Epithelium and Its Classification

Study Notes

Epithelium

Epithelium is one of the four basic tissues in the human body, covering the body surface and lining cavities.
Epithelial cells are closely packed, with minimal intercellular space, and rest on a basement membrane.
Epithelium is avascular; blood vessels do not penetrate the tissue, but nerves can be found between cells.
Epithelium is constantly undergoing degeneration and regeneration.

Classification of Epithelium

Epithelium is classified into three major types based on function:
- Covering epithelium: Protects and lines the body surface and cavities.
- Glandular epithelium: Produces and secretes substances.
- Neuroepithelium: Responsible for sensory perception.

Covering Epithelium

Based on cell layers, covering epithelium is categorized as:
- Simple: One layer of cells on the basement membrane.
- Stratified: Multiple layers of cells, the first layer resting on the basement membrane.

Simple Epithelium

Simple squamous epithelium: Flattened, scale-like cells with flat, bulging nuclei. Found in:
- Lining of blood vessels and heart.
- Lung alveoli.
- Small ducts of exocrine glands.
Simple cubical epithelium: Single layer of cube-shaped cells with central, rounded nuclei. Found in:
- Renal tubules.
- Medium-sized ducts of exocrine glands.

Stratified Epithelium

Stratified squamous epithelium: Multiple layers of cells with different shapes and nuclei arrangements:
- Basal layer: Columnar cells with basal oval nuclei (regenerates other layers).
- Middle layers: Polyhedral cells with central rounded nuclei.
- Top layer: Squamous cells.
Stratified squamous keratinized epithelium: Dry body surface, top layer covered by keratinized scales (epidermis of skin).
Stratified squamous non-keratinized epithelium: Wet surfaces leading to skin, no horny scales on top layer (mouth cavity, esophagus, external auditory canal, ear drum).
Transitional epithelium: 6-8 cell layers, cells glide over each other for flexibility:
- Full bladder: Two layers: basal cubical cells and top flat cells.
- Empty bladder: Full thickness showing multiple layers. Found in the pelvis of kidney, ureter, bladder, and prostatic urethra.

Simple Columnar Epithelium

Single layer of tall, column-shaped cells with oval nuclei near the base:
- Non-modified: Unspecialized cells (lining of large ducts of exocrine glands).
- Modified: Specialized for additional functions:
  - Simple columnar secretory epithelium: Production of mucus secretion (lining of the stomach).
  - Simple columnar absorptive epithelium: Absorption of soluble molecules (lining of the intestine).
  - Simple columnar ciliated epithelium: Cilia projecting from the free surface (lining of the fallopian tube and uterus).

Pseudostratified Columnar Epithelium

Single layer of cells that appear stratified due to cell crowding during development, nuclei at different levels:
- Pseudostratified columnar non-ciliated epithelium: Tall columnar cells without cilia (lining of large ducts of exocrine glands).
- Pseudostratified columnar ciliated epithelium: Tall columnar cells with cilia (lining of nasal sinuses, trachea, and bronchi).
- May contain goblet cells (mucus-secreting) for mucus production.

Glandular Epithelium

Modified epithelial tissue specialized in producing secretions.
Formed by proliferation of covering epithelium cells that penetrate connective tissue.

Classification of Glandular Epithelium

According to Presence or Absence of Duct

Exocrine glands: Possess ducts (e.g., salivary glands), secretion released onto a surface.
Endocrine glands: Lack ducts (e.g., thyroid, pituitary, adrenal glands), secretion released directly into blood.
Mixed glands: Have both exocrine and endocrine components (e.g., pancreas).

According to Duct and Secretory Part

Simple glands: Single unbranched duct and secretory part:
- Simple tubular: Tubular secretory part.
- Simple alveolar: Alveolar secretory part.
- Simple tubulo-alveolar: Combined tubular and alveolar secretory part.
Simple branched glands: Single unbranched duct, branched secretory units:
- Simple branched tubular: Branched tubular secretory units.
- Simple branched alveolar: Branched alveolar secretory units.
- Simple branched tubulo-alveolar: Branched tubulo-alveolar secretory units.
Compound branched glands: Branched duct system and branched secretory units:
- Compound branched tubular: Branched tubular secretory units.
- Compound branched alveolar: Branched alveolar secretory units.
- Compound branched tubulo-alveolar: Branched tubulo-alveolar secretory units.

Secretion Classification

Nature of secretion:
- Mucous glands: Produce viscous mucus low in enzymes (e.g., goblet cells, minor salivary glands).
- Serous glands: Produce watery solutions rich in enzymes (e.g., parotid glands, pancreas).
- Mucoserous glands: Produce both mucus and serous secretions (e.g., submandibular, sublingual glands).
- Sweat glands: Produce watery secretions with enzymes and waste products.

Description

Explore the fundamentals of epithelium, one of the four essential tissue types in the human body. This quiz covers the characteristics, classification, and functions of different types of epithelial tissues, including covering and glandular epithelium. Test your knowledge on how epithelial cells protect, secrete, and sense in the body.