Within the cell nucleus the main structures are: the nucleolus, an
optically dense region, spherical shaped, where there are concentrated
ribosomal RNA (rRNA) associated to proteins (there may be more than one
nucleolus in a nucleus); the chromatin, made of DNA molecules dispersed
in the nuclear matrix during the cell interphase; the karyotecha, or
nuclear membrane, the membrane that delimits the nucleus.
