A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen,[1] the structural framework (stroma) for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells of connective tissue in animals.
For example, a 700 x 20 C road tire would be a 20-622; a 700 x 38 hybrid tire would be a 38-622. The width difference between these sizes would make them less-than ideal replacements for one another, but any rim that could fit one of them would work after a fashion with the other.
R15 is the wheel size... any 15 tire can fit on a 15 wheel (well... not really, this depends on width of the tire and width of the wheel, but most consumer tires won't be a problem.) First number is the width in millimeters.