Histological Aspects of Cell Nucleus

Histological Aspects of Cell Nucleus

• The nucleus is the largest membrane-bound organelle in the cell.
• It houses nearly all the deoxyribonucleic acid (DNA) and the mechanisms for ribonucleic acid (RNA) synthesis.
• The nucleolus is the most prominent substructure within the nucleus.
• It is the site of ribosomal RNA (rRNA) transcription and processing, and ribosome assembly.

Nucleus Structures

• The nucleus is enclosed by a nuclear envelope composed of two concentric lipid membranes.
• The envelope has specialized components like nuclear pores.
• The inner and outer membranes are separated by a perinuclear space.
• The nucleus contains nucleoplasm, chromatin, and the nucleolus.

Nucleus Shape

• Typically, the nucleus is spherical and centrally located in the cell.
• However, it can vary in shape, including spindle-shaped, oblong shaped, twisted, lobulated, or disk-shaped.
• Some cells have multiple nuclei. Mature red blood cells, for example, have extruded their nuclei.
• The size, shape, and form of the nucleus can be diagnostic for cell type and malignancy in certain cancerous cells.

Nuclear Envelope

• The nuclear envelope is composed of two concentric, parallel lipid bilayer membranes.
• The inner and outer membranes are separated by the perinuclear cisterna,
• ranging from 20-40nm in space.
• The nuclear envelope is perforated by nuclear pores, allowing communication between the cytoplasm and nucleus.
• The nuclear pores also help organize the chromatin.

Outer Nuclear Membrane

• The outer nuclear membrane is continuous with the rough endoplasmic reticulum (RER).
• It's approximately 6nm thick.
• The outer membrane's cytoplasmic surface is lined with a loose meshwork of intermediate filaments, like vimentin.
• It has ribosomes that synthesize transmembrane proteins.

Inner Nuclear Membrane

• The inner nuclear membrane is around 6nm thick.
• It's in close contact with the nuclear lamina, a meshwork of intermediate filaments (80-300nm thick).
• Lamina is composed of lamins A, B1, B2, and C.
• It's located at the periphery of the nucleoplasm.

Nuclear Lamina

• The nuclear lamina helps organize and supports the lipid bilayer of the nuclear membrane.
• It also aids in the formation of nuclear pore complexes and assembly of vesicles for nuclear envelope reformation after cell division.

Nuclear Pores

• Nuclear pores are openings of about 80nm in diameter that traverse both the inner and outer nuclear membranes.
• They allow communication between the cytoplasm and the nucleus.
• The number of pores varies depending on the cell's metabolic activity.
• Pores are complex structures of hundreds of nucleoporins, arranged in three ring-like arrays.
• Pores have cytoplasmic fibers and a central transporter (plug) and nuclear basket.
• Pore complexes facilitate signal-mediated import and export processes.
• Molecules are transported through the nuclear pores via the interaction of importins and exportins with specific signals (NLS, NES) in the cargo.

Chromatin

• DNA, the genetic material, resides in the nucleus as chromosomes during cell division.
• Between divisions (interphase), DNA is unwound and forms chromatin.
• Different chromatin states (highly condensed heterochromatin or relaxed euchromatin) exist based on activity.
• The location (periphery vs. scattered) of chromatin can also indicate activity, as euchromatin is active in transcription.

Nucleolus

• The nucleolus is a dense, nonmembranous structure.
• It’s active during interphase (when the cell is not dividing) and disappears during cell division.
• Nucleoli have ribosomal RNA (rRNA) and protein.
• The nucleolus contains small amounts of DNA, which is not active.
• The number, size, and shape of nucleoli generally correlate with a cell's protein synthesis activity.

Nucleoplasm

• A somewhat viscous substance that surrounds chromosomes and nucleoli.
• It is made up of interchromatin granules, perichromatin granules, water, ribonucleoproteins (RNPs), and the nuclear matrix.

Chromosomes

• Chromosomes form from condensed chromatin fibers during mitosis or meiosis.
• Each species has a characteristic number of chromosomes.
• The total genetic material of a species is called its genome.

Telomeres

• Short, repeated DNA sequences at the ends of chromosomes.
• They protect the ends of chromosomes from degradation.
• Telomerase can maintain telomere length.
• Typically, somatic cells do not have telomerase so telomeres shorten with each division.

Sex Chromatin

• A condensed, inactivated X-chromosome.
• Only one of the two X chromosomes in female somatic cells is transcriptionally active.
• Inactive X chromosome, randomly determined early in development, remains inactive throughout the life of the individual.