English: In the small cnidarian Hydra vulgaris, a population of interstitial stem cells (indicated in yellow by cells expressing green fluorescent protein under control of the promoter for the gene nanos) gives rise to neurons, gland cells (for food digestion), gametes, and cnidocytes (stinging cells). These stem cells are located only in the the outer tissue layer (ectoderm) of the body column, the space between the head and the foot, but must make it to locations in the entire animal in both the outer and inner layers (endoderm). Thus, they migrate along and invade through the extracellular matrix, indicated here in magenta by an antibody to the protein Collagen I, to reach locations in the head, foot, and all through the endoderm. To me this image represents the dynamism of molecular structures--collagen can provide a roadway for cells to migrate along one tissue layer and a barrier keeping them from crossing through layers, but can also selectively create openings for cells to cross through those same layers.