10.7.
TWO-STAGE REPLICATION
339
with chromium, and finally covered with a replicating film of evaporated
carbon. Adhering biological material is later removed from this preshadowed replica by boiling it in concentrated sodium hydroxide. If the
original specimen is a frozen block of tissue with a shaved surface, the
final image will be that of a "pseudosection."
10.7. Two-Stage Replication
Single-stage replicas inevitably must be delicate objects, and are only
practical when a thin film can be stripped easily from the surface to be
replicated, or when the specimen can be dissolved or digested leaving the
replica behind. One can, fortunately, employ "two-stage" replication to
avoid these limitations. Then a suitably thick primary replica is made
first which can be mechanically stripped from the specimen surface. This
surface is in turn replicated to produce a second and final, thin, positive
replica. The primary replica is removed by differential solution.
It is obvious that the final replica may be either preshadowed to give
a negative image of the specimen surface, or shadowed as a final step to
give a positive image. Since preshadowing avoids distortion, it is more
commonly employed.
Nowadays evaporated carbon is almost always the material of choice
for the final replica. Its insolubility in almost all reagents allows it to be
isolated and cleaned. Primary replicas are most commonly made of
Formvar.
Two-stage replication is advantageous when it is desirable to remove
the replica by mechanical stripping and when the specimen is large
enough so that these operations can be performed under a binocular
dissecting microscope, or even without optical aids. Hard tissues such as
bone, teeth, and even hair can be handled easily (Fig. 41). Soft tissue
FIG. 41. The surface of the lunula of a
fingernail.
A fairly thick Parlodion replica
of a freshly exposed surface of the fingernail first was made, and then obliquely
shadowed in vacuo with chromium. This surface was secondarily replicated by evaporating carbon upon it from several directions and, particularly, vertically. Subsequently the Parlodion was dissolved away leaving the thin carbon replica for microscopy. This process is properly called two-stage replication. The first replica is a
negative, the second, a positive. The second replica can also be spoken of as being
preshadowed. This is a negative print, as is commonly done with shadowed material,
so that the "shadows" appear dark rather than light.
TWO-STAGE REPLICATION
339
with chromium, and finally covered with a replicating film of evaporated
carbon. Adhering biological material is later removed from this preshadowed replica by boiling it in concentrated sodium hydroxide. If the
original specimen is a frozen block of tissue with a shaved surface, the
final image will be that of a "pseudosection."
10.7. Two-Stage Replication
Single-stage replicas inevitably must be delicate objects, and are only
practical when a thin film can be stripped easily from the surface to be
replicated, or when the specimen can be dissolved or digested leaving the
replica behind. One can, fortunately, employ "two-stage" replication to
avoid these limitations. Then a suitably thick primary replica is made
first which can be mechanically stripped from the specimen surface. This
surface is in turn replicated to produce a second and final, thin, positive
replica. The primary replica is removed by differential solution.
It is obvious that the final replica may be either preshadowed to give
a negative image of the specimen surface, or shadowed as a final step to
give a positive image. Since preshadowing avoids distortion, it is more
commonly employed.
Nowadays evaporated carbon is almost always the material of choice
for the final replica. Its insolubility in almost all reagents allows it to be
isolated and cleaned. Primary replicas are most commonly made of
Formvar.
Two-stage replication is advantageous when it is desirable to remove
the replica by mechanical stripping and when the specimen is large
enough so that these operations can be performed under a binocular
dissecting microscope, or even without optical aids. Hard tissues such as
bone, teeth, and even hair can be handled easily (Fig. 41). Soft tissue
FIG. 41. The surface of the lunula of a
fingernail.
A fairly thick Parlodion replica
of a freshly exposed surface of the fingernail first was made, and then obliquely
shadowed in vacuo with chromium. This surface was secondarily replicated by evaporating carbon upon it from several directions and, particularly, vertically. Subsequently the Parlodion was dissolved away leaving the thin carbon replica for microscopy. This process is properly called two-stage replication. The first replica is a
negative, the second, a positive. The second replica can also be spoken of as being
preshadowed. This is a negative print, as is commonly done with shadowed material,
so that the "shadows" appear dark rather than light.