levels of organization...2012/02/01  · 1. tight junctions or zonulae occludens forms a band...

How do individual cells “connect” to form tissue?

Levels of Organization

http://animalcellbiology.wordpress.com/category/structure/

Desmosome

Tight Junction

1. Tight Junctions or Zonulae Occludens

forms a band completely around the cell

fusing cells together

and closing-off intercellular space.

Fusion site #: often correlates to leakiness of tissue.

The fewer fusion sites the more permeable to

water and solutes

More fusion sites less permeable…

Ex: Cells of the proximal renal tubule of the kidney where

filtration often takes place have few fusion sites to allow for

leakability, whereas, cells of the urinary bladder (that must hold

urine and not allow it to leak out) contains numerous fusion sites

or tight junctions to form a seal to prevent the flow of materials

between epithelial cells.

2. Gap junction (communicating junction)

a gap between the neighboring cell membranes

contains protein unit called connexin surrounds the pore,

which acts as a hydrophilic ion channel between two cells.

An individual unit of the gap junction is called a connexon .

3. Desmosome (anchoring junctions)does not surround the cell. It’s adhesion function is further

empowered by groups of intermediate keratin filaments which

provides firm adhesion.

Hemidesmosome literally means half a desmosome

Plant Cell Wall

From a functional point of view, junctions between cells can be classified as:

adhering junctions (desmosome and hemidesmosome), impermeable junctions (tight junctions) and communicating junctions(gap junctions).