Where can I find values, or estimates, of the density of cells in human tissues? Maybe an overall estimate, or distinct values for distinct tissues? Or maybe not human, but mammal tissues (which should be similar)?
Well, Sender, Fuchs, and Milo wrote a paper to discuss the total number of cells in the body and compare it to the number of bacteria in the body (http://dx.doi.org/10.1101/036103). Their discussion is quite in-depth.
There are a number of ways to calculate it, but an interesting one involved calculating mammalian cell density by a study measuring the DNA obtained from a 25 g mouse. Researchers were able to determine there were 3*109 cells in the mouse (Baserga, 1985). From this study we can simply divide to find a value for "typical" mammalian cell density--1.2*108 cells/g.
If you want to know the number of cells per mL, you'll need the density (g/mL) of the tissue you are measuring. As the comments have mentioned, this is typically around 1 g/mL and varies from person to person. There is a web page that has some tissue densities for various tissues.
Baserga, R. (1985). The Biology of Cell Reproduction (Harvard University Press).
The 'cleanest' data I am aware of is on studies using density gradient centrifugation. In density gradient centrifugation, a preparation of cells is layered on top of a density gradient media which has higher density than water (water=1g/ml). Centrifugation forces denser cells through the media whereas cells with equal or lower density remain above the solution. Common values of gradients are 1.084g/ml, 1.077g/ml and 1.073g/ml but a useful overview of the technique can be found here. According to a manual for one density gradient product, Ficoll-Paque PREMIUM (search the GE website), GE Lifesciences says:
Ficoll-Paque PREMIUM 1.073 can be used when isolating lower density human mononuclear cells, for example mesenchymal stromal/stem cells or monocytes. The higher density lymphocytes and granulocytes will sediment through Ficoll-Paque PREMIUM 1.073 to the bottom of the tube, thereby enriching the lower density cells at the interface. Ficoll-Paque PREMIUM 1.073 has been found superior to Ficoll-Paque PREMIUM for isolating mesenchymal stem cells from human bone marrow (56).
Ficoll-Paque PREMIUM 1,084 can be used for preparation of cell fractions including higher density human mononuclear cells or for isolating lymphocytes that form rosettes with autologous red blood cells (15). It can also be used for separating blood cells from mice and rats, since the lymphocytes in rodents have a slightly higher average density than lymphocytes in humans (50, 51).
If you wanted to try and evaluate density of isolated cells within 'solid' tissues your results would require careful (often enzymatic) release of cells from their integral contact with the extracellular matrix and other cells. I also presume you could try chunks of tissue.
Density gradient centrifugation can also be performed by layering proportions of gradient media of two densities, resulting in a continuous gradient and more nuanced separation.
TL;DR blood cells are slightly denser than water.
hi guys im new here and i was reading this and would add my own believs to this. and most wont follow this, but... Density is a meassure of complexity. Density equals energy + information. Based on our human build up its only logical our body density is close to that of water but with natural selection law of living things and the reproducing factor of non living things such as stars and galaxy's, i have come to conclude that the thing to understand behind it all is that there is that process, that something that drives energy or matter to change into something else, energy(or equal in matter) + information, that something is the next wheel in our understanding.