A method of treating brain tissue damage or a neurodegenerative disease in a subject by obtaining from a subject a bodily fluid sample containing a plurality of cells, incubating the sample with EDTA or heparin in a container until the sample is separated into an upper layer and a lower layer, collecting the upper layer, isolating from the upper layer a population of somatic stem cells that are 0.3-6.0 micrometers in size, and administering to a subject in need thereof an effective amount of the somatic stem cells.