Page 47 - Anatomy and Physiology of Farm Animals, 8th Edition

32 / Anatomy and Physiology of Farm Animals

This approach to the study of the animal Electron Microscopy
body has been standard for many years
VetBooks.ir and will continue to be useful regardless Electron microscopes do not use visible

of newer developments. However, some
factors should be kept in mind when study­ light for the delineation of structures as
in the light microscope; they use a beam
ing sections or photographs of sections. of electrons focused by electromagnetic
The relationship of the tissue sections lenses. The electron beam may pass through
to the actual tissue is about the same as a thin specimen in the transmission elec-
that of a bag of potato chips to a growing tron microscope or be reflected from the
potato. Both the sections and the potato surface of an object and studied with the
chips have been processed so that actual scanning electron microscope. The images
resemblance to the original structure is with the electron microscope, however, are
limited. Both are seen in two dimensions, only black and white. (For an example of an
length and width, with thickness relatively electron micrograph, see Fig. 1‐2.)
unimportant for visualization. Recent The scanning electron microscope is a
technological advances such as confocal versatile instrument with a magnification
microscopy and computer reconstruction range from × 15 to × 10,000 and a resolution
provide three‐dimensional views of cellular in the vicinity of 10 nm. Depth of field with
and tissue structure. the scanning electron microscope is much
The light microscope can magnify greater than with any light microscope.
objects to a maximum of about 1500 times Preparation of specimens for observation
the original size. This is known as the with the scanning electron microscope is
magnification, or power, of the microscope. relatively simple. Non metallic biologic mate­
Resolving power refers to the property of rial generally is dehydrated and coated with a
showing two objects as separate structures. thin layer of metallic gold before it is placed
The light microscope can resolve (separate) in the scanning electron microscope.
two structures that are as close as The transmission electron microscope
approximately 0.2 μm (about 200 nm). This is capable of much higher magnification
resolving power depends greatly on the (as much as × 1 million), with an effective
wavelength of the light used to observe resolution of 0.1 nm. By the use of photo­
the tissue and the optical quality of the graphic enlargement and projection tech­
objective lens of the microscope. niques, the magnifications can exceed 1
Other developments in light microscopy million and still show good detail. Although
include phase contrast and fluorescence much more detail can be seen in a small
microscopy. Phase contrast microscopy can area, tissue preparation for transmission
be used with unstained and/or living cells, electron microscopy is much more exact­
because it depends on differences in refrac­ ing and time‐consuming than for light
tion of various parts of a cell for image microscopy. (A typical transmission elec­
formation. Fluorescence microscopy is often tron micrograph is shown in Figure 2‐9.)
teamed with immunocytochemistry, in The best means of fixation is to apply a
which antibody‐labeled cells are identified fixative (commonly glutaraldehyde fol­
by causing them to fluoresce upon expo­ lowed by osmium tetroxide) to the living
sure to light of a specific wavelength. tissue or biopsy specimen. The time from
X‐ray diffraction is used to study the the living state to immersion in the fixative
structure of inorganic and organic crystals should not exceed 2 minutes, and the size
and the molecular structure of biologic of tissue should not exceed 1 mm on a side.
substances such as DNA, collagen, and Osmium tetroxide acts both as a fixative
hemoglobin. It consists of passing a beam and as a stain. Other heavy metals, includ­
of x‐rays through the substance and record­ ing lead, may be used as so‐called stains.
ing the diffraction pattern (scattering of The term stain may be used somewhat
the beam) on a photographic emulsion. loosely, because the areas where the