I can understand everything from this [article](http://www.bbc.com/news/health-32494846) except one single word that causes me trouble in getting the complete meaning of the following sentence:

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> The dietary swaps appeared to cause significant changes to the cells **lining the gut** as well as the bacteria that live in the bowel - with the US volunteers faring better.
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I understand that the new diet caused important changes to the cells, modifying their structure, but what exactly happened to the gut by **lining** it and also to the bacteria (were they **lined** too or what?) I quite can't get.

Checking [The Free Dictionary](http://www.thefreedictionary.com/line) for the verb **to line**, I couldn't find an appropriate sense for the context in question.

So, please let me know which one of the following dictionary definitions matches:

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> v. lined, lin·ing, lines
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> v.tr.
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> 1. To mark, incise, or cover with a line or lines.
> 2. To represent with lines.
> 3. To place in a series or row.
> 4. To form a bordering line along: Small stalls lined the alley.
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