Chromomeres

Chromomeres, histological term for the designation of the smallest grains of nuclear chromatin, introduced by Eisen (Eisen, 1910). At the beginning of karyokinesis, chromomeres combine into grains of larger size—chromomeres, or Pfitzner's spheres (Pfitzner, 1880), and from them are formed the chromosomes. The doctrine of chromomeres was developed in detail by M. Heidenhain (M. Heidenhain, 1907), who identified them with the cyanophilic granules of Altmann (Altmann, 1892) and considered them the simplest visible biosystems of the nucleus under the microscope, i.e., histomers of the same order as centrioles. In the resting nucleus, Heidenhain distinguishes two kinds of chromomeres: basi-chromomeres, which stain predominantly with basic dyes, and oxy-chromomeres—with acidic dyes; this difference is most clearly manifested when stained by Biondi; basi-chromomeres are usually connected into lumps and grains of larger size; during division, when chromomeres form compact Pfitzner spheres and chromosomes, the difference in staining disappears. Like all histomers, chromomeres multiply by division; this process already begins in the daughter nuclei and continues during the growth of the nucleus (see also Karyokinesis, Cell, Chromatin).