Table 1.

Summary of different permeable diffusion barriers in cells.

Permeable diffusion barrier	Key components of diffusion barrier	Functional pore size for passive diffusion	Diffusion tracers	methods used to study	References
Nuclear Pore Complex	FG-rich nucleoporins	9~10 nm	Dextran, gold particles, recombinant proteins	SPT, PAFP, FCS, FRAP, microinjection, CID	[12–15,40,44]
Dendritic spines neck	Unknown	<2.4 nm	GFP, Dextran, Ca2+ dye,	PAFP	[9]
Axon initial segment	Actin, ankyrin-G	>10 kDa, <70 kDa	Dextran, GFP	Microinjection, FRAP	[21]
Ciliary pore complex	Nucleoporin	~8.43 nm (~650 kD) [31] or <67 kD [30]	Dextran, recombinant proteins, YFP-FKBP-POIs	CID, Microinjection, FRAP	[30,31]
Connecting cilium	Unkown	>81 kD	GFP, PAFP, arrestin, transducin, recoverin	PAFP	[26,28,29]
Permeable diffusion barrier in cytoplasm	Actin	30~40 nm (average pore size)	Dextran, Ficoll, DNA, recombininat proteins, IgG	Microinjection, FRAP	[10,32,33]