Are all of the five aggregates saṅkhāras?

Are the five aggregates all saṅkhāras -- are they impermanent and dependently originated? I guess that "perceptions" and "feelings" are perhaps dependent on sensual contact. Is there anything else they're dependent on? Are "form" and "consciousness" dependently originated, and what (conditions or causes) do they depend on? Does "eye consciousness" exist when there's no contact? In a blind person, for example? If all five aggregates are indeed saṅkhāras, then what is the aggregate called "formation" (saṅkhāra)? What is the difference in the meaning in the word "saṅkhāra" when it's used to identify that one aggregate, as opposed to when it's used to characterise all aggregates? Is it right to call e.g. the doctrine of the "four noble truths" a dhamma instead of a saṅkhāra? Is the doctrine unconditioned and not subject to decay ... or is this a theory about Dhamma (i.e. a meta-dhamma) proposed by some schools and not others? Is awareness of (e.g. perception of) the dhamma a saṅkhāra, which depends on contact (e.g. contact between mind-object and mind-consciousness), and impermanent?