Epithelial Tissue Overview and Characteristics

Epithelial Tissue Overview

• Epithelial tissue is a diverse group of tissues, encompassing both surface epithelia and solid organs.
• These tissues are avascular, meaning they lack blood vessels and lymph nodes.
• Cells within epithelial tissue are tightly packed with minimal intracellular substance.
• It covers the body's surface, lines hollow organs (e.g., digestive, respiratory, urinary, and genital tracts), and forms the parenchyma of glands and their ducts.

Characteristics of Epithelial Tissue

• Cells are in close contact.
• Epithelial cells sit atop a basal lamina.
• Epithelial tissues exhibit functional and morphological polarity—apical and basal poles, and lateral domains.
• Epithelial tissues have a high regeneration capacity.
• The cytoskeleton is composed of cytokeratin intermediate filaments.

Classification of Epithelial Tissue

• Epithelial tissue classification considers cell arrangement (surface, trabecular, reticular), number of layers (simple, pseudostratified, stratified), and cell shape (cuboidal, columnar, squamous).
• Other factors also include dominant functions (sensory, transport, respiratory, secretory, etc.).

Simple Squamous Epithelium

• Passive transfer of substances via osmosis and diffusion.
• Specialized transporters in higher epithelia facilitate substance transfer across cell membranes.
• Active transport necessitates an energy source.

Simple Cuboidal Epithelium

• Composed of a single layer of cube-shaped cells.
• Primarily functions in secretion and absorption.

Simple Columnar Epithelium

• Composed of a single layer of column-shaped cells.
• Includes a ciliated form with hair-like projections (cilia).
• Functions include secretion, absorption, and movement of materials.

Pseudostratified Columnar Ciliated Epithelium

• Appears layered but all cells contact the basement membrane.
• Common in the respiratory system; cilia facilitate movement.

Pseudostratified Columnar Epithelium - Other

• Commonly found in the lining of the epididymis.
• Often includes specialized cells and stereocilia (hair-like structures that aid in absorption or secretion).
• Key location: epididymis duct.

Stratified Squamous Epithelium

• Multiple cell layers provide protection.
• Keratinized form (with keratin) is highly resistant.
• In keratinized form, nuclei are often absent.
• Found in areas requiring significant protection (skin, oral cavity, vagina, etc.).
• Non-keratinized form is less resistant, lacking keratin, and including nuclei.
• Includes areas such as oral cavity, esophagus, vagina, etc.

Transitional Epithelium

• Three layers of variable shape; this shape adapts to varying degrees of distension.
• Found in organs prone to expansion and contraction, such as the urinary bladder.
• Layers include basal, intermediate, and surface cells (umbrella cells).
• Umbrella cells adapt to changes in bladder volume and can lose contact with basement membrane when distended.

Types of Epithelial Tissue

• Trabecular Epithelium: Forms cords (trabeculae), common in endocrine glands (e.g., adrenal cortex).
• Follicular Endocrine Glandular Epithelium (Thyroid): Forms follicles; secretes hormones.
• Reticular Epithelium (Developing Tooth): Supports development.
• Stratified Squamous Non-Keratinized Epithelium: Found in oral cavity and other areas where distension is common.
• Transitional Epithelium: in bladder, ureters, renal calyces.

Epithelia and Glands

• Epithelial and glandular structures work together (e.g., epididymis duct with pseudostratified cylindrical epithelium).
• Glands come in two forms: apocrine (a portion of the cell is secreted) and holocrine (entire cell breaks up and is secreted).