FIGURE 2.58 • Electron micrographs of a liver cell with glycogen inclusions. a. Low-magnifcation electron micrograph showing a portion of a hepatocyte with part of the nucleus (N, upper left). Glycogen (G) appears as irregular electron-dense masses. Profles of rough endoplasmic reticulum (rER) and mitochondria (M) are also evident. 10,000. b. This higher-magnifcation EM reveals glycogen (G) as aggregates of small particles. Even the smallest aggregates (arrows) appear to be composed of several smaller glycogen particles. The density of the glycogen is considerably greater than that of the ribosomes (lower left). 52,000.
The Cell Nucleus
OVERVIEW OF THE NUCLEUS / 75 NUCLEAR COMPONENTS / 75 Chromatin / 75 Nucleolus / 79 Nuclear Envelope / 81 Nucleoplasm / 84 CELL RENEWAL / 84 CELL CYCLE / 86 Phases and Checkpoints Within the Cell Cycle / 86 Regulation of the Cell Cycle / 87 Mitosis / 89 Meiosis / 89 CELL DEATH / 93 Apoptosis / 94 Other Forms of Programmed Cell Death / 95 Folder 3.1 Clinical Correlation: Cytogenetic Testing / 80 Folder 3.2 Clinical Correlation: Regulation of Cell Cycle and Cancer Treatment / 81
The nucleus is a membrane-limited compartment that contains the genome (genetic information) in eukaryotic cells.
The nucleus contains genetic information, together with the machinery for DNA replication and RNA transcription and processing. The nucleus of a nondividing cell, also called an interphase cell, consists of the following components.
 Chromatin, is nuclear material organized as euchromatin or heterochromatin. It contains DNA associated with roughly an equal mass of various nuclear proteins (e.g., histones) that are necessary for DNA to function.
 The nucleolus (pl., nucleoli) is a small area within the nucleus that contains DNA in the form of transcription-ally active ribosomal RNA (rRNA) genes, RNA, and proteins. The nucleolus is the site of rRNA synthesis and contains regulatory cell-cycle proteins.
 The nuclear envelope is the membrane system that surrounds the nucleus of the cell. It consists of an inner and an outer membrane separated by a perinuclear cisternal space and perforated by nuclear pores. The outer membrane of the nuclear envelope is continuous with that of the rough-surfaced endoplasmic reticulum (rER) and is often studded with ribosomes.
 The nucleoplasm is nuclear content other than the chromatin and nucleolus.
A simple microscopic evaluation of the nucleus provides a great deal of information about cell well-being. Evaluation of nuclear size, shape, and structure plays an important role in tumor diagnosis. For instance, dying cells have visible nuclear alterations. These include karyolysis, or the disappearance of nuclei due to complete dissolution of DNA by increased activity of DNAase, pyknosis, or condensation of chromatin leading to shrinkage of the nuclei (they appears as dense basophilic masses), and karyorrhexis, or fragmentations of nuclei (these changes usually are proceeded by pyknosis).
Chromatin, a complex of DNA and proteins, is responsible for the characteristic basophilia of the nucleus.