Epithelial tissue

• Epithelial tissues cover body surfaces, line cavities and form glands.

• They function in protection, secretion, absorption, and transportation.

Epithelial tissues are classified based on the number of cell layers and cell shape:

1. Simple Epithelium:

• Single layers of cells primarily functions in absorption, secretion, and filtration.

Types:

1. Simple Squamous Epithelium:

   • Structure: Single layer of flat cells.

   • Location: Alveoli of lungs, glomeruli of kidneys, lining of blood vessels (endothelium), and lining of body cavities (mesothelium).

   • Function: Allows for rapid diffusion and filtration.

2. Simple Cuboidal Epithelium:

   • Structure: Single layer of cube-shaped cells.

   • Location: Kidney tubules, ducts of small glands, ovary surface.

   • Function: Secretion and absorption.

3. Simple Columnar Epithelium:

   • Structure: Single layer of tall, column-shaped cells.

   • Location: Lining of the gastrointestinal tract, gallbladder, and some ducts of glands.

   • Function: Absorption and secretion; may have microvilli for increased surface area or cilia for movement of substances

2. Stratified Epithelium:

• Multiple layers of cells, primarily functions in protection against abrasion.

Types:

1. Stratified Squamous Epithelium:

   • Structure: Multiple layers with flat apical cells.

   • Location:

     • Keratinized: Epidermis of the skin.

     • Non-keratinized: Lining of the mouth, esophagus, and vagina.

   • Function: Protects underlying tissues from abrasion and pathogens.

2. Stratified Cuboidal Epithelium:

   • Structure: Typically, two layers of cube-shaped cells.

   • Location: Ducts of sweat glands, mammary glands, and salivary glands.

   • Function: Protection and limited secretion/absorption.

3. Stratified Columnar Epithelium:

   • Structure: Multiple layers with columnar apical cells.

   • Location: Rare; found in the male urethra and large ducts of some glands.

   • Function: Protection and secretion.

Structure:

• Epithelial cells are closely packed together with little extracellular matrix, and they form a continuous sheet held together by cell junctions such as tight junctions, gap junctions, and desmosomes.

• They have an apical surface facing the body cavity or external environment and a basal surface attached to the underlying connective tissue via a basement membrane.

Location:

1. Surface epithelium: External body surfaces (skin), internal cavities (mouth, respiratory tract, and gastrointestinal tract)

2. Glandular epithelium: Forms glands (e.g., sweat glands, salivary glands, thyroid gland)

Functions:

1. Protection: Epithelial tissues protect the body from mechanical injury, harmful substances, and pathogens.

2. Secretion: Glandular epithelium secretes substances like hormones, enzymes, and mucus.

3. Absorption: Simple columnar and simple cuboidal epithelia help absorb nutrients in the gastrointestinal tract and kidney tubules.

4. Transportation: Ciliated epithelial cells help transport particles and mucus across their surfaces.