14: trash and unfinished Interactions between cells in multicellular systems

Tissues in the Body

• The four major tissue types:

• epithelial

• connective

• nervous

• muscular

Epithelial Tissue

• cover surfaces and line internal cavities

• organised into sheets

• sheets can be rolled up into tubes eg in kidney collecting duct

Functions of Epithelial Sheets

Act as barriers between the body and the environment.

types of epithelia

• columnar

• cuboidal

• squamous

• stratified

Tight Junctions

• Form a seal between epithelial cells to prevent diffusion / leakage across the sheet.

• allow the apical and basolateral membranes to be different in function and composition (meaning epithelia are functionally polarised)

Proteins in Tight Junctions

Occludin and claudin proteins

secretion from the apical surface in epithelia

• airways

• stomach

• intestine

• mammary gland

adherens junction

joins an actin bundle in one cell to a similar bundle in a neighbouring cell

desmosome

joins intermediate filaments in one cell to those in a neighbouring cell

gap junction

forms channels that allows small, water-soluble molecules to pass from cell to cell

hemidesmosome

anchors intermediate filaments in a cell to the basal lamina

Cytoskeleton-Linked Junctions

• Provide mechanical strength and stability to epithelial sheets.

• examples

  • adheren and desmosomes- link cytoskeleton of epithelial cells to their neighbours

    • use cadherins

  • hemidesmosomes - link cytoskeleton of epithelial cells to basal lamina

    • use integrins

Cadherins

• Transmembrane proteins that mediate attachment between epithelial cells.

• interaction needs calcium

Adherens Junctions mode of action

Cadherins link to actin filaments to form adhesion belts.

Properties of Adherens Junctions

Contractile because of presence of myosin II - allows epithelial sheets to move

examples of where contractility is important

• lens formation

• neurulation

Examples of Desmosome-Rich Tissues

Found in heart muscle and exposed epithelia under stress.

properties of intermediate filaments (desmosomes)

great tensile strength

Gap Junctions

Allow direct communication between neighboring cells by transferring ions and small molecules.

Connexons

Channel proteins that form gap junctions.

Gap Junction Function in the Heart

Enables electrical coupling

Plasmodesmata

The plant equivalent of gap junctions

Cadherins in Tissue Organization

Specific cadherins define which cells can interact to form tissues.

E-Cadherin vs. N-Cadherin

Epithelial cells express E-cadherin

Cadherins and Cancer

Cancer cells often lose cadherin expression

Extracellular Matrix (ECM) and Connective Tissue

Provides structural support to tissues and helps with cell communication.