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The tropocollagen subunits spontaneously self-assemble, with regularly staggered ends, into even larger arrays in the extracellular spaces of tissues. Additional assembly of fibrils is guided by fibroblasts, which deposit fully formed fibrils from fibripositors. In the fibrillar collagens, molecules are staggered to adjacent molecules by approximately 64-67 nm, although up to a 10 nm distribution is intrinsic to collagen fibrils. This variation is hypothesized to arise from differing mechanical stressors, expression of other collagen types nearby, post-translational modifications, and/or fibrillar tilting and supercoiling. The unit is referred to as 'D' and is also dependent on the tissue and hydration state of the aggregate; the longitudinal distance between repeating molecules is 40 nm. Each D-period repeat of the microfibril consists of an "overlap" zone and a "gap" zone. The overlap zone consists of five collagen molecules aligned laterally and the gap zone consists of only four collagen molecules in cross-section. These overlap and gap regions are retained as microfibrils assemble into fibrils, and are thus viewable using electron microscopy. The triple helical tropocollagens in the microfibrils are arranged in a quasihexagonal packing pattern. The D-period plays a significant role in the mechanical properties of fibrils and cell-collagen crosstalk, and has also been correlated with various pathologies. Studies employing atomic force microscopy (AFM) have found changes in the D-band length assoicated with aging, diabetes, disc degeneration, and numerous genetic disorders