Bright-field Microscope
The bright-field microscope is the workhorse of the clinical
laboratory (Figure 1-54). The microscope is named brightfield
because the object being viewed is seen against a bright
field of view. All routine clinical laboratory tests requiring a
microscope can be performed using the bright-field microscope.
Bright-field microscopes are especially well suited for
viewing stained specimens, such as stained blood smears.
Figure 1-55A shows an image as seen with a bright-field
microscope.
Phase-Contrast Microscope
The phase-contrast microscope provides an improved way of
viewing unstained cells, which are nearly transparent. By installing
special objectives and a phase condenser, bright-field microscopes
can be equipped for phase contrast. Phase contrast is
useful for viewing specimens such as urine sediments and for
performing platelet counts using the hemacytometer. With phasecontrast
microscopy, the background (field) appears grey and the
specimen is bright. Figure 1-55B shows an image viewed with
phase-contrast microscopy.
Epi-fluorescence Microscope
Epi-fluorescence microscopes use ultraviolet light to illuminate
the specimen. The epi-fluorescence microscope enables
objects that have been stained with fluorescent dyes to be
observed. When these dyes are combined with antibodies, it is
possible to identify specific areas of reaction within a cell or on
a cell surface. The epi-fluorescence microscope can be used to
identify microorganisms such as mycobacteria, and to detect
the presence of antibodies in certain diseases such as syphilis
and lupus erythematosus. Figure 1-55C shows Borrelia burgdorferi,
the cause of Lyme disease. The bacteria are stained
with a fluorescently labeled antibody and viewed with epifluorescence
microscopy.
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Electron Microscopes
Electron microscopes provide much greater magnification and
resolving power than light microscopes. The image from an electron
microscope is created by exposing specimens to an electron
beam, rather than illuminating them with a light source. Electron
microscopes have been used in medical research for several years,
but have been limited for the most part to pathology and virology.
However, with new knowledge and techniques, their use in clinical
medicine is increasing.
With the electron microscope, objects as small as 0.001 m
(too small to be seen with light microscopes) can be viewed. The
two types of electron microscopes are the transmission electron
microscope (TEM) and the scanning electron microscope (SEM),
shown in Figures 1-56 and 1-57.
Objects are visualized in the TEM by passing an electron
beam through the specimen. Minute details inside a cell, such
as nuclear structure, can be seen (Figure 1-58A). The image is
displayed on a phosphorescent screen and viewed through protective
glass or projected onto a monitor (Figure 1-58A). In the
Diopter
adjustment
Eyepieces
Body tube
Arm
Coarse
adjustment
knob
Fine
adjustment
knob
Mechanical stage
controls
Base
Interpupillary
distance
adjustment
Revolving
nosepiece
Stage with
slide clips
Condenser and
iris diaphragm
Light
Objectives
FIGURE 1-54 Binocular bright-field microscope with parts labeled
FIGURE 1-55 Microscopy images: (A) stained cells viewed with a bright-field microscope; (B) phase-contrast image;
(C) Borrelia burgdorferi stained with fluorescent antibody and viewed with epi-fluorescence microscopy
(Photo A courtesy of Abbott Laboratories, Abbot Park, IL; photo B courtesy of CDC, Atlanta, GA)
A B C
LESSON 1-10 The Microscope 111
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112 UNIT 1 Introduction to the Clinical Laboratory
SEM, the electron beam is scanned over the surface of a metalcoated
specimen, causing electrons to bounce off the specimen.
These deflected electrons are measured with a detector and
converted into a three-dimensional image similar to an image on a
TV screen. Figure 1-58B is a scanning electron microscope image
of the surface of blood cells.
Electron microscopes are very expensive and require
lengthy specimen preparation and special expertise to operate.
Reference laboratories, medical schools, and teaching hospitals
are the most likely clinical locations for electron microscopes.
PARTS OF THE MICROSCOPE
Microscope design can differ slightly from one model to another.
However, some parts are common to all microscopes. The microscope
shown in Figure 1-54 has the parts labeled.
Oculars
A microscope can be monocular or binocular. Monocular microscopes
have only one ocular, or eyepiece. Because of this, most
people find it difficult to use them without eyestrain. Binocular
microscopes have two eyepieces to allow viewing with both eyes,
resulting in less eyestrain (Figure 1-54).
The oculars, or eyepieces, located at the top of the microscope,
are attached to a barrel or tube connected to the micro-
FIGURE 1-57 Scanning electron microscope
FIGURE 1-56 Transmission electron microscope
FIGURE 1-58 Electron microscopy images: (A) cell viewed
with transmission electron microscope (Courtesy of C.A.
Sundermann, Auburn University, AL); (B) blood cells as
seen with the scanning electron microscope (Courtesy Philips
Electronics Instruments Co.)
A
B
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