Epithelial Tissue Overview

Epithelial Tissue

• Epithelial tissue forms a protective covering of body surfaces and lines body cavities.
• It's avascular; nourished by diffusion from underlying connective tissue.
• It always rests on a basement membrane, a specialized type of connective tissue.
• Cells display polarity (structural and functional differences between sides).

Epithelial Cell Polarity

• Apical Domain: Faces the lumen or external environment; possesses microvilli, cilia, stereocilia.
• Lateral Domains: Contact adjacent cells; responsible for cell-to-cell adhesion.
• Basal Domain: Faces the basement membrane; attaches to it, facilitates nutrient transport.

Epithelial Tissue Classification

• Simple epithelium: Single layer of cells.
• Stratified epithelium: Multiple layers of cells.

Classifying Epithelial Cells Based on Shape

• Squamous epithelium: Flattened cells, width greater than height.
• Cuboidal epithelium: Cube-shaped cells, equal height and width.
• Columnar epithelium: Tall, column-shaped cells, height greater than width.

Pseudostratified Epithelium

• Appears stratified due to varying cell heights and nuclei positions.
• All cells connect to the basement membrane.
• Primarily pseudostratified columnar epithelium, found in the respiratory system..

Transitional Epithelium

• Also known as urothelium, found in the ureters, bladder, urethra.
• Stratified epithelium where the top layer of cells changes shape based on organ distension.
• Top layer (umbrella cells) covers underlying cells.

Specialized Epithelial Cells

• Endothelial cells: Simple squamous, line blood vessels.
• Mesothelial cells: Simple squamous, cover organ surfaces.

Apical Domain Modifications

• Microvilli: Finger-like projections increasing surface area for absorption; found in intestines (striated border), kidneys (brush border).
• Cilia: Hair-like projections moving substances along the cell surface; found in the respiratory system.
• Stereocilia: Long, non-motile microvilli found in the inner ear and the male reproductive system.

Microvilli Structure and Proteins

• Core: Contains 20-30 parallel actin filaments.
• Actin Filaments: Connected by fimbrin, villin, and fascin.
• Myosin I: Links actin filaments to the plasma membrane
• Villin: Maintains actin filament arrangement at the microvillus tip.
• Terminal Web: Anchoring structure; composed of cross-linked actin filaments and myosin II.

Microvilli, Stereocilia, and Cilia

• Microvilli:

  • Projections of the apical domain for increased absorption.
  • Contain actin filaments, connected by fimbrin, villin, fascin.
  • Myosin I links actin to the membrane; villin anchors the core.
  • Terminal web anchors at the base, composed of cross-linked actin filaments, and myosin II.

• Stereocilia:

  • Highly modified microvilli, found in epididymis (absorb testicular fluid).
  • Actin filaments with alpha-actinin, but no villin.
  • Ezrin connects actin filaments to the plasma membrane
  • They do not contract due to the absence of myosin, in contrast to microvilli.

• Cilia:

  • Hair-like structures responsible for movement.
  • Composed of microtubules in a 9+2 axoneme arrangement; 9 peripheral doublets & 2 central microtubules.
    • 9+0 axoneme is also possible
  • Dynein: Motor protein involved in cilia motion.
  • Basal body: Microtubule organizing center located in the apical domain

Types of Cilia

• Motile Cilia:

  • 9+2 axoneme and dynein, moving substances.
  • Abundant in respiratory system to clear mucus/foreign material.

• Primary Cilia:

  • 9+0 axoneme, non-motile; sensory function.

• Nodal cilia:

  • 9+0 axoneme and dynein. Development of left/right body symmetry.

Other Relevant Tissues and Structures

• Simple Columnar Epithelium:

  • Single layer columnar cells found in the small intestine; possess a brush border (microvilli).
  • Goblet cells: Mucus-secreting cells, appear empty in H&E after clearing. PAS staining highlights them

• Mesentery:

  • Tissue covering small intestines; composed of mesothelial cells; visible in Silver & H stains (silver stains lipids)

• Blood vessels:

  • Endothelial cells (simple squamous) lining blood vessels.
  • Veins appear collapsed in H&E staining

• Lymphatic vessels:

  • Lined by endothelial cells, do not contain red blood cells.

• Basement membrane: A thin, red line underneath the epithelium in H&E staining.

• Terminal web: At the base of microvilli, composed of cross-linked actin filaments and myosin II. Not present under cilia.

This quiz focuses on the characteristics and functions of epithelial tissue, including its structure and polarity. Understand the significance of avascularity and the roles of the apical, lateral, and basal domains in epithelial cells. Test your knowledge on these fundamental concepts in histology.