Tissues, Histology, and Glands: Structure and Function in Anatomy & Physiology

Tissues, Organs, Histology, and Functions of the Four Tissue Types

Introduction

In multicellular animals, tissues are groups of cells specialized for specific functions. Organs are structures composed of two or more tissue types that work together to perform particular functions. Histology is the study of tissues. The four primary tissue types in the human body are: epithelial, connective, muscle, and nervous.

• Epithelial tissue: Covering and lining surfaces; protection, absorption, filtration, secretion.

• Connective tissue: Support, protection, insulation, transportation.

• Muscle tissue: Movement.

• Nervous tissue: Control and communication.

The arrangement of tissues determines the organ's structure and capabilities.

Special Characteristics and Classification of Epithelial Tissue

Specialized Characteristics of Epithelium

• Cellularity: Composed almost entirely of cells with minimal extracellular material.

• Specialized contacts: Cells are tightly joined by junctions for strength and communication.

• Polarity: Epithelial cells have an apical (free) surface exposed to the body exterior or cavity, and a basal surface attached to underlying tissue.

• Basement Membrane: Basal surface rests on a thin supporting layer separating epithelium from connective tissue.

• Regeneration: High regenerative capacity due to rapid cell division.

Classification of Epithelia

• Number of layers:

  • Simple epithelium: Single layer of cells; absorption and filtration.

  • Stratified epithelium: Multiple layers; protection in areas of abrasion.

• Shape of cells:

  • Squamous: Flat, scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall, column-like.

Major Classifications of Simple Epithelium:

• Simple squamous

• Simple cuboidal

• Simple columnar

• Pseudostratified/Highly modified

Major Classifications of Stratified Epithelium:

• Stratified squamous

• Stratified cuboidal

• Stratified columnar

• Transitional epithelium

Types of Simple Epithelium

• Simple Squamous: Single layer, flat cells; found in air sacs of lungs, lining of heart, blood vessels. Functions in diffusion and filtration.

• Simple Cuboidal: Single layer, cube-shaped cells; found in kidney tubules, ducts, glands. Functions in secretion and absorption.

• Simple Columnar: Single layer, tall cells; lines digestive tract. Functions in absorption and secretion. May have microvilli or goblet cells.

• Pseudostratified Epithelium: Appears layered but all cells touch basement membrane. Found in respiratory tract. Functions in secretion and absorption.

Types of Stratified Epithelium

• Stratified Squamous: Several layers; protects against abrasion. Found in skin, mouth, esophagus.

• Stratified Cuboidal: Two layers; found in ducts of sweat glands.

• Stratified Columnar: Rare; found in male urethra and some glands.

• Transitional Epithelium: Multiple layers; cells change shape. Found in urinary bladder.

Glandular Epithelium

Introduction

Glandular epithelium consists of cells that make and secrete a product called secretion. Glands can be classified as endocrine (secrete into blood) or exocrine (secrete onto body surfaces or into cavities).

• Endocrine glands: Ductless; secrete hormones directly into blood.

• Exocrine glands: Secrete products via ducts; e.g., sweat, salivary glands.

Types of Exocrine Glands

• Unicellular: Single cells, e.g., goblet cells producing mucus.

• Multicellular: Composed of a duct and secretory unit. Classified by duct structure:

  • Tubular: Secretory cells form tubes.

  • Alveolar: Secretory cells form sacs.

  • Tubuloalveolar: Combination of tubes and sacs.

Mode of Secretion

• Merocrine: Secrete by exocytosis; most common (e.g., sweat glands).

• Holocrine: Accumulate products until cell ruptures (e.g., sebaceous glands).

• Apocrine: Accumulate products and pinch off part of cell (rare in humans).

Characteristics of Connective Tissue

Introduction

Connective tissue is the most abundant and widely distributed tissue in the body. It supports, protects, insulates, and transports substances. Connective tissue has many forms, including bone, cartilage, blood, and fat.

• Binding and support

• Protection

• Insulation

• Transportation (blood)

Common Characteristics of Connective Tissue

• Common Origin: All connective tissues arise from mesenchyme (embryonic tissue).

• Degrees of Vascularity: Varying blood supply; cartilage is avascular, bone is highly vascular.

• Extracellular Matrix: Composed of ground substance and fibers; separates cells and allows tissue to bear weight and endure stress.

Structural Elements of Connective Tissue

• Ground Substance: Material that fills space between cells; contains interstitial fluid, cell adhesion proteins, and proteoglycans.

• Fibers:

  • Collagen fibers: Strongest and most abundant; provide tensile strength.

  • Elastic fibers: Long, thin, allow stretch and recoil.

  • Reticular fibers: Short, fine, form networks supporting soft tissue.

• Cells: Each class of connective tissue has a fundamental cell type (e.g., fibroblasts, chondroblasts, osteoblasts, hematopoietic stem cells).

Types of Connective Tissue

• Connective Tissue Proper:

  • Loose connective tissue: Areolar, adipose, reticular.

  • Dense connective tissue: Dense regular, dense irregular, elastic.

• Cartilage

• Bone

• Blood

Loose Connective Tissue

• Areolar: Gel-like matrix; wraps and cushions organs.

• Adipose: Fat cells; stores energy, insulates, supports organs.

• Reticular: Network of reticular fibers; supports lymphoid organs.

Dense Connective Tissue

• Dense regular: Parallel collagen fibers; found in tendons and ligaments.

• Dense irregular: Irregularly arranged collagen fibers; found in dermis, joint capsules.

• Elastic: High proportion of elastic fibers; found in walls of large arteries.

Additional info:

• Connective tissue cells include macrophages (defense), mast cells (inflammation), and white blood cells (immune response).

• Cartilage cells are called chondrocytes; bone cells are osteocytes.