lever for muscles.

Additional info: The periosteum and endosteum are essential for bone growth, repair, and remodeling, as they house osteogenic cells that can differentiate into bone-forming or bone-resorbing cells as needed.