Tissues: Structure, Function, and Classification in Human Anatomy & Physiology

Tissues

Introduction to Tissues

Tissues are groups of cells that are similar in structure and function. In the human body, tissues are fundamental building blocks that organize cells into functional units. There are four primary tissue types, each with distinct roles and characteristics.

• Epithelial tissue: Forms boundaries between different environments, protects, secretes, absorbs, and filters.

• Connective tissue: Supports, protects, and binds other tissues together.

• Muscle tissue: Contracts to cause movement.

• Nervous tissue: Enables internal communication by forming the brain, spinal cord, and nerves.

The Four Primary Tissue Types

Epithelial Tissue

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• Locations: Covers body surfaces (skin), lines internal organs and cavities (digestive, respiratory, reproductive, urinary tracts), and forms glands.

• Characteristics:

  • Cells are closely packed in continuous sheets held by tight junctions and desmosomes.

  • Exhibits polarity: has an apical (free) surface and a basal (attached) surface.

  • Supported by a basement membrane (composed of basal and reticular lamina).

  • Avascular (lacks blood vessels) but innervated (has nerve supply).

  • High regenerative capacity (mitotic).

  • Minimal extracellular matrix.

Connective Tissue

• Functions: Binding and support, protection, insulation, and transportation (blood).

• Characteristics:

  • Most abundant and widely distributed tissue type.

  • Varying degrees of vascularity (blood supply).

  • Composed of specialized living cells separated by a nonliving extracellular matrix.

  • Abundant extracellular material (matrix) defines the tissue, not the cells themselves.

Muscle Tissue

• Function: Has the ability to contract and cause movement.

• Characteristics:

  • Vascular (rich blood supply).

  • Defined by their contractile cells.

Nervous Tissue

• Function: Forms the brain, spinal cord, and nerves; responsible for internal communication.

• Characteristics:

  • Vascular.

  • Defined by excitable cells (neurons) and supporting cells (neuroglia).

Epithelial Tissue (Epithelium)

Types of Epithelial Tissue by Location

1. Covering & Lining Epithelia

   • External surfaces: Skin

   • Internal surfaces: Digestive, respiratory, reproductive, and urinary tracts (open to the outside); chest and abdominal cavities, endothelium of blood vessels and heart (closed).

2. Glandular Epithelia

   • Clusters deep to covering & lining epithelium.

   • Form the secretory tissue in glands.

Characteristics of Epithelial Tissue

• Defined by their cells, not by extracellular matrix.

• Continuous sheets held by tight junctions and desmosomes.

• Exhibit polarity: apical (upper, free) surface may have microvilli (e.g., small intestine) or cilia (e.g., respiratory tract); basal (lower, attached) surface rests on a basement membrane.

• Basement membrane consists of basal lamina and reticular lamina, anchoring epithelium to underlying connective tissue.

• Avascular but innervated.

• High rate of regeneration.

• Minimal extracellular matrix.

Classification of Epithelial Tissue

• By number of cell layers:

  • Single layer = Simple epithelium

  • Two or more layers = Stratified epithelium

• By cell shape:

  • Squamous: Flat, scale-like cells

  • Cuboidal: Cube-shaped cells

  • Columnar: Tall, column-like cells

• Named according to the shape of the apical (surface) cells.

Summary Table: Epithelial Tissue Types

Type	Structure	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration, secretion	Air sacs of lungs, lining of heart, blood vessels, serosae
Simple Cuboidal	Single layer, cube-like cells	Secretion, absorption	Kidney tubules, ducts, small glands
Simple Columnar	Single layer, tall cells (may have microvilli or cilia)	Absorption, secretion of mucus/enzymes	Digestive tract, gallbladder, uterine tubes
Stratified Squamous	Multiple layers, surface cells flat	Protection from abrasion	Skin (keratinized), mouth, esophagus, vagina (non-keratinized)
Stratified Cuboidal	Typically two layers, cube-like cells	Protection	Sweat glands, mammary glands (rare)
Stratified Columnar	Several layers, surface cells columnar	Protection, secretion	Male urethra, pharynx, some glandular ducts (rare)
Pseudostratified Columnar	Single layer, nuclei at different levels	Secretion, propulsion of mucus	Trachea, upper respiratory tract (ciliated); male ducts (non-ciliated)
Transitional	Multiple layers, surface cells dome-shaped or squamous depending on stretch	Stretches readily	Urinary bladder, ureters, part of urethra

Glandular Epithelium

• Glands: One or more cells that secrete an aqueous fluid.

• Unicellular glands: Single-celled (e.g., goblet cells).

• Multicellular glands: Composed of many cells.

Type	Secretion Method	Examples
Exocrine	Secrete into ducts or onto body surfaces/cavities	Salivary glands, sweat glands
Endocrine	Secrete hormones directly into blood or lymph (ductless)	Pituitary gland, thyroid gland

Additional info: Exocrine glands include both unicellular (goblet cells) and multicellular types. Endocrine glands secrete hormones that regulate various physiological processes.