CorpusHuman Anatomy
Cells & TissuesOrganelle

Nucleus

Nucleus

Also known as: cell nucleus, nucleoplasm, karyoplasm

The largest organelle (≈5–10 µm), enclosed by a double-membraned nuclear envelope, that houses the genome as chromatin and is the site of DNA replication, transcription, RNA processing and ribosome assembly.

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Overview

Diameter≈5–10 µm
DNA per diploid nucleus≈6.4 × 10⁹ bp (≈6.5 pg), ≈2 m long

Most cells have a single spherical or oval nucleus, but its shape and number are characteristic of cell type: flattened in squamous cells, elongated ("cigar-shaped") in smooth muscle, multilobed in neutrophils, and multiple in skeletal muscle fibres (a syncytium of fused myoblasts), osteoclasts and syncytiotrophoblast; some hepatocytes and urothelial umbrella cells are binucleate. Mature erythrocytes, lens fibres and cornified keratinocytes lose their nuclei, and platelets are anucleate cell fragments.

The nucleus is bounded by the nuclear envelope, perforated by nuclear pores, and supported internally by the nuclear lamina. Its contents (nucleoplasm) include chromatin (DNA with histones), one or more nucleoli, and nuclear bodies and speckles where RNA-processing factors concentrate. Separating the genome from the cytoplasm allows pre-mRNA to be fully processed (capped, spliced, polyadenylated) before it meets ribosomes, and allows gene expression to be regulated by controlling the nuclear entry of transcription factors.

In histopathology the nucleus is the key to diagnosis: malignant cells often show enlarged, hyperchromatic, pleomorphic nuclei with irregular outlines, coarse chromatin, prominent nucleoli and a high nuclear-to-cytoplasmic ratio, while dying cells show pyknosis (shrinkage), karyorrhexis (fragmentation) or karyolysis (fading).

Location

Usually central; basal in columnar epithelia; eccentric in plasma cells and signet-ring (white fat) adipocytes; peripheral in skeletal muscle fibres.

Parts & subdivisions

  1. Nuclear envelope
    Inner and outer membranes with nuclear pore complexes.
  2. Nuclear lamina
    Meshwork of lamins A, B and C (type V intermediate filaments) lining the inner membrane; shapes the nucleus and anchors chromatin.
  3. Chromatin
    DNA–histone complex; euchromatin and heterochromatin.
  4. Nucleolus
    Ribosome factory.
  5. Nucleoplasm
    Nuclear matrix with enzymes for replication, transcription and RNA processing.

Anatomy

Histology

Nuclei are basophilic (stained blue-purple by haematoxylin) because of the phosphate groups of DNA. Pale, vesicular nuclei with fine chromatin indicate active transcription (e.g. neurons); small dense nuclei indicate inactive cells (e.g. lymphocytes).

Function

  • Stores and protects the genome.
  • DNA replication and repair.
  • Transcription and processing of RNA.
  • Ribosome subunit assembly (nucleolus).
  • Controls cell activity by regulating gene expression.

Clinical significance

Nuclear features of malignancy

Pleomorphism, hyperchromasia, high nuclear-to-cytoplasmic ratio, prominent nucleoli and atypical mitoses are hallmarks of anaplasia and are used for tumour grading.

Antinuclear antibodies (ANA)

Autoantibodies to nuclear components are a screening test for systemic autoimmune disease: anti-dsDNA and anti-Smith (SLE), anti-histone (drug-induced lupus), anti-centromere (limited cutaneous systemic sclerosis), anti-Scl-70/topoisomerase I (diffuse systemic sclerosis).

Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.