If you want something to dwell on, to perhaps guide your search of cell types, here's this. Cell types arise from differentiation of DNA expression, i.e., naked DNA acquires epigene markers that irreversibly direct the expression of DNA along different paths required by different cells. As cells develop, they spin off RNA according to which active genes are expressed, but only germline transmission of expression/transfer vector DNA, not RNA occurs.ª
Hence, the DNA which is most pliable to expression, i.e. unmarked naked DNA is the best indicator for the presence in gametes. Here's your thought: find the cell type with the least RNA or RNA specific to the type in gametes and focus on creating your sperm or eggs. A cautionary note, however, I don't believe it is ethical to alter naked DNA before creating gametes, so a "clean" naked DNA is requisite.
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http://www.ema.europa.eu/docs/en_GB/document_library/Scientific_guideline/2009/10/WC500003982.pdf