I’m taking Anatomy and Physiology 1 this semester. One of the questions this week I only sort of understand.

The textbook says the epidermis layers have increasing amounts of keratin as their superficial position increases. (woo used a&p directional terms from chapter 1! I am learning.) The text also says the epidermis – most specifically the strateum corneum – prevents water loss from the body.

This seems to somewhat indirectly imply that water can’t get into the body through the epidermis, and thus water-soluble substances wouldn’t work in transdermal patches. What I have been able to find on fat-soluble solutes indicates that they can get through.

However, information I have found about transdermal administration of medication indicates that the substance needs to be both fat- and water- soluble, which means the original question from the textbook doesn’t have an explanation, if it means exclusively fat-soluble and not at all water-soluble.

My question is: Why? Why does the epidermis keep water out – is it because of the same properties that allow it to keep water in the body? What does keratin have to do with this property of the epidermis? Why and/or how do the cells secrete a water-repellent glycoprotein (which is only briefly mentioned in my textbook, and NOT explained any further)?

Maybe I’m just asking questions that we’re not supposed to think of asking until we get deeper into A&P or whatever specific field would be appropriate.