Nucleolus & Nucleus functions

• Every cell contains a nucleus, which carries almost all genetic information, characterizing it as a eukaryotic cell.
• The nucleus is the cell's "vital center".
• The nucleus is enclosed by a nuclear envelope during interphase and is vital for eukaryotic cell survival.
• The nucleus stores hereditary information as DNA, which is preserved through DNA replication.
• The nucleus is responsible for synthesizing mRNA, tRNA, and ribosomal RNA.

Visualization

• Standard staining with hematoxylin-eosin causes the nucleus to appear basophilic.
• Special staining with toluidine blue and Feulgen stain can also highlight the nucleus.

Structure

• The nucleus is surrounded by a nuclear envelope made of two membranes, creating a perinuclear space, and houses nucleoplasm, chromatin clusters, and nucleoli.

Morphology

• Nucleus shape and position varies based on cell type: central, basal, peripheral, or multiple as in hepatocytes.
• A complex structure separates chromatin from hyaloplasm and regulates exchanges between the nucleus and cytoplasm.
• The nuclear envelope has two trilamellar membranes, separated by a perinuclear space that stores calcium ions.
• Nucleus size varies by cell type, the nucleo-cytoplasmic ratio is specific to each cell type.
• Most cells have a single nucleus, with exceptions like anucleate red blood cells and keratinocytes, binucleate hepatocytes, and multinucleate osteoclasts.
• The nucleus can be centrally located in lymphocytes, fibroblasts, and endocrine gland cells
• The nucleus can be located at the base of the cell in mucous cells and exocrine glandular cells
• The nucleus can be peripherally located in striated muscle cells and adipocytes.

Nuclear Envelope

• The nuclear envelope is a complex membrane assembly in eukaryotic cells, separating chromatin from hyaloplasm during interphase and controlling exchanges between them.
• The envelope has two tri-layered membranes, each 75A° thick, separated by a perinuclear space of 200 to 400A°.
• The outer membrane is lined with ribosomes, continuous with the endoplasmic reticulum, and contains 70% protein and 30% fat.
• The outer membrane is rich in enzymes like glucose-6-phosphatase and electron transport chains.
• Perinuclear space, located between the two membranes, stores calcium ions.
• The inner membrane faces the nucleoplasm, is lined by the lamina, similar in structure to the outer membrane but with less enzymatic activity.
• The inner membrane has transmembrane proteins that act as binding sites for lamins and chromatin proteins and transmembrane calcium channels that release calcium ions (Ca++ATPase).
• The nuclear envelope acts as a barrier, controlling the passage of water, ions, and macromolecules, involved in nucleocytoplasmic exchanges and protein synthesis, and actively transports and stores Ca++ in the perinuclear space.

Nuclear Pores

• Nuclear pores are complex structures formed by nucleoporins, allowing nucleocytoplasmic exchanges.
• They're dynamic, ranging in number from 3,000 to 4,000 per nucleus based on the cell's physiological state.
• Pores vary in number depending on the physiological state of the cell (3,000 - 4,000 per nucleus).
• Composed of eight subunits that form two rings with perpendicular filaments, anchored to the nuclear envelope as a nuclear basket.
• The lateral tunnels measure around 10nm, and a central tunnel measures around 40nm.
• The pores control nucleocytoplasmic exchanges, enabling selective transit.
• Small molecules pass through lateral channels via passive diffusion while larger molecules, move through the central transporter, requiring energy.
• Transport involves an addressing system and an adapter (importin, exportine).

Nuclear Lamina

• The lamina is a dense fibrillar protein network beneath the inner nuclear envelope, made of lamin polypeptides.
• The nuclear lamina provides shape and maintains the rigidity of the nuclear envelope.
• Disassembly and reorganization occurs during lamin dissociation at mitosis.
• Progeria is associated with a mutation in the gene encoding lamin A on chromosome 1 and is a rare genetic premature aging disease

Chromatin

• Chromatin is the nucleoplasm content observed by microscopy in interphase nuclei.
• Heterochromatin is darkly stained, electron-dense representing 80-90% of the total chromatin in an adult cell, metabolically inactive regions (no transcription).
• Euchromatin appears clear, less dense after staining representing 10-20% of the total cell's chromatin and metabolically active regions (transcription).
• Euchromatin is spread throughout the nucleoplasm, while heterochromatin is found at the periphery, pressed against the nuclear membrane (forming the chromatic membrane), or in one or more juxta-nucleolar clumps.
• A somatic cell nuclei mass of heterochromatin is the Barr's corpuscle, or sex chromatin.
• Chromatin ultrastructure via electron microscopy shows tightly packed chromatin or nucleosomal fibers are made of nucleosomes, formed by a histone octamer with DNA wound around it.
• Chromatin condensation depends on cell activity.

Chromatin Composition

• Chromatin consists of DNA and histone proteins.
• Deoxyribonucleotides linked by phosphodiester bonds form DNA that contains the genetic information.
• Histones are basic proteins of five types: H1, H2A, H2B, H3, and H4.

Nucleolus

• The nucleolus appears as a dense spherical body and is considered a dynamic nuclear organelle.
• It is present during interphase, disappears during mitosis, varies in number, and is not bound by a membrane.
• Main function is ribosome biogenesis
• The nucleolus has a spherical or oval refractive body, surrounded by a ring of chromatin.
• Perinucleolar chromatin is in the form of a crescent, surrounding the nucleolar body.
• The components of the nucleolus visible by microscopy are fibrillar centers, a dense fibrillar component, and a granular component.
• The nucleolus has loops of DNA from acrocentric chromosomes (13, 14, 15, 21, 22), each contains rRNA genes, organized in nucleolar organizing centers (NOR).

Nucleolus Functions

• Fibrillar centers contain transcription proteins, while the dense fibrillar component consists of fibrillarin for rDNA transcription and pre-rRNA cleavage.
• The granular component houses prebosomes.
• Ribosome biosynthesis needs transcription of rDNA into 45S rRNA and proper neoformed maturation, and its cleavage into 5,8S, 18S, 28S fractions.
• The 18S RNA joins with the 40S small ribosomal subunit that exits the nucleoplasm.
• rRNA 5, 8, and 28s associate with each other and with ribosomal proteins and 5s RNA
• The large ribosome subunit (60S) then enters the cytoplasm.
• The nucleolus controls the cell cycle, working with regulatory proteins.

Nucleolus Related Diseases

• Werner syndrome is a hereditary premature aging disease caused by a mutation in a protein involved in nucleolar, DNA transcription.
• Treacher Collins syndrome is a genetic disorder affecting face and skull development, caused by a mutation in a nucleolar protein.

Cancer Cell Distinctions

• Cancer cells show numerous normal cell modifications, but cytological criteria unique to cancer are not seen.
• Anisokaryosis is unequal nucleus sizes in the same tissue.
• Irregular nucleus shape and contour.
• There can be multiple cores and an increased nucleocytoplasmic ratio.
• Chromatin condensation, or nuclear hyperchromatism.
• Nucleolar abnormalities (enlarged, irregular, or multinucleated nucleoli).
• Increased mitoses with shortened interphase.
• Abnormal, multipolar mitoses with chromosome distribution issues.
• Accumulation of certain elaboration products e.g. keratin, mucus, glycogen
• The plasma membrane appears thickened and irregular under light microscopy
• Under electron microscopy, the plasma membrane shows amorphous, irregular microvilli, and affected cell junctions.
• Cells of irregular size and shape define Anisocytosis, or cellular gigantism
• Reduced and basophilic cytoplasm

Explore the nucleus and nucleolus: rRNA production, ribosome assembly, and cell cycle regulation. Understand Werner and Treacher Collins syndromes. Learn to identify cell types based on nuclear characteristics and staining.