ROLE OF mRNA IN EMBRYONIC DEVELOPMENT
145
the diagram (Fig. 19) by stippled areas. This RNA turns over rapidly.
It is present in a much higher number of copies in late embryos, and
this increase must be related to the increasing number of cells that make
up the growing embryo (about 40,000 in gastrulae and 600,000 in differentiated tadpoles; Dawid, 1965). The presence of an mRNA fraction
with a rapid turnover rate is not exclusive to embryonic cells; it has
also been demonstrated in differentiated cells (Hoyer et al, 1963; Girard
et al, 1965; Henshaw et al, 1965; Yoshikawa-Fukuda, 1966; Ellem,
1966).
3. In postneurula
stages, some DNA
sites synthesize
RNA
for a
certain period of time and then stop functioning. The products of these
genes remain, however, present in subsequent stages in a stabilized form
and apparently in a reduced number of copies. The process of stabilization is probably accompanied by a reduction of the average size of the
mRNA molecules (Brown and Gurdon, 1966). Stable mRNA molecules
appear in early stages (between neurula and tail-bud stages) and accumulate slowly in later development. The appearance of stable mRNA
is concomitant with the process of differentiation. Since the full-grown
embryo is made up of many cell types, it is not possible to decide
whether both types of mRNA (stable and unstable) are present in the
same or in different cells.
The presence of long-lived mRNA molecules has been observed in
several differentiated cell types: erythrocytes (Marks et al, 1962; Wilt,
1965), liver cells (Revel and Hiatt, 1964; Henshaw et al, 1965), and
thyroid cells (Seed and Goldberg, 1963). The appearance of stable
mRNA in differentiated cells explains why the process of differentiation,
once initiated, cannot be stopped by actinomycin (Klein and Pierro,
1963; Yaffe and Feldman, 1964; Wessels, 1964; Wessells and Wilt, 1965),
although this antibiotic completely inhibits the early steps of differentiation (Denis, 1964a; Wessels and Wilt, 1965).
C. Messenger RNA and Differentiation
Three different models for the biochemical mechanism of cell differentiation have been proposed in the Introduction of the present review.
The second model fits best the experimental data that have just been
described. As we have seen, genetic information is released progressively
during embryonic development (Fig. 19). It seems, therefore, likely that
the type of information transcribed in embryonic cells results from a
selection at the chromosome level. The information transcribed is likely
to be different in each region of the early embryo, thus causing the
145
the diagram (Fig. 19) by stippled areas. This RNA turns over rapidly.
It is present in a much higher number of copies in late embryos, and
this increase must be related to the increasing number of cells that make
up the growing embryo (about 40,000 in gastrulae and 600,000 in differentiated tadpoles; Dawid, 1965). The presence of an mRNA fraction
with a rapid turnover rate is not exclusive to embryonic cells; it has
also been demonstrated in differentiated cells (Hoyer et al, 1963; Girard
et al, 1965; Henshaw et al, 1965; Yoshikawa-Fukuda, 1966; Ellem,
1966).
3. In postneurula
stages, some DNA
sites synthesize
RNA
for a
certain period of time and then stop functioning. The products of these
genes remain, however, present in subsequent stages in a stabilized form
and apparently in a reduced number of copies. The process of stabilization is probably accompanied by a reduction of the average size of the
mRNA molecules (Brown and Gurdon, 1966). Stable mRNA molecules
appear in early stages (between neurula and tail-bud stages) and accumulate slowly in later development. The appearance of stable mRNA
is concomitant with the process of differentiation. Since the full-grown
embryo is made up of many cell types, it is not possible to decide
whether both types of mRNA (stable and unstable) are present in the
same or in different cells.
The presence of long-lived mRNA molecules has been observed in
several differentiated cell types: erythrocytes (Marks et al, 1962; Wilt,
1965), liver cells (Revel and Hiatt, 1964; Henshaw et al, 1965), and
thyroid cells (Seed and Goldberg, 1963). The appearance of stable
mRNA in differentiated cells explains why the process of differentiation,
once initiated, cannot be stopped by actinomycin (Klein and Pierro,
1963; Yaffe and Feldman, 1964; Wessels, 1964; Wessells and Wilt, 1965),
although this antibiotic completely inhibits the early steps of differentiation (Denis, 1964a; Wessels and Wilt, 1965).
C. Messenger RNA and Differentiation
Three different models for the biochemical mechanism of cell differentiation have been proposed in the Introduction of the present review.
The second model fits best the experimental data that have just been
described. As we have seen, genetic information is released progressively
during embryonic development (Fig. 19). It seems, therefore, likely that
the type of information transcribed in embryonic cells results from a
selection at the chromosome level. The information transcribed is likely
to be different in each region of the early embryo, thus causing the