2.1 Epithelial cells

List the three key characteristics of epithelial cells.

1. They have a free (apical) surface exposed to the exterior or a lumen.

2. They adhere to a basement membrane at their basal surface.

3. They are tightly connected to neighboring cells via junctions.

How does simple epithelium differ from stratified epithelium?

• Simple epithelium consists of a single layer of cells, suited for absorption/secretion (e.g., intestines).

• Stratified epithelium has multiple layers, providing protection (e.g., skin).

Name two locations where simple squamous epithelium is found and state its

• Locations: Lungs (alveoli), blood vessels (endothelium), and body cavities (mesothelium).

• Function: Facilitates diffusion, osmosis, and filtration.

Compare the functions of mesothelial cells and endothelial cells.

• Mesothelial cells: Line closed body cavities (e.g., pleura, peritoneum) and provide a friction-free surface for organ movement.

• Endothelial cells: Line blood vessels and tubes (e.g., cardiovascular system) and regulate exchange between blood and tissues.

Why is transitional epithelium (uroepithelium) uniquely suited for the urinary bladder?

• It can stretch and change shape (e.g., when the bladder fills).

• Apical "umbrella cells" have thickened plasma membrane plaques that unfold to accommodate volume changes.

Describe the role of keratohyalin granules in the stratum granulosum.

• They contain proteins (e.g., filaggrin) that help bundle keratin filaments, contributing to the tough, waterproof barrier of the skin.

What structural change occurs in keratinocytes as they move into the stratum

• They lose their nuclei (die) and become flattened, keratinized cells filled with keratin, forming a protective outer layer.

Name the three main types of cell junctions in epithelial tissues and their primary functions.

1. Tight junctions (zonula occludens): Seal gaps to prevent leakage between cells.

2. Adherens junctions (zonula adherens): Provide mechanical strength via actin linkage.

3. Desmosomes (macula adherens): Anchor cells together via keratin filaments.

How do tight junctions contribute to epithelial polarity?

• They separate apical and basolateral membranes, ensuring specific proteins/receptors are localized to the correct side (e.g., growth factors on apical side).

What is the role of E-cadherin in adherens junctions?

It mediates calcium-dependent cell adhesion, linking to actin filaments to maintain tissue integrity. Loss of E-cadherin is linked to cancer metastasis.

Why are desmosomes particularly important in tissues like the skin and heart?

• They provide strong adhesion to withstand mechanical stress (e.g., skin stretching, heart contractions). Defects cause blistering diseases (e.g., pemphigus).

What happens to epithelial barrier function if tight junctions are disrupted?

• Loss of selective permeability, allowing harmful substances (e.g., bacteria, toxins) to leak between cells (e.g., inflammatory bowel disease).

How would you distinguish simple columnar epithelium from pseudostratified columnar epithelium under a microscope?

• Simple columnar: Single layer of tall cells with elongated nuclei aligned at the same level (base or center).

• Pseudostratified columnar: Nuclei at varying heights (appears layered), but all cells attach to the basement membrane (e.g., respiratory tract).

Why is stratified squamous epithelium further classified as keratinized or non-keratinized? Provide an example of each.

• Keratinized: Outer layers are dead, filled with keratin (e.g., skin).

• Non-keratinized: Living cells throughout; found in moist surfaces (e.g., mouth, esophagus).

Explain how the apical and basal surfaces of epithelial cells differ structurally and functionally.

• Apical surface: May have microvilli (e.g., intestines) or cilia (e.g., trachea) for absorption/movement.

• Basal surface: Attaches to basement membrane via hemidesmosomes; contains receptors for signaling.

What structural adaptation makes transitional epithelium ideal for the urinary bladder?

• Pliable apical plaques in umbrella cells that unfold when stretched (e.g., during bladder filling).

Compare the roles of tight junctions and gap junctions in epithelial tissues.

• Tight junctions: Prevent paracellular leakage (e.g., gut lining blocking bacteria).

• Gap junctions: Allow intercellular communication (e.g., ion exchange in cardiac muscle).

A patient has a mutation in desmoglein. What clinical symptom might occur, and why?

Blistering skin disorders (e.g., pemphigus) because desmoglein is critical for desmosome-mediated adhesion in stratified epithelia.

How does the loss of E-cadherin in epithelial cancers promote metastasis?

• Reduces cell-cell adhesion in adherens junctions, allowing cancer cells to detach and invade surrounding tissues.

Why are hemidesmosomes clinically significant in conditions like epidermolysis bullosa?

• They anchor epidermis to the basement membrane. Defects cause skin fragility and blistering upon minor trauma.

You observe a tissue sample with cube-shaped cells forming a single layer around a duct. What epithelium is this, and where might it be located?

• Simple cuboidal epithelium; found in kidney tubules or pancreatic ducts.

A slide shows a multi-layered epithelium with surface cells resembling flattened scales. Is this likely keratinized or non-keratinized? How could you confirm?

• Keratinized if the outer layers lack nuclei (e.g., skin). Non-keratinized if nuclei are visible (e.g., vaginal lining). Confirm with histological stains (eosinophilia in keratinized layers).