3. Introduction To Microanatomy: Epithelial Tissue

Question 1

1. Identify and describe the different types of epithelial cells and give an example of where each type is found (objective)

Answer 1

Answer later

Question 2

2. Identify and describe the specialized epithelial cell junctions and the role they play in forming epithelial tissue (objective)

Answer 2

Answer later

Question 3

Epithelial Cells: Overview (1/3)

Answer 3

Form continuous sheets that cover and line body’s free surfaces

• Provide protection (skin)
• Mediate transport of nutrients (intestines), gases (lungs), waste material (kidneys)
• Secrete lubricating fluid (serosal surfaces)

Question 4

Epithelial Cells: Overview (2/3)

Answer 4

Make up functional tissues of organs (parenchyma) and are key elements of many glands
-Produce and export enzymes and hormones (pancreas, liver, salivary gland)

Question 5

Epithelial Cells: Overview (3/3)

Answer 5

Specialized epithelial cells serve as sensory receptors

-Convert external stimuli to electrical signals (olfactory sensory neurons, auditory hair cells, photoreceptors)

Question 6

Epithelial Tissue Characteristics

Answer 6

1. Location: cover external/internal surfaces
2. Organization: maintain cohesion so that they form continuous sheets
3. Attached to basement membrane: cells have structural and functional polarity
4. Avascular: tissue closely supported by underlying connective tissue
5. Capable of repair and replacement: cells can be renewed continuously by mitotic activity and stem cell populations

Question 7

Intercellular Adhesion (1/3)

Answer 7

Via membrane-associated structures that provide connection and communication between lateral surfaces of cells
-Tight junctions (occludens) and adherent junctions form adjacent bands encircling apical end of cell (six-pack of canned drinks)

Question 8

Tight Junctions

Answer 8

• Opposing cell membranes held in tight contact by transmembrane adhesive proteins (claudin, occludin, junctional adhesion molecule)
• Physical/chemical barrier to control paracellular movement of substances
• Define and maintain separation between apical and basal areas of cell membrane to maintain cell polarity

Question 9

Adherent Junctions

Answer 9

• Opposing membranes held close with thin sliver of intercellular space
• Space bridged by cadherins (Ca2+)
• Cadherins attach to cytoplasmic proteins (catenins) which in turn attach to actin filaments/microtubules.
• Transmembrane protein complexes interact with signaling molecules (tumor suppressor)
• Role in regulating cell cell contact (morphogenesis, remodeling of tissue/organs, controlled proliferation of cells)

Question 10

Intercellular Adhesion (2/3)

Answer 10

• Desmosomes: anchoring junctions that occur sporadically (do not form band, function as “spot welds” to reinforce cellular adhesion), sheering force
• Transmembrane proteins (desmosomal cadherins) attach to intermediate filaments of cytoskeleton via cytoplasmic proteins for additional structural support

Question 11

Desmosomes

Answer 11

• Prominent in tissues under mechanical stress (heart, skin)
• Dysfunction of desmosomal cadherins associated with cell adhesion cardiomyopathy (arrhythmogenic right ventricular dysplasia), blistering diseases (pemphigus vulgaris)

Question 12

Intercellular Adhesion (3/3)

Answer 12

-Gap junctions: clusters of intercellular channels that allow for rapid transmission of electrical or chemical information from cell to cell, promote coordinated action

Question 13

Gap Junctions

Answer 13

• Intercellular gap is spanned by channel-forming transmembrane proteins (connexins)
• Connexins form hexameric complexes called connexons, which align with connexons from neighboring cell (each gap junction has dozens to hundreds of aligned pairs)

Question 14

Basilar Adhesion

Answer 14

• Basal surface of cells rests on thin felt-like sheet of macromolecules which is outside cell but secreted from cell (basement membrane)
• Basement membrane composed of 2 layers (basal and reticular lamina)
• Attached to cell via hemidesmosome

Question 15

Hemidesmosome

Answer 15

• Transmembrane protein integrin interacts with extracellular protein laminin to link intermediate filaments of cytoskeleton to extracellular basement membrane
• Integrin plays role in cell-cell communication, modulation of cell proliferation and differentiation

Question 16

Basement Membrane (not actually a membrane)

Answer 16

• Complex mixture of macromolecules that bind hormones and other signaling molecules
• Serves as semi-permeable filter for substances coming to cell from underlying tissue
• Epithelial cells require contact with it to function normally and establish polarity

Question 17

Specializations of Apical Cell Surface (1/2)

Answer 17

Apical vs. Basal

• basal is by basement membrane, apical is opposite, usually facing a space
• Different in structure/function, uneven distribution of organelles and proteins

Question 18

Specializations of Apical Cell Surface (2/2)

Answer 18

Specialized structures function to increase the apical surface area for better absorption (microvilli) or to move substances along the epithelial surface (cilia)
Microvilli- absorption organs
Villi- respiratory tract and lungs, fallopian tubes (longer projections, microtubules)

Question 19

Epithelial Tissue Type I- Covering (Lining)

Answer 19

Classified according to cell shape and number of layers

Question 20

Epithelial Tissue Type II- Secretory (Glandular)

Answer 20

Classified by structure

Question 21

Arrangement of Cells (classification)

Answer 21

Simple (single layer)
Stratified (multiple layers)
Pseudostratified (single layer but looks multilayered)

Question 22

Shape of Cells (classification)

Answer 22

Squamous
Cuboidal
Columnar

Question 23

Simple Squamous

Answer 23

Structure: one layer with flat nuclei

Function: allow passage via diffusion/filtration, secretes lubricating substances

Location: lining blood vessels/lymphatics, alveoli/lungs , renal glomeruli, heart, serous lining of cavities (mesothelium)

Question 24

Simple Cuboidal

Answer 24

Structure: cube like cells with large spherical central nucleus

Function: secretion and absorption

Location: renal tubules, ducts and glands

Question 25

Simple Columnar

Answer 25

Structure: single layer of tall cells with large round nucleus Function: protection, lubrication, absorption, secretion Location: lining of intestine, gallbladder, female reproductive tract

Question 26

Stratified Squamous

Answer 26

Structure: two or more layers of thin cells with flat nuclei Function: protection, secretion, prevention of fluid loss Location: skin, mouth, esophagus, larynx, vagina, anus

Question 27

Stratified Cuboidal

Answer 27

Structure: 2+ (usually 2) layers of cube like cells with large central nuclei Function: protection, secretion Location (rare): sweat glands, mammary glands, testes, developing ovarian follicles

Question 28

Transitional

Answer 28

Structure: top has dome-shaped (umbrella) cells with membranes that allow them to withstand hypertonic effects of urine and protect underlying cells from toxins; cells can adjust with each other as urinary bladder fills and wall is distended. Function: protection and distensibility Location: bladder, ureters, renal calyces

Question 29

Stratified Columnar

Answer 29

Structure: 2+ layers of columnar (top columnar, lower usually cuboidal) Function: protection and secretion Location (rare): parts of male urethra, some glands

Question 30

Pseudostratified | Clue is mucus white spots

Answer 30

Structure: single layer of cells with nuclei at varying heights (mucus in between, also look for cilia) Function: protection, secretion, cilia-mediated transport of mucous and trapped particles Location: lining of trachea, bronchi, nasal cavity

Question 31

Glands

Answer 31

Begins as epithelium, proliferation of cells and growth into connective tissue. 1. Exocrine: secretory portion and duct leading back to epithelium (ie acini in pancreas) 2. Endocrine: no duct cells, secrete products into bloodstream (ie islets of Langerhans in pancreas)