Chapter 1 - The Cell
Ultrastructure is the architecture of cells that is visible at higher magnifications than found on a standard light microscope. Electron microscopes are widely used to investigate the ultrastructure of biological specimens.
This chapter demonstrates the utility of electron microscopy in understanding histology of cells and tissues.
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) to enhance contrast. The images are colorless and have a resolution of ~0.1 nm (more than 1,000x better than a light microscope).
Example micrograph of a cell imaged by TEM that explains how these images can be interpreted.
Schematic comparison of a cell imaged by TEM and light microscopy.
Schematic representations of a simple columnar cell from images acquired by TEM, SEM, and freeze fracture.
Ultrastructure of Cells TEM
Scanning Electron Microscopy SEM
Scanning electron microscope SEM produces three-dimensional images of a specimen 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 tissues and cells to be visualized. The images are colorless and have a resolution of ~1.0 nm.
Freeze fracture is a technique that reveals the internal structure of biological specimens. Specimens are rapidly frozen and physically broken apart (fractured). The exposed surface is coated with evaporated platinum/carbon to create a replica that is viewed by transmission electron microscopy TEM.
Freeze fracture forms enface views of membrane-bound compartments that give striking three-dimensional representations.
Thin Section versus Freeze Fracture
Mast cell as seen in a thin sections and by freeze fracture.
Ultrastructure of Cells Freeze Fracture
Freeze fracture reveals the internal surfaces of cells.
Cells undergoing mitosis as seen by TEM.