The version you wanted was “Laminin 332 is secreted only by mature corneal epithelial cells.”
If you were saying this outloud you would use inflection in your voice to draw the distinction. Because you can’t put that emphasis in your writing, you should be careful about where you place your “only”!
If you say “Laminin 332 is only secreted by mature corneal epithelial cells.” The emphasis is placed on the “secreted” therefore this sentence says that mature cells only secrete the laminin, they don’t synthesize, remodel, transport or do anythign else with it.
In contrast, when you say “laminin 332 is secreted only by mature corneal epithelial cells.” then emphasis is placed on the mature corneal epithelial cells. It says that immature cells don’t secrete laminins.
Various or varying?
Which of the following could you use to mean that there was a range of sampling times?