Pyknosis

PYKNOSIS (from Greek pyknoo—I condense), condensation with shrinkage, observed during necrobiosis (see) in the protoplasm and nuclei of cells. Pyknosis of the nucleus in some cases precedes its disintegration (see Karyorrhexis). The gradual condensation is associated with a decrease in the size of the cell and its transformation into a compact mass. At the same time, the protoplasm becomes darker, homogeneous, or exhibits very densely arranged granulation, showing a strong tendency to stain with nuclear stains. In many cases, the pyknotic cell stains so diffusely and intensely that the nucleus within it is no longer distinguishable. In this stage, the cells assume an irregular shape, their contours become serrated, and cracks appear inside. Pyknotic changes of the nucleus (karyopyknosis), accompanying pyknosis of the protoplasm, are more complex and, in general, reduce to the fact that the nucleus becomes smaller due to the absorption of liquid components and the agglutination of chromatin; the nucleus often loses its round shape and acquires a particularly intense and diffuse coloration. With deeper disturbances of cell nutrition, the stainable content of the nucleus (chromatin and the substance of the nucleolus) collects on the nuclear membrane (marginal hyperchromatosis of the nucleus). Subsequently, karyopyknosis passes into the splitting of the nucleus (karyorrhexis). Sometimes, during pyknosis, the nucleus becomes irregularly rounded, angular, lobed, or notched. In some nuclei that have still retained their shape, light bubble-like vacuoles of various sizes are encountered. Further, during pyknosis, forms of nuclei resembling mulberries are observed, and finally, in pyknotic cells, nuclei are encountered whose chromatin takes the form of rings, oblong, oval, or irregularly shaped formations. A good object for observing pyknotic changes of the cell and nucleus can be the desquamated epithelium of the renal tubules of the kidneys (epithelial casts), as well as cells of malignant tumors.