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Tissues and Epithelial Tissue: Structure, Classification, and Function

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Tissues

Definition and Overview

  • Tissues are groups of cells that are similar in structure and perform a common or related function.

  • Histology is the study of tissues.

  • There are four basic tissue types in the human body:

    • Epithelial (covers)

    • Connective (supports)

    • Muscle (moves)

    • Nervous (controls)

  • Most organs contain all four tissue types.

Microscopy

Studying Tissue Structure

  • Microscopy allows for the study of tissue structure.

  • General steps in tissue preparation:

    1. Fixation (preserved)

    2. Sectioned (sliced thinly to allow light transmission)

    3. Stained (enhances contrast, reveals structures)

  • Light microscopy uses stains that bind to acidic or basic macromolecules, providing contrast.

  • TEM (Transmission Electron Microscopy) uses heavy metal stains for contrast by deflecting electrons to different extents.

  • SEM (Scanning Electron Microscopy) provides 3D images of an unsectioned tissue surface.

Epithelial Tissue

Definition and Functions

  • Epithelial tissue is a sheet of cells that covers a body surface or lines a cavity.

  • Main functions include:

    • Absorption (e.g., intestines)

    • Filtration (e.g., kidneys)

    • Excretion

    • Secretion (forms glands)

    • Sensory reception (supplied with nerves that detect changes in environment)

Characteristics of Epithelial Tissue

  • Polarity: Has two surfaces:

    • Apical surface: Not attached to surrounding tissue; exposed to outside of body or cavity of internal organ. May have microvilli to increase surface area for absorption.

    • Basal surface: Attached to underlying connective tissue; supported by a thin sheet called the basal lamina (consists of glycoproteins and collagen fibers).

  • Specialized contacts: Cells fit closely together to form continuous sheets.

  • Supported by connective tissue: Reinforced by a basement membrane (basal lamina + reticular lamina).

  • Avascular but innervated: No blood vessels; nourished by diffusion from underlying connective tissue; supplied by nerve fibers.

  • Regeneration: High regenerative capacity; can replace lost cells by cell division if nutrients are available.

Classification of Epithelial Tissue

  • Classified by two criteria:

    1. Number of cell layers:

      • Simple epithelium: Single cell layer; found where absorption, secretion, and filtration occur.

      • Stratified epithelium: Two or more cell layers; common in high-abrasion areas for protection.

    2. Cell shape:

      • Squamous: Flattened, scale-like

      • Cuboidal: Boxlike, as tall as they are wide

      • Columnar: Tall, column-shaped

Types of Epithelial Tissue

Type

Shape

Features

Function

Location/Action

Simple Squamous

Flattened laterally

Cytoplasm is sparse; thin and often permeable

Filtration, diffusion

Air sacs (lungs), lining of blood vessels, serosae

Simple Cuboidal

Boxlike, as tall as wide

Central nuclei; may have microvilli

Secretion, absorption

Small ducts of glands, kidney tubules

Simple Columnar

Tall, closely packed

Some have cilia or microvilli; may contain goblet cells (secrete mucus)

Absorption, secretion

Digestive tract, gallbladder, some ducts, bronchi, uterine tubes

Stratified Squamous

Surface cells are squamous; deeper layers are cuboidal/columnar

Thick, forms protective barrier

Protection

Subject to wear and tear (skin, mouth, esophagus)

Stratified Cuboidal

Two layers of cuboidal cells

Rare

Protection

Large ducts of some glands (sweat, mammary)

Stratified Columnar

Only apical layer is columnar

Rare

Protection, secretion

Male urethra, pharynx, some glandular ducts

Transitional Epithelium

Basal cells cuboidal/columnar; apical cells vary

Stretches readily

Permits stored urine to distend urinary organ

Ureters, bladder, part of urethra

Pseudostratified Columnar Epithelium

  • Single layer of cells of differing heights; appears multilayered.

  • Most ciliated; may contain goblet cells (secrete mucus).

  • Function: Absorption, secretion.

  • Location: Respiratory passages.

Glandular Epithelium

  • Consists of one or more cells that make and secrete a particular product (secretion).

  • Classified by:

    • Endocrine glands: Ductless, produce hormones, secrete into blood or lymph.

    • Exocrine glands: Secrete products onto body surfaces (skin) or into body cavities; may be unicellular (e.g., goblet cells) or multicellular (e.g., sweat glands).

Type

Unicellular

Multicellular

Cells/Parts

Mucous cells, goblet cells (produce mucin)

Duct + secretory unit (acinus); supported by connective tissue; supplied by blood vessels and nerve fibers

Modes of Secretion

Secrete via exocytosis

Merocrine (secrete by exocytosis), holocrine (accumulate product until rupture), apocrine (accumulate product, apex pinches off)

Comparison: Epithelial vs. Connective Tissue

  • Epithelial: Many cells tightly packed, little to no extracellular matrix (ECM).

  • Connective: Few scattered cells, surrounded by large amount of ECM.

ECM = extracellular matrix

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