The starting material for de novo isolation of stem cell lines can be either normal 3.5-days post coitum (dpc) expanded blastocysts or "delayed" blastocysts. Delayed blastocysts are usually collected 4-6 days after ovariectomy. For both groups of blastocysts, tissue culture procedures are similar. The only difference is the timing of the first disaggregation, because delayed blastocysts will initially grow more slowly.