Fig. 3

A conceptual ideograph representing the three distinct architectures for the stem cell niche notion in metazoans. A, B, and C refer to three structurally defined states assigned to describe the progressively complex architecture of niches and their cognate locations in an animal’s body. State A represents statuses with no connatural niche, where individual cells bear stem cell properties in their own existence, and cell fates are regulated through interactions with abutting cells. A cellular example that fills this criterion is: CTVT = canine transmissible venereal tumor. State B extends conceptual niches to the level of the whole tissue or the whole animal, which, by abductive reasoning, provide the appropriate habitat and foundation for the numerous SCs that reside and proliferate within a permanently existing niche holograph. The State C prototype epitomizes the well-structured and enduring model SCNs (typical of vertebrates) along with cases of ephemeral SCNs. Transitory niches = niches that are functional for a short period of time (about 1 or 2 weeks in botryllid ascidians), prior to ASCs departing the SCN—in concert with SCN degradation—and moving through vasculature to newly developed SCNs. Note that although sponges are presented here within the state A category, different clades bear state A and B architectures