Tissue - Epithelial

Introduction to Tissues

Tissues are groups of cells that perform common or related functions. In the human body, tissues are specialized to maintain homeostasis and support organ function. The study of tissues is called histology.

• Four basic tissue types:

  1. Epithelial

  2. Connective

  3. Muscle

  4. Nervous

• Structural organization:

  • Chemical level: atoms, molecules, organelles

  • Cellular level: single cell

  • Tissue level: groups of similar cells

  • Organ level: contains two or more types of tissues

  • Organ system level: organs that work closely together

  • Organismal level: all organ systems combined

Epithelial Tissue

Definition and General Features

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Two main forms:

  • Covering and lining epithelium: Covers external and internal surfaces (e.g., skin).

  • Glandular epithelium: Secretory tissue in glands (e.g., salivary glands).

Functions of Epithelial Tissue

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

Distinguishing Characteristics of Epithelial Tissue

• Polarity: Cells have an apical (top) and basal (bottom) surface. The apical surface is exposed to the exterior or cavity, while the basal surface is attached to underlying tissue via the basal lamina.

• Specialized contacts: Epithelial cells fit closely together, forming continuous sheets. Specialized junctions (tight junctions, desmosomes) bind adjacent cells.

• Supported by connective tissues: All epithelial sheets rest upon and are supported by connective tissue, including the reticular lamina (network of collagen fibers).

• Avascular, but innervated: Epithelial tissues lack blood vessels and receive nutrients by diffusion from underlying connective tissues. They do have a supply of nerve fibers.

• Regeneration: Epithelial cells have high regenerative capacity, especially when exposed to friction or hostile environments. Adequate nutrients and cell division are required for regeneration.

Additional info: The basal lamina and reticular lamina together form the basement membrane, which reinforces the epithelial sheet and resists stretching and tearing.

Classification of Epithelia

Epithelial tissues are classified based on the number of cell layers and the shape of the cells in the apical layer.

• Number of cell layers:

  • Simple epithelia: Single layer of cells; involved in absorption, secretion, and filtration.

  • Stratified epithelia: Two or more layers; more durable and involved in protection.

• Cell shape:

  • Squamous: Flattened and scale-like

  • Cuboidal: Box-like, cube

  • Columnar: Tall, column-like

Note: In stratified epithelia, cell shape can vary in each layer, but the tissue is named according to the shape in the apical layer.

Types of Epithelial Tissue

Simple Epithelia

• Simple squamous epithelium:

  • Cells are flattened laterally; cytoplasm is sparse.

  • Function: rapid diffusion (e.g., kidney, lungs).

  • Special types:

    • Endothelium: Lining of lymphatic vessels, blood vessels, and heart.

    • Mesothelium: Serous membranes in the ventral body cavity.

• Simple cuboidal epithelium:

  • Single layer of cube-like cells.

  • Function: secretion and absorption.

  • Location: forms walls of ducts of glands and kidney tubules.

• Simple columnar epithelium:

  • Single layer of tall, closely packed cells; may have cilia.

  • Function: absorption, secretion of mucus, enzymes, and other substances.

  • Location: digestive tract, gallbladder, ducts of glands, bronchi, uterine tubes.

• Pseudostratified columnar epithelium:

  • Cells vary in height and appear multi-layered, but all cells rest on the basement membrane.

  • Function: secretion, movement of mucus via ciliary action.

  • Location: upper respiratory tract, ducts of large glands, tubules in testes.

Stratified Epithelial Tissues

• Stratified squamous epithelium:

  • Most widespread stratified epithelium.

  • Free surface cells are squamous; deeper layers are cuboidal or columnar.

  • Function: protection against abrasion.

  • Location: skin (keratinized), moist linings of mouth, esophagus, vagina (nonkeratinized).

• Stratified cuboidal epithelium:

  • Rare; found in some sweat and mammary glands.

  • Typically two cell layers thick.

• Stratified columnar epithelium:

  • Limited distribution; found in pharynx, male urethra, some glandular ducts.

  • Usually occurs at transition areas between other types of epithelia.

  • Only apical layer is columnar.

• Transitional epithelium:

  • Forms lining of hollow urinary organs (bladder, ureters, urethra).

  • Basal layers are cuboidal or columnar; apical cells change shape when stretched.

  • Function: allows increased flow of urine and storage space.

Glandular Epithelia

A gland is one or more cells that makes and secretes an aqueous fluid called a secretion. Glands are classified by site of product release and number of cells forming the gland.

• Site of product release:

  • Endocrine glands: Internally secreting (e.g., hormones); ductless; secrete into blood or lymph.

  • Exocrine glands: Externally secreting (e.g., sweat); secrete onto body surfaces or into body cavities; have ducts.

• Relative number of cells forming the gland:

  • Unicellular: e.g., goblet cells (mucus-secreting cells in intestinal and respiratory tracts).

  • Multicellular: composed of a duct and secretory unit; usually surrounded by supportive connective tissue.

Exocrine Gland Types

• Unicellular exocrine glands: Mucous cells and goblet cells; produce mucin, a glycoprotein that forms mucus.

• Multicellular exocrine glands: Classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, tubuloalveolar).

Summary Table: Epithelial Tissue Classification

Key Terms and Definitions

• Apical surface: The upper free surface exposed to the body exterior or cavity.

• Basal surface: The lower attached surface facing the underlying tissue.

• Basal lamina: Noncellular, adhesive sheet supporting the basal surface.

• Reticular lamina: Layer of extracellular material containing collagen fibers.

• Endothelium: Simple squamous epithelium lining blood vessels and heart.

• Mesothelium: Simple squamous epithelium lining serous membranes.

• Mucin: Glycoprotein that dissolves in water to form mucus.

Relevant Equations

• Surface area of absorption (with microvilli): Where is total surface area, is number of microvilli, is area of each microvillus.

Summary

• Epithelial tissue is essential for protection, absorption, secretion, and sensation.

• Classified by cell layers and cell shape, with specialized forms for different functions and locations.

• Glandular epithelia form both endocrine and exocrine glands, vital for body regulation and homeostasis.