Tissue Level of Organization: Structure and Function of Human Tissues

Tissue Level of Organization

Introduction to Tissues

The human body is composed of approximately 75 trillion cells, which are organized into groups called tissues. A tissue is a group of cells performing similar functions. All human body cells belong to one of four basic tissue groups, each with distinct structures and functions.

• Tissue: A group of similar cells and their extracellular products that perform a common function.

• Extracellular matrix: Substance produced by tissue cells, consisting of protein fibers, salts, water (H2O), and dissolved macromolecules, located outside of cells.

Tissue Types

Overview of the Four Basic Tissue Types

There are four main categories of tissues in the human body, each with unique characteristics and functions. These tissues originate from specific embryonic germ layers.

Tissue Type	General Characteristics	General Functions	Primary Germ Layer Derivative
Epithelial tissue	Cellular, polar, attached, avascular, innervated, high regeneration capacity	Covers surfaces; lines insides of organs and body cavities	Endoderm, mesoderm, ectoderm
Connective tissue	Diverse types; all contain cells, protein fibers, and ground substance	Protects, binds together, and supports organs	Mesoderm
Muscle tissue	Contractile; receives stimulation from nervous system and/or endocrine system	Facilitates movement of skeleton or organ walls	Mesoderm
Nervous tissue	Neurons: excitable, high metabolic rate, extreme longevity, nonmitotic Glial cells: nonexcitable, mitotic	Neurons: control activities, process information Glial cells: support and protect neurons	Ectoderm

Characteristics of Epithelial Tissue

Structural and Functional Features

Epithelial tissue is specialized to cover body surfaces, line cavities, and form glands. It is composed almost entirely of tightly packed cells with minimal extracellular matrix.

• Cells are joined by intercellular junctions.

• Exhibits polarity: has an apical (top) surface and a basal (bottom) surface attached to underlying tissues.

• The basal surface is attached to a thin, acellular basement membrane (produced by both epithelial and connective tissue).

• Lacks blood vessels (avascular); nutrients diffuse from underlying tissues.

• Richly innervated to detect environmental changes.

• Cells are frequently damaged or lost and are replaced rapidly by cell division.

Epithelial Tissue Structure

Cellular Arrangement and Surfaces

Epithelial cells are arranged in continuous sheets, either as a single layer or multiple layers, with very little intercellular space.

• Apical surface: Faces the body surface, organ lumen, or duct; may have cilia or microvilli.

• Lateral surface: Faces adjacent cells; contains tight junctions and other cell junctions.

• Basal surface: Opposite the apical surface; deepest cells attach to the basement membrane.

• Basement membrane: Composed of basal lamina (from epithelial cells) and reticular lamina (from connective tissue); provides support and anchorage for the epithelium.

Intercellular Junctions

Types and Functions of Cell Junctions

Epithelial cells are strongly bound to each other on their lateral surfaces by specialized membrane structures called intercellular junctions. These junctions maintain tissue integrity and regulate the passage of substances.

• Tight junctions: Seal adjacent cells near their apical surface, preventing passage of substances between cells.

• Adherens junctions: Form adhesion belts that provide mechanical attachment between cells via actin filaments.

• Desmosomes: Button-like junctions that provide strong adhesion by linking intermediate filaments of adjacent cells.

• Hemidesmosomes: Attach epithelial cells to the basement membrane using integrins and intermediate filaments.

• Gap junctions: Allow direct communication between adjacent cells through connexons, permitting the passage of ions and small molecules.

Example: Tight junctions in the intestinal epithelium prevent leakage of digestive enzymes and pathogens into underlying tissues.