to its counterpart in vivo. The origin of this unit is today multiple, as it can come from a fragment of tissue, a stem cell located in an adult organ, an embryonic stem cell, or an induced pluripotent stem cell. However, it is critical to remember that the relevance or necessity of stem cells to production of organoids may be overstated to say the least. Even unselected single human breast cells were shown to be capable of making beautiful clonal acini, which of course are organoids when in 3D cultures, as early as 1992 (Petersen et al., 1992). Even more astonishing, single malignant breast cells under de right conditions can make phenotypically normal organoids that do not make tumors in animals (Weaver et al., 1997; Bissell and Hines, 2011). We should be mindful of claims of “stemness” being necessary for formation of organoids. The moral here is that when the right context is created, a single cell derived from a frozen mammary gland of a sheep can become a dolly! The complexity and paradoxes of biology are also its beauty, which never ceases to beckon us to go deeply in search of anwers.
