. Author manuscript; available in PMC: 2021 Jan 1.

Published in final edited form as: J Invest Dermatol. 2020 May 22;141(1):203–205. doi: 10.1016/j.jid.2020.04.023

Table 2.

Toolbox with Validation Parameters Specific to Type of Epidermal Barrier Studied

Barrier Type	Essential Assays	Expected Result	Optimal and Specialized Assays	Expected Result
Inside-out: water barrier	TEWL	TEWL¹: < 15 mg H₂0/cm²	Lanthanum penetration	Passage is blocked at the SC/SG interface (for lipid barrier) or SG1 or SG2 level (for TJ barrier)
Inside-out: water barrier	TEER	TEER¹: 1,200–2,500 Ohm/cm²	Lanthanum penetration
Inside-out: barrier to larger molecules	Dye studies: larger dye molecules that model substances of interest	Dye penetration reveals an incompetent barrier to large molecules that may model passage of large molecules such as proteins	Dye studies: e.g., biotin tracers for TJ function	Passage is blocked at the SG1 or SG2 level
Outside-in: barrier permeation	Tracking labeled molecules: e.g., Lucifer yellow (see text)	Tracers will penetrate the viable epidermis if no SC is present or when the SC is severely compromised; however, it is not possible to rule out defects in barrier function with histologic exam of tracer alone	Diffusion studies	Competent or incompetent barrier will yield concentrations of permeants in the acceptor fluid comparable to that seen in diffusion cell studies of human skin. Can combine with ELISA assays
Outside-in: barrier permeation	Tracking labeled molecules: e.g., Lucifer yellow (see text)		Lipid composition: e.g., HPTLC, LC-MS	Lipid composition matches that seen in normal or diseased skin, depending on the model
Outside-in: barrier to microbes	Detection of microbes: DAPI, FISH, or fluorescence-labeled	Localization of microbes on top of SC or in viable epidermis	Epidermal expression of host defense molecules: AMPs, cytokines, chemokines
	Inoculum and viability of microbes before and after coculture	CFU counts
	Infection control	Culture medium in basolateral chamber should remain sterile (no CFU on agar plates)

Abbreviations: AMP, antimicrobial peptide; CFU, colony forming units; HPTLC, high-performance thin layer chromatography; LC-MS, liquid chromatography–mass spectrometry; SC, stratum corneum; SG, stratum granulosum; TEER, transepithelial electrical resistance; TEWL, transepidermal water loss; TJ, tight junction.

Values are for information only. They might vary depending on the environment and how measurements are performed as well as device used.