9.12.
INTERPRETATION OF ELECTRON IMAGES
323
tions one observes, it is quite legitimate to use the information qualitatively.
This basic characteristic of the electron image has far-reaching consequences. For example, it is theoretically impossible to see sharply the
edge of a wedge or a sphere suspended in space, for as one approaches
the edge the density does not abruptly drop, but rather tapers gradually
down to nothing. This has had more than theoretical interest for bacteriologists and virologists since their small organisms and particles presented a very serious problem in visualization until "shadowing" with
heavy atoms was developed as a method of outlining them (cf. Chapter
10). It remains a problem when looking at particles and, more particularly, membranous structures that are embedded in the plastic of
a section.
When a membrane is sectioned transversely, it appears as a line. When
it is sectioned in the horizontal plane, it appears as a smudge. Oblique
sections of membranes look like pale ribbons. The uninitiated may overlook the smudge, and interpret the ribbon an an out-of-focus image, or,
perhaps, as an image spoiled by drift. Figure 39 shows at high magnification numerous cytomembranes. In many places the lines are sharp and
obviously in focus. The rest of the micrograph must be also! Regions of
apparent local fuzziness and lack of contrast thus require an explanation
unrelated to the question of focus. The explanation lies in the geometry
of the membranes. When they are viewed vertically in the section, their
substance provides a maximal total density, and their edges appear sharp.
However, when the membranes are viewed obliquely, or particularly
when viewed horizontally, there is less scattering material in the path of
the electrons, and hence a reduced density. Thus their images appear
relatively pale, and without sharp delineation. Thus orientation becomes a matter of considerable importance in electron microscopy, and it
FIG. 39. Sensory cells of a cat retina. In some places the cytomembranes appear
sharp and with good contrast. In other areas the same membranes become pale and
lose definition. It is absurd to think that some portions of the micrograph are in focus
while other portions are out of focus. The explanation lies in the geometry of the
membranes. When they are viewed vertically in the section as a result of being sectioned
transversely, their substance provides a maximum total density, and the boundaries
are sharp. Remembering, though, that an electron micrograph is fundamentally a
shadowgram, when the membranes are viewed obliquely, and particularly tangentially
in the horizontal plane, there is less scattering material in the path of the electrons,
and hence a reduced density. Also edges tend to fade away in the image rather than
being sharply delineated.
Tissue first fixed by perfusion with buffered formaldehyde, and then treated with
osmium tetroxide. Embedded in Araldite. Stained with alkaline lead hydroxide.
INTERPRETATION OF ELECTRON IMAGES
323
tions one observes, it is quite legitimate to use the information qualitatively.
This basic characteristic of the electron image has far-reaching consequences. For example, it is theoretically impossible to see sharply the
edge of a wedge or a sphere suspended in space, for as one approaches
the edge the density does not abruptly drop, but rather tapers gradually
down to nothing. This has had more than theoretical interest for bacteriologists and virologists since their small organisms and particles presented a very serious problem in visualization until "shadowing" with
heavy atoms was developed as a method of outlining them (cf. Chapter
10). It remains a problem when looking at particles and, more particularly, membranous structures that are embedded in the plastic of
a section.
When a membrane is sectioned transversely, it appears as a line. When
it is sectioned in the horizontal plane, it appears as a smudge. Oblique
sections of membranes look like pale ribbons. The uninitiated may overlook the smudge, and interpret the ribbon an an out-of-focus image, or,
perhaps, as an image spoiled by drift. Figure 39 shows at high magnification numerous cytomembranes. In many places the lines are sharp and
obviously in focus. The rest of the micrograph must be also! Regions of
apparent local fuzziness and lack of contrast thus require an explanation
unrelated to the question of focus. The explanation lies in the geometry
of the membranes. When they are viewed vertically in the section, their
substance provides a maximal total density, and their edges appear sharp.
However, when the membranes are viewed obliquely, or particularly
when viewed horizontally, there is less scattering material in the path of
the electrons, and hence a reduced density. Thus their images appear
relatively pale, and without sharp delineation. Thus orientation becomes a matter of considerable importance in electron microscopy, and it
FIG. 39. Sensory cells of a cat retina. In some places the cytomembranes appear
sharp and with good contrast. In other areas the same membranes become pale and
lose definition. It is absurd to think that some portions of the micrograph are in focus
while other portions are out of focus. The explanation lies in the geometry of the
membranes. When they are viewed vertically in the section as a result of being sectioned
transversely, their substance provides a maximum total density, and the boundaries
are sharp. Remembering, though, that an electron micrograph is fundamentally a
shadowgram, when the membranes are viewed obliquely, and particularly tangentially
in the horizontal plane, there is less scattering material in the path of the electrons,
and hence a reduced density. Also edges tend to fade away in the image rather than
being sharply delineated.
Tissue first fixed by perfusion with buffered formaldehyde, and then treated with
osmium tetroxide. Embedded in Araldite. Stained with alkaline lead hydroxide.