EM 007 Nucleus

Nucleus

The nucleus of a pancreatic β cell (which synthesizes large amounts of the protein insulin) as seen by transmission electron microscopy (TEM).

Switch between the grayscale and color images (see above) to identify these components of a nucleus.

• Chromatin (blue) - DNA bound to histones and other proteins. Exists in two forms:
  • Heterochromatin (dark blue) - condensed chromatin that is transcriptionally inactive (intensely stained)
  • Euchromatin (light blue) - dispersed regions of unpacked chromatin that is often actively transcribed (lightly stained)
• Nucleolus (yellow) - site of ribosomal RNA transcription and production of ribosomes (intensely stained)

Nuclear Envelope

The nucleus is enclosed by a nuclear envelope that consists of two membranes. The inner membrane is in contact with chromatin. The outer membrane is continuous with the endoplasmic reticulum. The space between these membranes is continuous with lumen of the endoplasmic reticulum.

Identify connections between the outer membrane (purple) and the endoplasmic reticulum (cyan). The outer membrane (but not the inner membrane) often has bound ribosomes.

(Ribosomes also occur "free" in the cytoplasm or "membrane-bound" to the endoplasmic reticulum.)

Nuclear Pores

Nuclear pores are channels that fuse together the inner and outer nuclear membranes. The large complexes (~120 nm diameter) regulate the transport of materials between the nucleus and the cytoplasm. Their number varies between cell types and the metabolic activity of cells.

Examine the nuclear envelope (purple) for nuclear pores cut in cross section. Heterochromatin (dark blue) is not bound to the inner nuclear membrane where nuclear pores are found.