20
2. TISSUE EXPOSURE
Formaldehyde, glutaraldehyde, and some of the other smaller aldehydes
are splendid penetrants. Preservation can be excellent if buffered solutions of these are used as a primary fixative, followed by a secondary
fixative
with osmium tetroxide. Their use in this manner allows a
technical latitude that would be quite impossible with osmium tetroxide
alone. Thus slabs of tissue, several times as thick as would be acceptable
with osmium tetroxide, can be cut free-hand, and given a short (10-30
minutes) fixation in suitably buffered formaldehyde or glutaraldehyde
(Chapter 3.9). After some hardening, these then must be trimmed so that
at least one dimension is sufficiently thin for the subsequent penetration
of osmium tetroxide. It is interesting and curious that the latter agent
will not penetrate any better through a block of formaldehyde-fixed tissue
than through fresh tissue. But the hardened tissue at least can be trimmed
more precisely than fresh tissue, and furthermore, one then is not working under any pressure of time. Presumably, also, one has the advantage
of well-arranged lighting and whatever magnifying aids he needs, set up
in an optimal fashion.
It is of course also desirable for some types of work to use tissue fixed
only in an aldehyde. Such tissue can be prepared by cutting a slab freehand, and putting it into an aldehyde bath. Then the tissue would
receive prolonged fixation, perhaps overnight, before proceeding with
dehydration and embedding.
2.4. Topical, in Situ, Preservation of Natural Surfaces
It is often possible to take advantage of naturally occurring surfaces to
preserve tissue in a living animal while blood continues to circulate
through the tissue in question. Under these circumstances mechanical
damage can be kept to an absolute minimum. It is a particularly appropriate means of preservation in an organ like the kidney which
receives its blood supply from within. It is less appropriate in anatomical
situations like the adrenal gland which receives blood by way of its
capsule, and thus would lose its blood supply before fixation begins. It
has proved an important way of preserving tissues which are too delicate
to withstand well the mechanical disturbance of mincing, such as those of
the central nervous system. It is also a good way to fix muscle in a
normally extended state.
If an organ surface is to be fixed in situ, it has been found almost
2. TISSUE EXPOSURE
Formaldehyde, glutaraldehyde, and some of the other smaller aldehydes
are splendid penetrants. Preservation can be excellent if buffered solutions of these are used as a primary fixative, followed by a secondary
fixative
with osmium tetroxide. Their use in this manner allows a
technical latitude that would be quite impossible with osmium tetroxide
alone. Thus slabs of tissue, several times as thick as would be acceptable
with osmium tetroxide, can be cut free-hand, and given a short (10-30
minutes) fixation in suitably buffered formaldehyde or glutaraldehyde
(Chapter 3.9). After some hardening, these then must be trimmed so that
at least one dimension is sufficiently thin for the subsequent penetration
of osmium tetroxide. It is interesting and curious that the latter agent
will not penetrate any better through a block of formaldehyde-fixed tissue
than through fresh tissue. But the hardened tissue at least can be trimmed
more precisely than fresh tissue, and furthermore, one then is not working under any pressure of time. Presumably, also, one has the advantage
of well-arranged lighting and whatever magnifying aids he needs, set up
in an optimal fashion.
It is of course also desirable for some types of work to use tissue fixed
only in an aldehyde. Such tissue can be prepared by cutting a slab freehand, and putting it into an aldehyde bath. Then the tissue would
receive prolonged fixation, perhaps overnight, before proceeding with
dehydration and embedding.
2.4. Topical, in Situ, Preservation of Natural Surfaces
It is often possible to take advantage of naturally occurring surfaces to
preserve tissue in a living animal while blood continues to circulate
through the tissue in question. Under these circumstances mechanical
damage can be kept to an absolute minimum. It is a particularly appropriate means of preservation in an organ like the kidney which
receives its blood supply from within. It is less appropriate in anatomical
situations like the adrenal gland which receives blood by way of its
capsule, and thus would lose its blood supply before fixation begins. It
has proved an important way of preserving tissues which are too delicate
to withstand well the mechanical disturbance of mincing, such as those of
the central nervous system. It is also a good way to fix muscle in a
normally extended state.
If an organ surface is to be fixed in situ, it has been found almost