2.3.
MINCED TISSUE
17
But in terms of tissue blocks, how small is small? This writer is reminded of chatting with a colleague at a national meeting when a novice
inquired about the sizes of pieces of tissue that might be well preserved.
My experienced friend asked him to visualize the head of a pin, and then
commented that we divide that into six parts! Although this may be a bit
of an exaggeration, the beginner has trouble grasping how truly tiny
these blocks must be.
Probably the simplest and best way of mincing tissue in osmium
tetroxide fixatives is to place a drop or two of fixative on a piece of
cardboard with a relatively nonabsorbent surface (ordinary filing cards
serve very well), or upon a thin sheet of dental wax. A small slab of tissue
is excised from the animal as quickly as possible, and transferred into the
drop of fixative with a toothpick. This slab or ribbon of tissue should be
as thin as possible, certainly less than a millimeter thick, and perhaps 2
or at the most 3 mm long. The toothpick then serves to pin the tissue
down while the tissue is minced with a razor blade (Fig. 1). The latter
must previously have been cleaned of the protective oils which the
manufacturer puts upon it.
The actual mincing must proceed with the utmost speed and be
stopped within 15 or 20 seconds even though not all of the tissue may be
finished. It is possible later to select only those pieces of satisfactory size
so it is not important at this stage to have uniformly small tissue blocks.
One reason for halting the mincing quickly is that osmium tetroxide
reacts with the steel of the razor blade so that a black precipitate appears.
No doubt it is advisable to stop as soon as this becomes evident. But even
more important seems to be that tissue fluids and plasma destroy within
seconds the desirable properties of buffered osmium tetroxide fixative
unless these substances are greatly diluted. Thus, the single drop of
fixative in which one minces will very quickly do more harm than good.
It is therefore essential to get the tissue into a proportionately large
volume of fixative within seconds.
The transfer of the tissue to a container with a large volume of fixative
can be done most easily if the original mincing is done near the tip of a
fairly narrow strip of cardboard, a strip which will fit into the vial of
fixative (Fig. 1). Ordinarily most of the tissue pieces will simply float away
when the tip of the cardboard is immersed in the vial. Any pieces that
remain adherent to the cardboard can be flicked off with a toothpick.
Strips of dental wax may be substituted for cardboard. Hard or soft grades
are obtainable from any dental supply house. In this procedure, metal
does not come in contact with the main volume of the fixative at all. If
MINCED TISSUE
17
But in terms of tissue blocks, how small is small? This writer is reminded of chatting with a colleague at a national meeting when a novice
inquired about the sizes of pieces of tissue that might be well preserved.
My experienced friend asked him to visualize the head of a pin, and then
commented that we divide that into six parts! Although this may be a bit
of an exaggeration, the beginner has trouble grasping how truly tiny
these blocks must be.
Probably the simplest and best way of mincing tissue in osmium
tetroxide fixatives is to place a drop or two of fixative on a piece of
cardboard with a relatively nonabsorbent surface (ordinary filing cards
serve very well), or upon a thin sheet of dental wax. A small slab of tissue
is excised from the animal as quickly as possible, and transferred into the
drop of fixative with a toothpick. This slab or ribbon of tissue should be
as thin as possible, certainly less than a millimeter thick, and perhaps 2
or at the most 3 mm long. The toothpick then serves to pin the tissue
down while the tissue is minced with a razor blade (Fig. 1). The latter
must previously have been cleaned of the protective oils which the
manufacturer puts upon it.
The actual mincing must proceed with the utmost speed and be
stopped within 15 or 20 seconds even though not all of the tissue may be
finished. It is possible later to select only those pieces of satisfactory size
so it is not important at this stage to have uniformly small tissue blocks.
One reason for halting the mincing quickly is that osmium tetroxide
reacts with the steel of the razor blade so that a black precipitate appears.
No doubt it is advisable to stop as soon as this becomes evident. But even
more important seems to be that tissue fluids and plasma destroy within
seconds the desirable properties of buffered osmium tetroxide fixative
unless these substances are greatly diluted. Thus, the single drop of
fixative in which one minces will very quickly do more harm than good.
It is therefore essential to get the tissue into a proportionately large
volume of fixative within seconds.
The transfer of the tissue to a container with a large volume of fixative
can be done most easily if the original mincing is done near the tip of a
fairly narrow strip of cardboard, a strip which will fit into the vial of
fixative (Fig. 1). Ordinarily most of the tissue pieces will simply float away
when the tip of the cardboard is immersed in the vial. Any pieces that
remain adherent to the cardboard can be flicked off with a toothpick.
Strips of dental wax may be substituted for cardboard. Hard or soft grades
are obtainable from any dental supply house. In this procedure, metal
does not come in contact with the main volume of the fixative at all. If