Epithelium (Anatomy)

Question 1

Describe the cell layers of epithelium and explain regional variations

Answer 1

• Cells are contiguous (closely apposed),
• Cells are polarised as they have apical and basal regions.
• Apical membranes face a free surface.
• Epithelia are avascular as there is not space for blood vessels between the cells.

Question 2

Explain how epithelium may be classified

Answer 2

Based on 3 morphological features.

Number of cell layers

• One layer of cells = simple epithelium
• Many layers = stratified epithelium

Cell shape

• Squamous, cuboidal or columnar.
• In a stratified epithelium, the name is based on the shape of the most superficial cell in the epithelium.

Presence of surface specialisations
- Cilia or keratin.

Question 3

Describe the various types of epithelium, their location and functions

Answer 3

Simple squamous epithelium

• Single layer of thin cells.
• Allows rapid diffusion of substances through membrane.
• Lungs and kidneys.

Simple cuboidal epithelium

• Single layer of cuboidal cells.
• Role in absorption and secretion (production of protective mucous coat).
• Liver and lining of small ducts and tubules.

Simple columnar epithelium

• Single layer of tall narrow cells.
• Absorption and secretion of mucus.
• Lining of gastrointestinal tract.

Ciliated pseudostratified columnar epithelium

• Some cells do not reach the apical surface but all cells are attached to the basement membrane. Cilia on apical surface of cells.
• Secretes and propels mucus.
• Almost exclusively found in respiratory tract (trachea and bronchus). Also found in portions of male urethra.

Stratified squamous keratinising epithelium

• Multiple cell layers with cells becoming increasingly flat toward the surface. Surface covered with a layer of compact dead cells.
• Resists abrasion and penetration, and prevents water loss through skin.
• Epidermis, palms and soles.

Stratified squamous non-keratinising epithelium

• Multiple cell layers with cells becoming increasing flat toward the surface.
• Resists abrasion and penetration.
• Oral cavity, oesophagus, and vagina.

Stratified cuboidal epithelium

• Multiple layers of cuboidal cells.
• Contributes to sweat secretion and produces sperm.
• Sweat gland ducts, seminiferous tubules of testis.

Transitional epithelium

• Somewhat resembles stratified squamous epithelium but surface cells are rounded.
• Stretches to allow filling or urinary tract and protects underlying tissue from osmotic damage by urine.
• Ureter and bladder (urinary tract).

Question 4

Describe how the epithelium adheres to the underlying connective tissue

Answer 4

• The base of the epithelium sits on the basement membrane which anchors the epithelium to the underlying connective tissue.
• The basement membrane contains collagen, glycoproteins, and blends into other proteins of the connective tissue.
• The epithelium is attached to the basement membrane by hemidesmosomes.
• The basement membrane controls exchange of material between the epithelium and the underlying tissues, and binds growth factors from below that regulate epithelial development.

Question 5

Describe glandular epithelium

Answer 5

• Glandular epithelium is found in sweat, sebaceous and salivary glands, and in the pancreas.
• Usually stratified cuboidal epithelium.

Question 6

Explain how the classification of glandular epithelium may be based upon structure and function

Answer 6

• Exocrine glands secrete product on to a surface via ducts.
• Endocrine glands secrete product into blood stream (no ducts).

Exocrine gland classification

• Simple ducts = us branched ducts, compound ducts = branched ducts.
• Acinar secretory units = shaped like a hollow berry, tubular secretory units = test-tube shaped.
• Mucous glands secrete a viscous fluid rich in mucopolysaccharides which lubricate and protect. Serous glands secrete a watery fluid rich in enzymes. Seromucous (mixed) glands contain both serous and mucous cells.
• Holocrine glands discharge the whole secretory cell. Merocrine (eccrine) glands discharge by exocytosis of vesicles. Apocrine glands discharge unbroken, membrane-bound vesicles.

Question 7

What are the adhesion specialisations of Epithelia

Answer 7

• Tight junction seals membranes of adjacent cells together to prevent transport of substances between cells. Maintains apical and lateral domains.
• Intermediate junction anchors to terminal we underlying apical membrane.
• Desmosome anchors to cell cytoskeleton.