Epithelial Tissue: Quick Revision
Structure and General Features
- Epithelial tissue: avascular; aggregates of cells in continuous sheets; minimal intercellular extracellular material
- Covers body surfaces; lines internal cavities and tubes; secretory portions of glands and ducts
- Functions: protection; reduction of friction; absorption; secretion; excretion; synthesis of proteins; sensory capacity
Classification of Epithelial Tissue
- By shape: squamous (flat), cuboidal (cube-like), columnar (tall)
- By number of layers: simple (one layer), stratified (two or more)
- Other types mentioned: pseudostratified epithelium (appears stratified but is not); transitional epithelium (special, can stretch)
Simple Epithelia
- Simple squamous epithelium
- Structure: flattened cells; flattened nuclei; cytoplasm indistinct
- Subtypes mentioned: mesothelium; endothelium
- Examples: simple squamous lining Bowman's capsule
- Simple cuboidal epithelium
- Structure: height ≈ width; round, centrally placed nucleus
- Examples: proximal and distal tubules
- Simple columnar epithelium
- Structure: height > width; nuclei basally located and arranged at the same level
Pseudostratified Epithelium
- Appears stratified due to nuclei at different levels, but all cells contact basement membrane
- Not true multiple-layered epithelium
Stratified (Complex) Epithelium
- Two or more cell layers; durable and protective in wear-and-tear areas
- Subtypes:
- Stratified squamous
- Stratified cuboidal
- Stratified columnar
Transitional Epithelium
- Special type of stratified epithelium; cells become flattened when stretched
- Lines organs subjected to contraction and stretching (e.g., urinary bladder)
Types of Epithelium (Overview)
- Simple squamous
- Simple cuboidal
- Simple columnar
- Stratified squamous
- Stratified cuboidal
- Pseudostratified columnar
- Transitional