Bones and Skeletal Tissues: Structure, Function, and Classification

Bones and Skeletal Tissues

Introduction

The study of bones and skeletal tissues is fundamental to understanding the human body's support, movement, and protection systems. Knowledge of bone anatomy and the process of bone modeling is essential for diagnosing and treating bone diseases such as osteoporosis.

Skeletal Cartilages

Overview of Skeletal Cartilage

• Skeletal cartilage forms the initial human skeleton, which is later replaced by bone except in areas requiring flexibility.

• Cartilage is a resilient, molded tissue consisting primarily of water, providing flexibility and cushioning.

• It contains no blood vessels or nerves, making it avascular and aneural.

Perichondrium

• The perichondrium is a dense connective tissue layer surrounding cartilage, acting like a girdle.

• Functions:

  • Resists outward expansion of cartilage.

  • Contains blood vessels that deliver nutrients to cartilage cells.

Cartilage Cells and Matrix

• Cartilage is composed of chondrocytes, which are cells encased in small cavities called lacunae within a jelly-like extracellular matrix.

Types of Cartilage

• Hyaline cartilage:

  • Provides support, flexibility, and resilience.

  • Most abundant type; contains only collagen fibers.

  • Locations: articular (joints), costal (ribs), respiratory (larynx), and nasal cartilage (nose tip).

• Elastic cartilage:

  • Similar to hyaline cartilage but contains elastic fibers for greater flexibility.

  • Locations: external ear and epiglottis.

• Fibrocartilage:

  • Contains thick collagen fibers, providing great tensile strength.

  • Locations: menisci of the knee and intervertebral discs.

Growth of Cartilage

Mechanisms of Cartilage Growth

• Appositional growth:

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

  • Results in increased thickness of cartilage.

• Interstitial growth:

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

  • Results in increased length of cartilage.

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

Functions of Bones

Major Functions

• Support: Provides structural framework for the body and supports soft organs.

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

• Anchorage: Serves as levers for muscle action, enabling movement.

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

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

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

• Hormone production: Produces osteocalcin, which regulates insulin secretion, glucose levels, and metabolism.

Classification of Bones

By Location

• Axial skeleton: Long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular skeleton: Bones of the upper and lower limbs and girdles attaching limbs to the axial skeleton.

By Shape

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

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

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

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

Bone Structure

General Organization

• Bones are organs composed of various tissues: bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels.

• Three levels of structure: gross, microscopic, and chemical.

Gross Anatomy

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

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

Structure of Short, Irregular, and Flat Bones

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

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

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

• Hyaline cartilage covers areas of bone involved in movable joints.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis; consists of compact bone surrounding a central medullary cavity (yellow marrow in adults).

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

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

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces (except joints).

  • Fibrous layer: Dense irregular connective tissue.

  • Osteogenic layer: Contains osteoprogenitor stem cells.

  • Rich in nerve fibers and blood vessels; perforating fibers anchor periosteum to bone.

  • Serves as anchoring points for tendons and ligaments.

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

Blood Vessels and Nerves

• Bones are well vascularized.

• Nutrient arteries and veins serve the diaphysis, entering through the nutrient foramen.

• Epiphyseal arteries and veins serve the epiphyses.

• Nerves accompany blood vessels through the nutrient foramen.

Hematopoietic Tissue in Bones

• Red marrow is 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.

• In adults, red marrow is mainly in flat bones and some irregular bones (e.g., hip bones); yellow marrow can revert to red if needed (e.g., in anemia).

Bone Markings

• Serve as sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves.

• Three types:

  • Projections: Attachment sites for muscles and ligaments.

  • Surfaces: Help form joints.

  • Depressions and openings: Passageways for vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

• Five major types, all specialized forms of the same basic cell type:

  1. Osteoprogenitor cells (osteogenic cells): Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

  2. Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed of collagen and calcium-binding proteins; actively mitotic.

  3. Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress/strain sensors; communicate with osteoblasts and osteoclasts for bone remodeling.

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

  5. Osteoclasts: Derived from hematopoietic stem cells; giant, multinucleate cells responsible for bone resorption; located in resorption bays with ruffled borders to increase surface area for enzyme activity.

Microscopic Anatomy of Compact Bone

• Also called lamellar bone.

• Consists of:

  • Osteon (Haversian system): Structural unit; elongated cylinder running parallel to bone's long axis; composed of concentric rings (lamellae) with alternating collagen fiber orientation for strength.

  • Canals and canaliculi:

    • 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 between osteocytes.

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

  • Circumferential lamellae: Extend around the entire diaphysis, just deep to periosteum and superficial to endosteum; help resist twisting.

Microscopic Anatomy of Spongy Bone

• Appears disorganized but is structured along lines of stress for strength.

• Composed of trabeculae (like cables on a suspension bridge) with irregularly arranged lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Include osteoprogenitor cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.

• Osteoid: Makes up one-third of organic bone matrix; secreted by osteoblasts; consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Resilience is due to sacrificial bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone by mass; mainly tiny calcium phosphate crystals in and around collagen fibers.

• Responsible for bone's hardness and resistance to compression.

• Mineral composition allows bones to last long after death and provides information about ancient peoples.

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; collagen fibers only	Joints, ribs, larynx, nose tip
Elastic	Like hyaline, but with elastic fibers	External ear, epiglottis
Fibrocartilage	Thick collagen fibers; high tensile strength	Menisci of knee, intervertebral discs

Summary Table: Bone Cell Types

Cell Type	Function
Osteoprogenitor	Stem cells; differentiate into osteoblasts
Osteoblast	Bone formation; secrete osteoid
Osteocyte	Maintain bone matrix; act as stress sensors
Bone-lining cell	Maintain bone matrix on surfaces
Osteoclast	Bone resorption (breakdown)

Key Equations

• Bone strength (compression):

  • $\text{Strength} \propto \text{Mineral content}$

• Bone flexibility (tension):

  • $\text{Flexibility} \propto \text{Collagen content}$