Figure 4. Levels of B2M in young muscle and brain correlate positively with the heterochronicity of blood exchange.

(a) Muscle cryosections of 10 μm and 25 μm brain-SGZ cryosections of isochronically and heterochronically apheresed mice (that had experimental muscle injury) were immuno-stained for B2M and counter-stained for Hoechst to label all nuclei. Representative images were acquired at the sites of muscle injury (Mu in) outside the injury-repair (Mu out) and at the hippocampi-DG areas (brain DG), scale bar is 50 μm for muscle and liver, and 100 μm for brain. (b) Pixel density of B2M was quantified using Image J from serial cryosections represented in a; and shown are the means and standard errors. In muscle: ***,**P<0.005. Significant differences were observed between YY and YO (P=0.004), OY and OO (0.001), YO and OY (P=0.0007), and YY and OO (P=0.006), N=5–7 per group. In brain: ****P<0.00005. Significant differences were observed between YY and YO (P=0.00001), and YY and OO (P=0.004). (c) Western SDS–polyacrylamide gel electrophoresis was used to analyse B2M levels in one microlitre of cell-free blood serum from 5 young (Y) and 5 old (O) mice. ECL images were quantified by ImageJ and expressed as background-corrected pixel volume. N=5. P=0.5. B2M becomes increased with age in muscle and brain but it is not elevated in old blood serum as compared with young. After heterochronic blood exchange B2M is increased by old blood in young muscle and decreased by young blood in old muscle (regionally, outside of the injury site). B2M is also increased in young hippocampi-DG after exchange with old blood, but B2M is not diminished in the old DG after young blood exchange. Shown are means±s.e.m. for all histograms.