Cell nucleus

📌 Definition & Significance

• The nucleus contains all genetic information necessary to initiate and control differentiation, maturation, and metabolic activities of each cell. (Exam Point)

🖇 Site (Location within Cells)

• Essential organelle present in all complete (true) cells.
• Exceptions: Mature erythrocytes (RBCs) and blood platelets lack nuclei. (Exam Point)
• Position varies: central, eccentric, basal, or peripheral.

🟠 Number of Nuclei per Cell

• Single nucleus: Most cells.
• Binucleated cells: Parietal cells of stomach, cardiac muscle cells, and liver cells. (Exam Point)
• Multinucleated (Giant cells): Osteoclasts (bone), megakaryocytes (bone marrow), skeletal muscle cells. (Exam Point)

🟣 Shape

• Variable; corresponds to cell shape (spherical, ovoid, elongated).
• May be lobulated in granular leukocytes (WBCs). (Exam Point)

📌 Ultrastructure of the Non-Dividing Nucleus
Main components (under EM):

1. Nuclear Envelope
2. Chromatin Material
3. Nucleolus
4. Nuclear Sap (Nucleoplasm)

🖇 Light Microscope (LM) Appearance

• Prominent basophilic (dark blue with H&E) structure.
• Two appearances:
• 1. Pale-stained (Vesicular nucleus): Extended chromatin.
  2. Dark-stained (Condensed nucleus): Condensed chromatin.

📌 Detailed Components of the Nucleus

🟣 1. Nuclear Envelope

• Two concentric unit membranes (each 7.5 nm thick) separated by perinuclear space (40–70 nm wide).
• Outer membrane: Continuous with rough Endoplasmic Reticulum (rER).
• Inner membrane: Smooth.
• Nuclear pores: Openings where membranes close; contain nuclear pore complex. (Exam Point)
• Nuclear lamina: Thin filamentous meshwork (lamins) providing structural support and maintaining nuclear shape in interphase. (Exam Point)

🟣 2. Nucleolus

• Site: Present only in interphase nuclei; especially in active protein-synthesizing cells. (Exam Point)
• LM appearance: Dense, well-defined, basophilic body (1–3 µm).
• EM structure: Non-membrane-bound; composed of:
• 1. Pars Granulosa (Granular Center - GC / Nucleolonema): RNA granules (13–15 nm).
  2. Pars Fibrosa (Fibrillar Center - FC): Filaments (5 nm).
  3. Nucleolus-associated chromatin: Regions organizing ribosomal RNA (rRNA) synthesis. (Exam Point)

🟣 3. Chromatin Material

• Definition: Chromosomal substance of nucleus (DNA + histones + proteins).
• LM appearance: Dark basophilic material filling nucleus (H&E: dark blue).
• EM classification:
  A. Heterochromatin
  B. Euchromatin
• • Found in metabolically inactive cells (small lymphocytes, sperm) or cells producing one main product (plasma cells). (Exam Point)
  • Distributed in 3 forms: (Exam Point)
  • 1. Marginal chromatin: Peripheral.
    2. Karyosomes: Irregular chromatin bodies within nucleus.
    3. Nucleolar-associated chromatin: Around nucleolus.
• • Dispersed, extended form; nucleus appears pale. (Exam Point)
  • Contains active genes for protein synthesis.
  • Found in metabolically active cells (spermatocytes, nerve cells). (Exam Point)

🟣 4. Karyolymph / Nuclear Matrix (Nucleoplasm)

• Semi-fluid colloidal solution filling spaces between chromatin and nucleolus.
• Composition: extended chromatin (invisible form), lipoproteins, enzymes.
• Supports nuclear structures and biochemical reactions.