Nucleus

NUCLEUS, an essential structural component of animal and plant cells. The question of the presence of a nucleus in bacteria and lower algae is somewhat controversial, yet even here, judging by recent data, its existence must be acknowledged in the form of a substance diffusely distributed in the plasma. Most commonly, a cell has a single nucleus, but binucleate and multinucleate cells are also found. The shape of the nucleus is quite diverse; as a rule, it corresponds to the shape of the cell, yet alongside the most widespread round-oval shape, nuclei of rather bizarre shapes are encountered, e.g., the multilobed nuclei of leukocytes, annular nuclei, etc. In the same way, the position of the nucleus within the cell also varies: as a rule, it is located in the center or closer to the base, though sometimes it occupies another position as well. The nucleus is separated from the protoplasm by a thin but distinctly noticeable nuclear membrane. Disruption of the integrity of this membrane leads to the fusion of the nuclear substance with the protoplasm, which in turn sometimes produces a picture of dissolution of the cell plasma. In the question of the structure of the nucleus, one must distinguish between the structure of the nucleus that can be detected on fixed and histologically processed preparations and the vital (in vivo) structure of the nucleus. Histologically, one distinguishes within the nucleus a liquid protein substance—nuclear juice—in which firmer elements are immersed: a delicate, weakly staining network of finest threads, the so-called linin or achromatin network, as well as clumps and granules of very diverse shape and size, which vary greatly in individual nuclei. These granules, which intensively take up histological stains, are designated by the morphological concept of chromatin. Depending on their ability to stain with acidic or basic stains, basichromatin and oxychromatin are distinguished. In addition to the above-mentioned formations, a clearly demarcated and also intensively staining body—the nucleolus—is located in the nucleus. The number and size of the nucleoli vary considerably. The question of the physico-chemical structure of the living nucleus cannot be considered finally resolved at the present time. According to some researchers, the nucleus is an optically empty accumulation of protein bodies in a colloidal state, devoid of any structure; according to others, when observed in vivo, a very delicate fibrous structure can be detected in the nucleus (P. I. Zhivago). Chemically, the nucleus is a complex mixture of protein substances, among which phosphorus-rich nucleoproteins predominate. The nucleus undergoes very significant changes in the process of division, which always precedes cell division; these changes are especially great in the process of karyokinesis (see), when the chromatin substance of the nucleus takes the form of special, clearly demarcated sections—chromosomes. The physiological significance of the nucleus is very clearly illustrated by merogony experiments, i.e., cutting a cell into parts with the creation of nucleated and enucleated fragments. In this case, only sections provided with a nuclear fragment prove viable. The nucleus apparently bears the general regulation of enzymatic processes in the cell, as well as participation in regenerative processes. This is illustrated, for example, by the data of Klebs, who demonstrated in plants the migration of the nucleus toward the damaged and regenerating section of the cell. The chromosomes present in the nucleus are considered carriers of hereditary material. In protozoa, generative (micronucleus) and somatic (macronucleus) nuclei are distinguished. The ubiquitous distribution of nuclear substance indicates its high importance for biological processes.