Complex sentence: The tail is shortened over time and when it is short enough the mRNA is enzymatically degraded, although a few cell types store mRNAs with short poly (A) tails for later activation by polyadenylation in the cytosol.
Split into simpler sentences: The tail is shortened over time and when it is short enough, the mRNA is enzymatically degraded. However, in a few cell types, mRNAs with short poly (A) tails are stored for later activation by re-polyadenylation in the cytosol.