Histological Aspects of Cell Nucleus PDF

Histological aspects of cell nucleus Nuriye Ezgi BEKTUR AYKANAT Faculty of Medicine Department of Histology and Embryology ATILIM UNIVERSITY 1  The nucleus is the largest and membrane- bound organelle of the cell....

Histological aspects of cell nucleus Nuriye Ezgi BEKTUR AYKANAT Faculty of Medicine Department of Histology and Embryology ATILIM UNIVERSITY 1  The nucleus is the largest and membrane- bound organelle of the cell. It contains nearly all of the deoxyribonucleic acid (DNA) possessed by the cell as well as the mechanisms for ribonucleic acid (RNA) synthesis. The most prominent substructure within the nucleus is the nucleolus, which is the site of rRNA transcription and processing, and of ribosome assembly. 2  NUCLEUS STRUCTURES 3  The nucleus, bounded by the nuclear envelope composed of two concentric lipid membranes, houses three major components: ❑Nucleoplasm, containing macromolecules and nuclear particles involved in the maintenance of the cell. ❑Chromatin, the genetic material of the cell ❑Nucleolus, the center for ribosomal RNA (rRNA) synthesis 4  The nucleus is usually spherical and is centrally located in the cell; however, in some cells, it may be spindle shaped to oblong shaped, twisted, lobulated, or even disk shaped. 5  Although, usually, each cell has a single nucleus, some cells (such as osteoclasts) possess several nuclei, whereas mature red blood cells have extruded their nuclei. The size, shape, and form of the nucleus are generally constant for a particular cell type, a fact useful in clinical diagnoses of the degree of malignancy of certain 6 cancerous cells. NUCLEAR ENVELOPE The nucleus is surrounded by the nuclear envelope, composed of two concentric parallel unit membranes: the inner and outer nuclear membranes, separated from each other by a 20- to 40-nm space called the perinuclear cisterna. The nuclear envelope is perforated at various intervals by nuclear pores that permit communication between the cytoplasm and the nucleus and assists in organizing the chromatin. 7 Outer Nuclear Membrane The outer nuclear membrane is also about 6 nm thick, faces the cytoplasm, and is continuous with the rough endoplasmic reticulum (RER) and is considered by some authors as a specialized region of the RER whose lumen is continuous with the perinuclear cisterna. Its cytoplasmic surface is surrounded by a thin, loose meshwork of the intermediate filaments, vimentin. Its cytoplasmic surface usually possesses ribosomes actively synthesizing transmembrane proteins that are destined for the outer or inner nuclear membranes. 8 Inner Nuclear Membrane The inner nuclear membrane is about 6 nm thick and faces the nuclear contents. It is in close contact with the nuclear lamins, an interwoven meshwork of intermediate filaments, 80 to 300 nm thick, composed of lamins A, B1, B2, and C and located at the periphery of the nucleoplasm. 9  The nuclear lamins help in 1organizing and providing support to the lipid bilayer membrane and the 2 perinuclear chromatin as well as 3playing a role in the formation of nuclear pore complexes and 4assembly of vesicles to reform the nuclear envelope subsequent to cell division. 10  Nuclear Pores At certain locations on the surface of the nuclear envelope, the outer and inner nuclear membranes are continuous with each other, creating openings about 80 nm in diameter known as nuclear pores, which permit communication between the nuclear compartment and the cytoplasm. The number of nuclear pores in a nucleus ranges between a few dozen to several thousand, correlated directly with the metabolic activity of the cell. 11  Each of the nuclear pore complexes is in communication with the others via the nuclear lamina and certain pore- connecting fibers. 12 Nuclear Pore Complex The nuclear pore complex is about 100 to 125 nm in diameter and spans the two nuclear membranes. The nuclear pore complex is composed of about a hundred different proteins, collectively known as nucleoporins, arranged in three ring-like arrays of proteins stacked on top of the other. In addition, the nuclear pore complex has cytoplasmic fibers, a central transporter (plug) , 13 and a nuclear basket.  Each subunit possesses a cytoplasmic fiber (cytoplasmic filament). It has been suggested that these fibers may mediate import into the nucleus through the nuclear pore complex by moving substrates along their length toward the center of the pore. 14 15 Function of the Nuclear Pore Complex The nuclear pore complex functions in bidirectional nucleocytoplasmic transport. The bidirectional traffic between the nucleus and the cytoplasm is mediated by a group of target proteins containing nuclear localization signals (NLSs) known as importins and nuclear export signals known as exportins (also known as karyopherins, PTACs, transportins, and Ran-binding proteins). Exportins transport macromolecules (e.g., RNA) from the nucleus into the cytoplasm, whereas importins transport cargo (e.g., protein subunits of ribosomes) from the cytoplasm into the nucleus. 16  Exportin and importin transport is regulated by a family of GTPbinding proteins known as Ran. These specialized proteins along with other nucleoporins located along receptor sites in the nuclear pore complex facilitate the signal-mediated import and export processes. 17 18 CHROMATIN DNA, the cell’s genetic material, resides in the nucleus in the form of chromosomes, which are clearly visible during cell division. In the interval between cell divisions (=interphase), the chromosomes are unwound in the form of chromatin. Depending on its transcriptional activity, chromatin may be condensed as heterochromatin or extended as euchromatin. 19  Heterochromatin is located mostly at the periphery of the nucleus and comprises almost 90% of the total chromatin of the nucleus. The remainder of the chromatin scattered throughout the nucleus and not visible with the light microscope is euchromatin (=less condensed chromatin). This represents the active form of chromatin where the genetic material of the DNA molecules is being transcribed into RNA. 20 21 22 23 24 25 Chromosomes As the cell leaves the interphase stage and prepares to undergo mitotic or meiotic activity, the chromatin fibers are extensively condensed to form chromosomes, visible with light microscopy. The number of chromosomes in somatic cells is specific for the species and is called the genome, the total genetic makeup. 26  In humans, the genome consists of 46 chromosomes, representing 23 homologous pairs of chromosomes. One member of each of the chromosome pairs is derived from the maternal parent; the other comes from the paternal parent. Of the 23 pairs, 22 are called autosomes; the remaining pair that determines gender are the sex chromosomes. The sex chromosomes of the female are two X chromosomes (XX); those of a male are the X and Y chromosomes (XY). 27  Giemsa reagent stains the adenine-thymine–rich regions of chromosomes, producing a pattern of G bands that is unique for each chromosome pair and is characteristic for each species. Careful analysis of the G bands can help reveal deletions of certain portions of the chromosome, nondisjunctions, translocations, and so on, that may assist in the diagnosis of certain genetic disorders or diseases resulting from chromosomal anomalies. 28 29 30  Telomeres are short, repeated, DNA sequences at the ends of chromosomes. They appear to protect the ends of the chromosomes from degradation, and in oocytes and spermatogonia as well as in stem cells an enzyme–RNA complex, known as telomerase, maintains the telomere length. Interestingly, the RNA portion of the enzyme is used as a template to synthesize the additional DNA necessary to maintain telomere length. Somatic cells do not possess telomerase, and, with each successive cell division, the telomeres become shorter, and eventually they become short enough that they can no longer protect the chromosome, and the cell becomes unable to replicate itself. This built in senescence is absent in cancer cells because many malignant cells are able to express the gene that codes for telomerase. 31 Sex Chromatin Only one of the two X chromosomes in female somatic cells is transcriptionally active. The inactive X chromosome, randomly determined early in development, remains inactive throughout the life of that individual. 32  Microscopic study of interphase nuclei of cells from females displays a very tightly coiled clump of chromatin, the sex chromatin (Barr body), the inactive counterpart of the two X chromosomes. Epithelial cells obtained from the lining of the cheek and neutrophils from blood smears are especially useful for studying sex chromatin. The sex chromatin is observed at the edge of the nuclear envelope in smears of the oral epithelial cells and as a small drumstick-like evagination of the nuclei of the neutrophils. 33 Nucleoplasm Nucleoplasm, separated from the cytoplasm by the nuclear envelope, is a somewhat viscous substance that surrounds the chromosomes and the nucleoli and is composed of interchromatin and perichromatin granules, water, ribonucleoproteins (RNPs), and the nuclear matrix. 34 35  It has an amorphous structure. It helps maintain the shape and structure of the nucleus. It plays an important role in the transport of materials that are vital for cell metabolism 36 and function.  Nucleolus The nucleolus, a dense nonmembranous structure located in the nucleus, is observed only during interphase because it dissipates during cell division. It stains basophilic with hematoxylin and eosin, being rich in rRNA and protein. The nucleolus contains only small amounts of DNA, which is also inactive. Usually, there are no more than two or three nucleoli per cell; however, their number, size, and shape are generally species specific and relate to the synthetic activity of the cell. In cells that are actively synthesizing protein, the nucleolus may occupy up to 25% of the nuclear volume. 37  Four distinct areas of the nucleolus have been described: ❑A pale-staining fibrillar center (FC), containing inactive DNA (not being transcribed) ❑Pars fibrosa (PF), containing nucleolar DNAs being transcribed into rRNA as well as the protein nucleolin, small nucleolar ribonuclear proteins, and ribonucleoprotein enzyme fibrillarin, necessary for the converting of the pre-rRNA into mature rRNA ❑Pars granulosa (PG), in which maturing ribosomal subunits. ❑Nucleolar matrix, a network of fibers active in nucleolar organization ❑Also located in the pale-staining regions are the tips of chromosomes 13, 14, 15, 21, and 22 (in humans), containing the nucleolar-organizing regions (NORs), where gene loci that encode rRNA are located. 38 39 Video links https://www.youtube.com/watch?v=yxy59vpd33I https://www.youtube.com/watch?v=rjaudsjiGV8 40 41