Record it? Inert gas in cellular tissue? (10)

I believe the answer is:

epithelium

I'm a little stuck... Click here to teach me more about this clue!

'cellular tissue?' is the definition.
'epithelium' can be an answer for 'tissue?' (I've seen this before). I am not certain of the 'cellular' bit.

'record it? inert gas' is the wordplay.
'record' becomes 'ep'.
'inert gas' becomes 'helium' (I've seen this before).
'ep'+'it'+'helium'='EPITHELIUM'

'in' is the link.

Can you help me to learn more?

(Other definitions for epithelium that I've seen before include "tissue" , "outer layer" .)

I've seen this clue in the Sydney Morning Herald.

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