The shelf life for tissues for human transplantation differs significantly depending on which tissue, for what purpose. For the long term, you could direct your heirs to remove your bone marrow and preserve your bones for potential use in the far future, but the expense and emotional load seems excessive for a very low probability of utility to any of your heirs or descendants. Directing your heirs to donate all your tissues appropriate for use upon your demise saves multiple recipients immediately, improving society incrementally by increasing useful lifetimes, as well as incrementally expanding positive progressive altruism as a societal norm. Much surer positive impact on your loved ones, immediately and in the long term. Best wishes.