The effect of section thickness and embedding media on the observed S-phase labelling index of artificially selected cell populations from neonatal mouse liver and spleen.

Following an intraperitoneal injection of tritiated thymidine to neonatal mice, livers and spleens were removed and their labelling indices were derived autoradiographically. This was done in a number of ways: (1) from tissue imprints on gelatinised glass slides; (2) from tissue embedded in JB4 plastic sectioned at thicknesses of 2, 5 and 7 micron; and (3) from tissue embedded in paraffin wax and sectioned at 7 micron. The results show that the indices from the JB4 embedded sections increase as the section thickness decreases, and that this relationship persists down to the notional section thickness of zero in the tissue imprints (in which all the cells are in contact with the autoradiographic emulsion). Indices from the 7 micron paraffin wax embedded sections are surprisingly close to the values from the imprints, are higher than indices from the 5 and 7 micron JB4 embedded sections, and are not significantly different (at the 2% level) from those from 2 micron JB4 embedded sections. Possible reasons for these results are discussed in respect of the autoradiographic process and in relationship to various mathematical correction factors which have been proposed to take account of beta-particle self-absorption in thick sections. It is concluded that none of these correction factors is of value and that the embedding medium has an important effect on the observed labelling indices. Comparisons between labelling indices, therefore, should be made only when they are derived from similarly embedded material at the same section thickness.