Epithelial Tissue

Basal Feature: The Basal Lamina

The basal lamina is a supporting sheet of proteins secreted by epithelial cells. It forms part of the basement membrane, providing structural support and regulating cell behavior.

• Composition: Proteins secreted by epithelial cells.

• Function: Supports epithelial cells, acts as a selective filter, and provides a surface for cell migration during repair.

Types of Epithelial Tissue

Epithelial tissues are classified based on cell shape and arrangement. Each type has distinct functions and locations in the body.

• Simple Squamous: Single layer of flat cells; lines blood vessels and air sacs of lungs. Permits exchange of nutrients, wastes, and gases.

• Simple Cuboidal: Single layer of cube-shaped cells; lines kidney tubules and glands. Secretes and reabsorbs water and small molecules.

• Simple Columnar: Single layer of tall cells; lines most digestive organs. Absorbs nutrients, produces mucus.

• Stratified Squamous: Multiple layers; outer layer of skin, mouth, vagina. Protects against abrasion, drying out, infection.

• Stratified Cuboidal: Multiple layers of cube-shaped cells; lines ducts of sweat glands. Secretes water and ions.

• Pseudostratified Columnar: Appears layered but all cells touch the basement membrane; lines epididymis, mammary glands, larynx. Secretes mucus.

• Transitional: Specialized for stretching; lines urinary bladder, urethra, ureters.

Properties of Epithelial Tissue

Epithelial tissue exhibits several distinguishing characteristics that enable its functions in protection, absorption, and secretion.

• Cellularity: Cells are tightly bound together in sheets.

• Polarity: Distinct apical (exposed) and basal (attached) surfaces.

• Attachment: Cells rest on a basement membrane.

• Avascularity: Lacks blood vessels but is innervated.

• Regeneration: Capable of rapid cell division.

Polarity of Epithelial Cells

Epithelial cells are polar, with distinct surfaces:

• Apical Surface: Faces the exposed outer or inner surface of the organ.

• Basal Surface: Attached to underlying connective tissue via the basement membrane.

Support by Connective Tissue

All epithelial sheets are supported by connective tissue, which provides structural integrity and nourishment.

• Reticular Lamina: Deep to basal lamina; consists of collagen fibers.

• Basement Membrane: Composed of basal and reticular lamina; reinforces epithelial sheet, resists stretching and tearing, defines boundary.

Subdivision of Epithelial Tissue

Epithelial cells are subdivided into:

• Glandular Epithelium: Specialized for secretion (endocrine and exocrine glands).

• Surface Epithelium: Covers surfaces and lines cavities.

Classification and Naming of Epithelia

Epithelia are classified by arrangement (layers) and shape of cells.

• Arrangement: Simple (one layer) or Stratified (multiple layers).

• Shape: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall).

• Naming: First name indicates layers, second name indicates cell shape. Accessory structures (e.g., goblet cells, cilia, microvilli, keratin) may be included.

• Special Types: Pseudostratified and Transitional epithelia.

Classification of Epithelial Tissues

Simple Squamous Epithelium

Simple squamous epithelium forms the thinnest membranes in the body, facilitating rapid passage of materials and secretion of lubricating substances.

• Special Types: Endothelium (lining blood vessels), Mesothelium (lining body cavities).

• Function: Passage of materials, secretion of lubricants.

• Location: Air sacs of lungs, lining of heart, blood and lymphatic vessels.

Simple Cuboidal Epithelium

Simple cuboidal epithelium is adapted for secretion and absorption.

• Function: Secretion and absorption.

• Location: Kidney tubules, pancreatic ducts.

Simple Columnar Epithelium

Simple columnar epithelium lines the digestive tract and some exocrine glands, specializing in nutrient absorption and mucus secretion.

• Function: Absorption of nutrients, secretion of mucus.

• Location: Digestive tract, exocrine glands.

Pseudostratified Columnar Epithelium

Pseudostratified columnar epithelium appears layered due to nuclei at varying heights, but all cells contact the basement membrane. It is specialized for mucus secretion and propulsion.

• Function: Secretion and movement of mucus by cilia.

• Location: Trachea, bronchi, nasal cavity (ciliated); epididymis, mammary glands, larynx (non-ciliated).

Stratified Epithelia

Stratified epithelia contain two or more layers of cells, providing protection and regeneration from below. They are named according to the shape of cells at the apical layer.

• Function: Protection against abrasion, drying out, infection.

• Regeneration: Cells regenerate from basal layers.

Stratified Squamous Epithelium

Stratified squamous epithelium is the thickest epithelial tissue, with many layers. Deeper layers appear cuboidal or columnar.

• Types: Keratinized (epidermis of skin), Non-keratinized (mucous membranes lining body orifices).

• Function: Protection.

• Location: Skin, mouth, vagina, esophagus, rectum.

Transitional Epithelium

Transitional epithelium is specialized for stretching, with basal cells usually cuboidal or columnar and superficial cells dome-shaped.

• Function: Allows stretching.

• Location: Urinary bladder, urethra, ureters.

Specializations of Epithelial Cells

• Microvilli: Finger-like projections (brush border) that increase surface area for absorption. Found in the small intestine.

• Cilia: Hair-like appendages, short and numerous, move substances across the cell surface. Line respiratory tract (move mucus) and oviduct (move ova).

• Keratin: Protein providing strength and waterproofing. Found in skin, mouth, esophagus, vagina, rectum, hair, nails, horns.

Glandular Epithelium

Glandular epithelium forms glands that secrete substances. Glands are classified as endocrine (ductless) or exocrine (with ducts), and as unicellular or multicellular.

• Exocrine Glands: Secrete products onto body surfaces or into cavities via ducts. Examples: mucus, sweat, oil, salivary glands, liver, pancreas, mammary glands.

• Endocrine Glands: Ductless; release hormones directly into interstitial spaces and bloodstream. Examples: pineal gland, pancreas, ovaries, testes, thyroid, adrenal gland.

Exocrine Gland Classification

Exocrine glands are classified by structure and mode of secretion.

• Structure: Simple (unbranched ducts) or compound (branched ducts); tubular (secretory cells form tubes), alveolar (secretory cells form sacs), tubule alveolar (both types).

• Mode of Secretion:

  • Merocrine: Secrete products by exocytosis (e.g., sweat, pancreas).

  • Apocrine: Apical portions of cells are pinched off (e.g., mammary glands).

  • Holocrine: Cells accumulate products and then rupture (e.g., sebaceous oil glands).

Epithelial Tissue Pathology

Cancer originating in epithelial tissue is called carcinoma. It is the most common type of cancer in adults, accounting for 80-90% of cases.

• Common Sites: Lung, breast, prostate, bowel.

• Benign Tumors: Adenoma (e.g., polyp in the colon).

Additional info:

• Endothelium: Specialized simple squamous epithelium lining blood vessels.

• Mesothelium: Specialized simple squamous epithelium lining body cavities.

• Brush Border: Term for microvilli-rich apical surface in absorptive epithelia.