EM 013 Endoplasmic Reticulum

Freeze Fracture

Freeze fracture of a more complex specimen as seen by scanning electron microscope (SEM).

The absence of nuclei identifies this view as only the cytoplasm of one or more cells.

• Plasma Membranes - a pair cross the middle of the image with another one at the bottom of the image.
  • Protoplasmic face (or P-face; cyan) - most proteins (or complexes of proteins) remain embedded in the inner half of the plasma membrane adjacent to the cytoplasm.
  • Extracellular face (or E-face; yellow) - fewer particles are bound to the outer half of the plasma membrane adjacent to the extracellular space
• Cytoplasm (green) - upper cell and lower cell.

The a small region of the cytoplasm adjacent to the plasma membrane is seen for the upper cell, while a narrow cross-section of the cytoplasm is seen for the lower cell.

Endoplasmic Reticulum

Examine the cytoplasm of the upper cell.

The most prominent feature is a stack of flattened, membrane-enclosed structures known as cisternae. They are a characteristic feature of endoplasmic reticulum in many cell types. The apparent absence of ribosomes attached to their surface identifies them as smooth endoplasmic reticulum.

• Plasma Membranes - protoplasmic face (or P-face) (cyan) and extracellular face (or E-face) (yellow)
• Endoplasmic Reticulum (purple)
• Cytoplasm (green)

Mitochondria

Several organelles can be identified in the lower cells. Mitochondria (red) and endoplasmic reticulum (green) can be seen in the lower cell.

A circular structure (~500 nm in diameter) contains internal folds of membrane. This suggests it is a mitochondria fractured in cross-section revealing cristae.

A longitudinal depression appears to contain fragments from two parallel membranes. This suggests the fracture jumped between the inner and outer mitochondrial membranes on the underside of the missing mitochondria.

• Plasma Membranes - upper membrane and lower membrane [protoplasmic faces (or P-face) (cyan) and extracellular faces (or E-face) (yellow)].
• Mitochondria (red)
• Endoplasmic Reticulum (purple)
• Cytoplasm (green)