generic protein number density ≈2.7×10^6/µm^3: collagen bulk number density ≈105/µm^3 1/µm^3
||Rutenberg AD, Brown AI, Kreplak L. Uniform spatial distribution of collagen fibril radii within tendon implies local activation of pC-collagen at individual fibrils. Phys Biol. 2016 Aug 25 13(4):046008. doi: 10.1088/1478-3975/13/4/046008. p.4 left column 2nd paragraphPubMed ID27559989
|| Milo R 2013 What is the total number of protein molecules per cell volume? A call to rethink some published values BioEssays 35 1050–5 doi: 10.1002/bies.201300066.PubMed ID24114984
||P.4 left column 2nd paragraph: "Whole cell proteomics can estimate collagen abundance in vivo. PaxDB [refs 16, 17] provides an online resource for protein abundances, and integrated values over all listed studies estimate the abundance of the α2 chain collagen I in human cell lines to be 39 ppm, which is comparable to the whole-body average of 115 ppm (or 81 ppm for mice). [Investigators] take the smaller cell-line abundance as a crude upper bound on non-fibrillar collagen abundance. By using the protein number density of approximately 2.7×10^6/µm^3 in mammalian cells [primary source], they then obtain a bulk number density of collagen of ρ≈105/µm^3 . [They] see that [their] estimate of ρlower is two orders of magnitude less than, and so consistent with, the estimated bulk number densities from cell-line studies."