CorpusHuman Anatomy
Cells & TissuesOrganelle

Nucleolus

Nucleolus

Also known as: nucleoli, nucleolar organiser region, NOR

A dense, non-membranous nuclear body that forms around the ribosomal RNA genes and is the site of rRNA transcription, processing and ribosome subunit assembly.

The Eukaryotic Cell

Overview

NOR-bearing chromosomes13, 14, 15, 21, 22 (acrocentric)
Pre-rRNA45S → 18S + 5.8S + 28S
rRNA gene copies≈200 per haploid genome

Nucleoli form around nucleolar organiser regions (NORs) — tandem repeats of the rRNA genes (≈200 copies per haploid genome) on the short arms of the five acrocentric chromosomes 13, 14, 15, 21 and 22. RNA polymerase I transcribes them into a 45S pre-rRNA, which is chemically modified (2′-O-methylation, pseudouridylation, guided by small nucleolar RNAs) and cleaved into 18S, 5.8S and 28S rRNA. These combine with 5S rRNA (made by RNA polymerase III outside the nucleolus) and with ≈80 ribosomal proteins imported from the cytoplasm to form 40S and 60S subunits, which are exported separately through nuclear pores.

In EM three regions are seen: pale fibrillar centres (rDNA and RNA polymerase I), the surrounding dense fibrillar component (active transcription and early processing; fibrillarin) and the outer granular component (maturing pre-ribosomal particles). The nucleolus is a phase-separated condensate: it disperses in prophase when rRNA transcription stops and re-forms at the NORs in telophase.

Nucleolar size reflects protein-synthetic activity — nucleoli are large in neurons, oocytes, secretory cells and cancer cells. The nucleolus also senses stress: disruption of ribosome biogenesis releases ribosomal proteins that inhibit MDM2 and so stabilise p53.

Parts & subdivisions

  1. Fibrillar centre
    rDNA not yet being transcribed, RNA polymerase I.
  2. Dense fibrillar component
    Nascent pre-rRNA, snoRNPs (fibrillarin).
  3. Granular component
    Assembling ribosomal subunits.

Anatomy

Histology

Basophilic (RNA-rich), often 1–3 per nucleus. A large, prominent "cherry-red" (eosinophilic) macronucleolus is a feature of some malignancies (e.g. melanoma, prostatic adenocarcinoma) and of Reed–Sternberg cells.

Function

  • Transcription of rRNA genes by RNA polymerase I.
  • Processing and modification of pre-rRNA.
  • Assembly of 40S and 60S ribosomal subunits.
  • Stress sensing and sequestration of regulatory proteins.

Clinical significance

Prominent nucleoli in cancer

Increased ribosome biogenesis supports rapid growth; nucleolar prominence is part of nuclear grading (e.g. WHO/ISUP grading of renal cell carcinoma).

Anti-nucleolar antibodies

A nucleolar ANA pattern (anti-fibrillarin/U3-RNP, anti-PM/Scl, anti-RNA polymerase I/III) is associated with systemic sclerosis.

Robertsonian translocation

Fusion of two acrocentric chromosomes at their centromeres loses only the rDNA-containing short arms (redundant), so carriers are healthy but at risk of unbalanced offspring — e.g. translocation Down syndrome (often 14;21).

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