Units of Measurement

UNITS OF MEASUREMENT

n  1 m = 1000 mm (millimeters)

n  1000 mm = 1 µm (microns)

n  Bacteria are about 1µm or smaller

n  1000 µm = 1nm (nanometers)

n  Viruses are about 1nm

n  1000 viruses can fit into one bacterium

Protozoa are fairly large single-celled animals. You can see them with the naked eye.

Bacteria are so small, they are measured in µm. They are the smallest things you can see under a microscope with the oil immersion lens.

Viruses are even smaller, so they are measured in nm.

MICROSCOPY: VOCABULARY

Immersion oil: keeps light from bending and allows lens to be refracted.

Resolution: ability of two lenses to distinguish two points.

Parfocal: focused in all lenses.

Depth of field: how much of the background is in focus at the same time that the foreground is in focus.

RESOLUTION: the ability of the lenses to distinguish two points. A microscope with a resolving power of 0.4 nm can distinguish between two points greater than or equal to 0.4nm. When you go to the eye doctor, you look at the chart (Snellen chart) and read it from 20 feet away. If you can read what a normal sighted person can read from 20 feet away, it is called 20/20 vision. If you can’t read it well, your eyesight has less resolution than normal.

TYPES OF MICROSCOPES

SIMPLE MICROSCOPE: Has only one lens, like an ocular (eyepiece)

COMPOUND MICROSCOPE: More than one lens, like an ocular and an objective. An example is the Brightfield microscope.

There are two main types of compound microscopes: Light Microscopes and Electron Microscopes.

Light Microscopy
Light microscopes have an RP of about 0.25 µm, about the size of the very largest viruses and the very smallest cells. They extend the resolving power of the human eye by about 1000x. One major advantage of light microscopes is their low cost and relative ease of use. Another is that it is possible to view living tissue in some microscopes, especially with phase optics. For many applications, light microscopy is the preferred tool, such as in screening tissues for signs of cancer or other pathology. A disadvantage of light microscopes is that they do not have good depth of field; when you look at the foreground, the background is out of focus.

Light microscopes can have a variety of configurations. Some popular types are:

Dissecting

Brightfield
Darkfield
Phase-contrast

Differential Interference contrast
Fluorescence

DISSECTING MICROSCOPE

This type of microscope is for viewing in 3D objects that are very large (visible to the naked eye) but in greater detail than the naked eye can see. It shows a view like a powerful magnifying glass. It typically magnifies 20x to 40x. It requires an external light source (plug in a lamp and face it to the scope).

BRIGHTFIELD ILLUMINATION

Most student microscopes (including the ones we use in lab) are Brightfield microscopes, requiring that objects to be viewed either have natural contrast or be stained to provide sufficient contrast for viewing. Some stains, called vital stains, can be used on living tissue; most require the sample to be dried and fixed.

In general, the Brightfield microscope is not very good for looking at live cells (without stain) because the contrast is not good; cells look white on a white background. You can attempt to look at live cells with this microscope by opening up the iris all the way to let all the light in to improve contrast. This microscope is very good for looking at stained cells. When you apply stain to cells it kills them, but the contrast is improved and you can see them well. This type of light microscope produces the most realistic images of a cell, compared to other light microscopes.

DARKFIELD ILLUMINATION

This has limited applications, but it is good for looking at live cells because the cells look white against a dark background. You can even see the cilia and flagella.

PHASE CONTRAST MICROSCOPY

This is the most sensitive type of microscopy. Live cells are visible because contrast is increased. You can even see cilia and flagella.

DIFFERENTIAL INTERFERENCE CONTRAST

This shows depth in 3D. It has two beams of light and a prism. By moving the prism, you see different colors. This is a good way to view live cells; it has the best resolution of the light microscopes. To get better resolution, you need to use electron microscopy.

FLUORESCENCE MICROSCOPY

In this technique, ultraviolet light (invisible) is used. Cells are stained with a fluorescent dye call fluorochromes, which energizes electrons to create visible light. You can see the light given off by cells. It is not possible to see live cells with this. It is often used for viewing syphilis and TB.

ELECTRON MICROSCOPY

Electron microscopes use a beam of electrons rather than light to illuminate an object. Magnification of up to a million-fold is possible. Creating an electron beam requires very high voltages, typically 100,000 volts, and the electron beam works well only in a vacuum, so air must be removed prior to specimen examination. Electrons are not easily scattered by the light atoms of common cellular materials: C, H, O, and N. To improve electron scattering and increase contrast, heavy metals are used, either as coatings over exposed surfaces or as stains applied to tissues.

The primary advantage of all electron microscopes is the added resolving power and the extraordinary clarity of the images.

Disadvantages include: the cost and complexity of the equipment; the need to isolate them from vibrations (vibrations from a passing truck can be easily visible when viewing an object at 1,000,000x) the inability to study living tissue; the very thin samples required; and the need for heavy metals to obtain good contrast, with possible damage or distortion of the sample.

The two major forms of electron microscope, TEM and SEM, are very different. One views surfaces or "outsides," the other views "insides." Together, they reveal an extraordinary world of very small objects.

Transmission Electron Microscope (TEM)
Transmission electron microscopes (the most common types of electron microscopes) transmit a beam of electrons through the sample. To achieve this, samples must be very small and very thin. Therefore, the specimen is sliced thin using an instrument called an ultramicrotome. Most samples are mounted on tiny circular grids with a very fine mesh screen. The grid is coated with a thin plastic layer, and sample is either adsorbed directly (as in a virus sample) or prepared as an ultra thin section of tissue that has been fixed, stained, dehydrated, and embedded in resin to form a hard block of plastic. This gives a view of sections of the specimen’s cells, revealing the structures of its organelles.

This type of microscope uses electromagnetic lenses and creates a flat image, and since there is no light, there is no color. Electrons float better in a vacuum, so it has a vacuum pump. Staining is done with heavy metal salts instead of dyes. Areas with low levels of impregnation of these metal salts produce bright regions on the screen, and heavily impregnated areas produce dark ones (the electrons can't get through). This improves how thing look with this microscope.

Advantages of TEM are the ability to view objects at a very high level of resolution, and to see the insides of common biological materials. Disadvantages include the limit of only viewing a small sample at one time, and the need for very thin specimens. If the specimen is thicker than 0.25 µm, too many electrons are adsorbed to obtain a good image. The TEM and the SEM have the highest resolution of all the microscopes.

Scanning Electron Microscope (SEM)

Scanning electron microscopes work by scanning a beam of electrons across the surface of an object. Each area so illuminated discharges a small shower of electrons, called secondary electrons. These are trapped by a detector that converts electrons into light signals, which in turn are trapped by a photomultiplier and used to drive a cathode-ray tube (similar to a television tube). The resulting image can be viewed and photographed.

A unique feature of an SEM is its great depth of focus; objects in both foreground and background appear equally sharp and crisp, giving the best 3D image because it has the highest resolution. By contrast, light microscopes have a very shallow depth of focus, so as one moves to high-power lenses, only a small part of the image is in focus at any one time.

Another advantage of SEM is that sample size can be very large. For example, an entire insect or small flower can be viewed at one time. The SEM gives the most realistic view of a specimen.

This provides a view of the outside of the cell only; it cannot penetrate to the inside of cells like the TEM. It also needs a vacuum, and cells are never alive. Its resolution is 10x better than a Brightfield microscope.

A special type of SEM is called the Scanning Probe Microscope.

Scanning Probe Microscope

This microscope forms images of surfaces using a physical probe that scans the specimen. An image of the surface is obtained by mechanically moving the probe in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position. A Raster scan, or raster scanning, is the pattern of image detection and reconstruction in television, and is the pattern of image storage and transmission used in most computer image systems. The word raster comes from the Latin word for a rake, as the pattern left by a rake resembles the parallel lines of a scanning raster.

In a raster scan, an image is cut up into successive samples called pixels, or picture elements, along scan lines. Each scan line can be transmitted as it is read from the detector, as in television systems, or can be stored as a row of pixel values in an array in a computer system. On a television receiver or computer monitor, the scan line is turned back to a line across an image, in the same order. After each scan line, the position of the scan line is advanced, typically downward across the image in a process known as vertical scanning, and a next scan line is detected, transmitted, stored, retrieved, or displayed. One advantage of scanning probe microscopy is that the resolution of the microscopes is not limited by diffraction, but only by the size of the probe-sample interaction volume. Disadvantages of scanning probe microscopy are that the scanning techniques are generally slower in acquiring images due to the scanning process, and the maximum image size is generally smaller. This type also gives a 3D view of the specimen.

COMPARISON OF MICROSCOPES

BRIGHTFIELD / Dark objects are visible against a bright background.
Light reflected off the specimen does not enter the objective lens
Not for looking at live cells
Maximum resolution is 0.2µm and maximum magnification is 2000x
Stains are used on specimens
DARKFIELD / Light objects are visible against dark background
Used for live cells, cilia, flagella
Especially good for spirochetes
Uses special condenser with an opaque disc that eliminates all light in the center
PHASE-CONTRAST / No staying required
Accentuates diffraction of the light that passes through a specimen
Good for live cells; good contrast
Most sensitive; cilia shows up
Not three-dimensional
DIFFERENTIAL
INTERFERENCE
CONTRAST / Uses two beams of light
Shows three dimensions
Has a prism to get different colors
Good for live cells (unstained)
Best resolution
FLUORESCENCE / Uses ultraviolet light
Stained cells with fluorescent dye; energizes electrons and creates visible light
No live cells
Quick diagnosis of TB and syphilis
TRANSMISSION
ELECTRON / Get flat images
Have vacuum pumps to allow electrons to float better
Stain with heavy metal salts
Shows sections of cell, revealing organelles
Requires an ultramicrotome
Best resolution of all microscopes
SCANNING
ELECTRON / Surface view only
Needs a vacuum
No live cells
Three-dimensional view
SCANNING PROBE / Physical probe scans the specimen
Raster scan: image is cut up into pixels and transmitted to computer
Not limited by diffraction
Slower in acquiring images
Maximum image size is smaller