5.19.
ORIENTATION OF BLOCK IN MICROTOME
173
between no thin sections and perfect cutting. The minimum possible
angle for sectioning naturally is very critical, but fortunately there is a
leeway of 2-3° beyond it.
Standardizing the knife height and clearance angle accomplishes little
if the position of the block is not reasonably standardized also. If for one
reason or another it is necessary to cut a block that points decidedly
upwards or downwards, appropriate allowance in the adjustment of the
knife height and angle should also be made.
5.19. Preferred Orientation of Block in Microtome
In mounting the block in the microtome, usually the primary consideration should be to minimize the forces which tend to deform the knife
edge. Therefore, in general, the short axis of the block face should be
parallel to the knife edge. The narrower the block, the less chance there
will be for "chatter" and "digging" (Chapter 5.22).
Dense connective tissue with its quantities of tough collagenous fibers
is particularly difficult to cut smoothly. If this involves only part of a
block it should be placed on the trailing side. Thus, to cut skin, epidermis
should reach the knife edge first, followed by the dermis. In cutting a
portion of a large artery, endothelium should be first and the adventitia
last, etc. In this way coarse chatter effects and tears are at least confined to
the connective tissue zone, and the rest of the tissue spared.
On occasion one wishes to make transverse sections of a thin sheet of
tissue so that the rectangle of the block face becomes very disproportionate, with one very narrow axis, and the other as long as possible. It may
be impractical to mount the long axis vertically, and it would be far too
wide to mount it parallel to the knife edge. Such very narrow blocks can
be cut by mounting them with the long axis somewhat oblique to the
knife edge. As the knife sections, then, only a small part of its edge is
being used at any given moment. This technique combines nicely with
the problems discussed in the previous paragraph. This writer, for
instance, has successfully sectioned long sheets of the anterior cornea by
mounting them diagonally, with the epithelium facing the knife edge,
and the lamina propria trailing.
Ordinarily the side faces of a block will be nearly parallel and should
be oriented vertically. If this is not the case, an overhanging corner of one
section may not be pushed cleanly away from the knife edge by the
ORIENTATION OF BLOCK IN MICROTOME
173
between no thin sections and perfect cutting. The minimum possible
angle for sectioning naturally is very critical, but fortunately there is a
leeway of 2-3° beyond it.
Standardizing the knife height and clearance angle accomplishes little
if the position of the block is not reasonably standardized also. If for one
reason or another it is necessary to cut a block that points decidedly
upwards or downwards, appropriate allowance in the adjustment of the
knife height and angle should also be made.
5.19. Preferred Orientation of Block in Microtome
In mounting the block in the microtome, usually the primary consideration should be to minimize the forces which tend to deform the knife
edge. Therefore, in general, the short axis of the block face should be
parallel to the knife edge. The narrower the block, the less chance there
will be for "chatter" and "digging" (Chapter 5.22).
Dense connective tissue with its quantities of tough collagenous fibers
is particularly difficult to cut smoothly. If this involves only part of a
block it should be placed on the trailing side. Thus, to cut skin, epidermis
should reach the knife edge first, followed by the dermis. In cutting a
portion of a large artery, endothelium should be first and the adventitia
last, etc. In this way coarse chatter effects and tears are at least confined to
the connective tissue zone, and the rest of the tissue spared.
On occasion one wishes to make transverse sections of a thin sheet of
tissue so that the rectangle of the block face becomes very disproportionate, with one very narrow axis, and the other as long as possible. It may
be impractical to mount the long axis vertically, and it would be far too
wide to mount it parallel to the knife edge. Such very narrow blocks can
be cut by mounting them with the long axis somewhat oblique to the
knife edge. As the knife sections, then, only a small part of its edge is
being used at any given moment. This technique combines nicely with
the problems discussed in the previous paragraph. This writer, for
instance, has successfully sectioned long sheets of the anterior cornea by
mounting them diagonally, with the epithelium facing the knife edge,
and the lamina propria trailing.
Ordinarily the side faces of a block will be nearly parallel and should
be oriented vertically. If this is not the case, an overhanging corner of one
section may not be pushed cleanly away from the knife edge by the