TRANSPLANTATION OF CELL NUCLEI
31
D. Intracellular Communication
The nuclear transfer experiments described in this section have contributed to the identification and tracing of substances that pass between nucleus and cytoplasm.
1. Morphogenetic Substances in Acetabularia
Using the method of rhizoid grafts described in Section II, Hämmerling (1953) has obtained some results which strongly suggest that
substances promoting specific protein synthesis remain in the Acetabularia stalk after removal of the nucleus. He grafted a rhizoid (with
nucleus) of one species into an anucleate stalk of another and at the same
time cut off the cap. The type of cap which was regenerated resembled
that of the nuclear or cytoplasmic species according to the relative
amount of cytoplasm that was grafted with the nucleus or which was
left with the host stalk. We now know from the work of Schweiger and
Bremer (1961) that an Acetabularia from which the rhizoid has been
cut off can synthesize RNA and protein so long as it has been kept in
the dark for several days before enucleation. The rhizoid therefore
releases substances which promote RNA synthesis and which accumulate in the cytoplasm in the dark. These substances are distributed in a
gradient towards the cap end of the stalk ; the apical concentration of
these substances was demonstrated by transplanting a nucleus from a
uracil- and serine-labelled individual to an anucleate host, and finding
that the label accumulated in the cap end of the stalk (Werz and Zetsche,
1963). Zetsche (1962) made use of the fact that a very small amount of
these morphogenetic substances usually exists in the basal part of the
stalk to show that the substances derive from the nucleus and not from
the rhizoid cytoplasm. He took the basal part of the stalk (with no
nucleus) from two algae. In one he implanted an isolated nucleus (Section II), and the other he left without a nucleus. After both pieces had
been kept in the dark for 8 days, their capacity for regeneration showed
that the anucleate pieces had very little morphogenetic substance,
while the other piece (from which the nucleus had been removed before
regeneration) had accumulated much of these substances. They must
therefore have come from the nucleus itself.
This series of investigations on Acetabularia has established that the
nucleus gives out, into the cytoplasm, some substances which promote
RNA and protein synthesis. These substances appear to be broken
down when promoting the syntheses for which they are intended. I t is
possible that the release of morphogenetic substances from nuclei into
the cytoplasm is a normal process taking place in all synthesizing cells.
The persistence of these substances may vary considerably from one
31
D. Intracellular Communication
The nuclear transfer experiments described in this section have contributed to the identification and tracing of substances that pass between nucleus and cytoplasm.
1. Morphogenetic Substances in Acetabularia
Using the method of rhizoid grafts described in Section II, Hämmerling (1953) has obtained some results which strongly suggest that
substances promoting specific protein synthesis remain in the Acetabularia stalk after removal of the nucleus. He grafted a rhizoid (with
nucleus) of one species into an anucleate stalk of another and at the same
time cut off the cap. The type of cap which was regenerated resembled
that of the nuclear or cytoplasmic species according to the relative
amount of cytoplasm that was grafted with the nucleus or which was
left with the host stalk. We now know from the work of Schweiger and
Bremer (1961) that an Acetabularia from which the rhizoid has been
cut off can synthesize RNA and protein so long as it has been kept in
the dark for several days before enucleation. The rhizoid therefore
releases substances which promote RNA synthesis and which accumulate in the cytoplasm in the dark. These substances are distributed in a
gradient towards the cap end of the stalk ; the apical concentration of
these substances was demonstrated by transplanting a nucleus from a
uracil- and serine-labelled individual to an anucleate host, and finding
that the label accumulated in the cap end of the stalk (Werz and Zetsche,
1963). Zetsche (1962) made use of the fact that a very small amount of
these morphogenetic substances usually exists in the basal part of the
stalk to show that the substances derive from the nucleus and not from
the rhizoid cytoplasm. He took the basal part of the stalk (with no
nucleus) from two algae. In one he implanted an isolated nucleus (Section II), and the other he left without a nucleus. After both pieces had
been kept in the dark for 8 days, their capacity for regeneration showed
that the anucleate pieces had very little morphogenetic substance,
while the other piece (from which the nucleus had been removed before
regeneration) had accumulated much of these substances. They must
therefore have come from the nucleus itself.
This series of investigations on Acetabularia has established that the
nucleus gives out, into the cytoplasm, some substances which promote
RNA and protein synthesis. These substances appear to be broken
down when promoting the syntheses for which they are intended. I t is
possible that the release of morphogenetic substances from nuclei into
the cytoplasm is a normal process taking place in all synthesizing cells.
The persistence of these substances may vary considerably from one