Chapter 1 - Introduction
Ultrastructure is the architecture of cells that is visible at higher magnifications than found on a standard optical light microscope. Electron microscopes are used to investigate the ultrastructure of biological specimens.
This chapter demonstrates the potential of electron microscopy in understanding histology.
Transmission Electron Microscopy (TEM)
TEM produces two-dimensional images of a specimen by imaging a thin section with a beam of electrons. Ultrathin tissue sections are stained with heavy metals (such as osmium tetroxide, uranium or lead salts). The images are colorless and have a resolution of ~0.1 nm (or ~1000x greater than a light microscope).
Example micrographs of cells imaged with TEM.
Schematic comparison of a cell imaged by TEM and light microscopy.
Ultrastructure of cells as seen by TEM.
Schematic representations of a simple columnar cell from images acquired by TEM, SEM, and freeze fracture.
Scanning Electron Microscopy (SEM)
Scanning electron microscope (SEM) produces three-dimensional images of a speciment by measuring the relative differences in the reflection of a focused beam of electrons scanned across a specimen. Biological specimens are usually coated with a thin layer of metal (such as platinum) to form a replica that is then imaged. This allows the surface structures of organelles, cells, and tissues to be visualized. The images are colorless and have a resolution of ~10 nm (or ~100x greater than a light microscope).
Freeze fracture reveals the internal structure of biological specimens. Frozen specimens are physically broke apart (fracturing) and the exposed surface is coated with evaporated platinum. This replica is then imaged by TEM.
Freeze fracture forms enface views of membrane-bound compartments that give striking three-dimensional representations.
Comparison of Ultrathin Sections and Freeze Fracture
Micrographs of a mast cell as seen by ultrathin sections and freeze fracture.
Examples of Freeze Fracture
The interior structure of cells and organelles as seen by freeze fracture.
Cells undergoing mitosis as seen by transmission electron microscopy (TEM).