The extraction behavior of thymus and the composition of fractions prepared from this organ has been studied. Sequential extraction methods using 0.15 M NaCl followed by water gave information with respect to the weight fraction of cytoplasmic and nuclear constituents. Lipide, nucleic acid, and electrophoretic analysis of the extracts provided additional information. A less complex electrophoretic pattern was obtained from subsequent extracts in the sequence.

Sucrose and saline dispersates obtained from tissue fragmented with either the Potter-Elvehjem homogenizer or in a Waring blendor were fractionated, using standard differential sedimentation methods. The fractions obtained by means of four different dispersion procedures were compared in terms of yield, chemical analysis, and electrophoretic composition.

The quantity of material in thymus having the sedimentation characteristics of liver mitochondrial and microsomal fractions was remarkably small. Both the suspension medium employed and the method used to bring about a disruption of the cells in the tissue affected the yield of "particulate" material. The components present in the later extracts in the sequence, E4 to E7, in the case of sequential extraction study resembled with respect to chemical composition and electrophoretic characteristics, the microsome fraction prepared by differential sedimentation methods.

About 76 per cent of the PNA in the tissue appeared to be in the cytoplasm. The remaining 24 per cent PNA was found in the nucleus and accounted for 1.7 per cent of nucleus on a dry weight basis.

From 75 to 88 per cent of cytoplasmic PNA was extracted from the tissue and 76 to 94 per cent of the PNA in the extract was found in the final supernatant solutions, depending upon the dispersion methods and suspension medium used in the extraction procedure.

The composition of the final supernatant fractions using differential sedimentation methods were comparable in terms of electrophoretic properties, protein concentration, nucleic acid content, and fractionation behavior to saline extracts E1 to E3, of thymus used in earlier studies.

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