Epithelia II

Question 1

Cell junctions purpose?

Answer 1

Adherence and communication. On lateral surfaces (towards apical side).

Question 2

Tight junctions

Answer 2

Selective barrier that limits or prevents diffusion of substances.
Force transcellular rather than paracellular transport. Key core proteins: occludins and claudins.
Belt around the cell (?)

Question 3

Adherence junctions

Answer 3

Promote attachment and polarity, morphological organization, and stem cell behavior in the epithelial sheet.
More towards basal side than TJ’s.
Cadherins that link to actin and other signaling proteins in cytoplasm.

Question 4

Cadherins

Answer 4

TM proteins with ECM domains that interact with each other, cytoplasmic tails that bind adapters and actin filaments.
Some cadherin associated proteins (like beta catenin) control various aspects of polarity, development, and fx.

Question 5

Desmosomes

Answer 5

Promote mechanical strength and resist shearing forces and promote structural organization of the epithelial sheet.
Cadherins that link IF.
Local patches that hold cells together.

Question 6

Gap junctions

Answer 6

Promote rapid communication b/w epithelial cells through diffusion of ions and small mlcs.

Question 7

2 key aspects of polarity?

Answer 7

1. ) PM composition is segregated (diff on apical vs. basolateral)
2. ) Cytoplasm is polarized

Question 8

PM polarity

Answer 8

Transporters, ion channels, receptors for endo/exocytosis, signaling receptors/effectors, proteins that mediate cell-cell and cell-lamina attachments.
TJ near apical, so often lateral side is similar to basal. But there are some diffs.

Question 9

Cytoplasm poliarty

Answer 9

Particularly MT’s. Organelle distribution. Secretory vesicles in apical domain are diff than those in basal, move in specific directions.

Question 10

Polarity fx.?

Answer 10

Crucial for unidirectional secretion/absorption to/from one side of epithelium. DIRECTIONAL TRAFFIC.
Necessary for transcytosis (endocytosis on one side, exocytosis of it on the other mem).
Also important for localizing and orienting intercellular signaling among epithelial cells or b/w epithelial cells and other cell types.

Question 11

Apical surface modifications of microvilli

Answer 11

THeir fx is to increase SA for transport/secretion. One type of microvillus the stereocilia (NOT related to cilia) are found in the epididymis and sensory cells in the ear. Avtin filled. Fx. in reception of sound.

Question 12

Apical surface modifications of cilia

Answer 12

3 types of cilia:

1. ) Primary: single cell, non-motile. MT based. Organize and promote signal transduction systems that promote division, differentiation, and fx.
2. ) Motile: respiratory tract, oviduct. wave like a boat.
3. ) Sensory: non-motile, sensory reception. (likely specific variations of primary cilia)

Question 13

Basolateral surface modifications

Answer 13

Infolds and outfolds. They increase surface area.

Question 14

Basal lamina

Answer 14

Underlies basal surface of each epithelial tissue. Linked to epithelia physically and functionally. Also surround blood vessels etc.
Formed by collagen (IV) in sheets that is interwoven with glycoproteins. Glycoprotines like laminin and enactin are common.

Question 15

Where is basal lamina made?

Answer 15

Epithelial cells secrete/synthesize most basal laminal components.
But some extracell components of the connective tissue (made by fibroblasts) bind to and organize elements of the basal lamina.

Question 16

Basal Lamina fx?

Answer 16

• Mediate attachment of epithelia to underlying connective tissue (basal lamina separate epithelia from connective tissue but also attach them to extracell matrix of connective tissue)
• Contribute to selective filtration of diffusing substances
• Necessary for establishment/maintenance of epithelial cell polarity
• Serve as highways for the migration of cells through connective tissue (but also a barrier to cell movement)
• Control gene expression of cells to affect prolif/devel.
• Control development, morphogenesis and organization of epithelial cells (like a scaffold)
• Important for repair after damage

Question 17

How do epithelia attach to basal lamina?

Answer 17

Directly connect to basal lam. by attachment of hemidesmosomes and focal adhesions on the basal surface of the epithelial cell.
Key class forming these connections are integrins.

Question 18

Integrins

Answer 18

Main fx: attachment of cell to ECM and signal transduction from ECM to cell. 
TM receptors that are the bridges for cell-cell and cell- ECM interactions. 
When triggered (ligands include fibronectin and collagen), integrins in turn trigger chemical pathways to the interior (signal transduction), such as the chemical composition and mechanical status of the ECM, which results in a response (activation of transcription).

Question 19

Where do integrins attach?

Answer 19

Specific integrins of hemidesmosomes attach to IF (strength).
Integrins of focal adhesions attach to actin. (Regulate epithelial polarity and fx. through signaling… important in epithelial turn over after damage or normal cell turn-over).