E M B R Y O N I C H E M O G L O B I N S Y N T H E S I S
105
red cell lines present in 7-day blood. Fraser (1963a) has shown erythrocytes of 5-day blood incorporate uridine-H
3
, and some cells incorporate
thymidine-H
3
. As the cells mature the ability to incorporate both these
nucleosides decreases, but some incorporation of thymidine is seen in
polychromatic cells as late as 13 days. Cameron and Prescott (1963)
found in an examination of the chick (2 days after hatching) that the
capacity to synthesize RNA dropped off rapidly between mid and late
polychromatic stages and was absent in mature cells. Protein synthesis
was retained through the reticulocyte stage, and decayed less rapidly
than RNA synthesis. These observations on nucleated red blood cells
parallel those found with enucleate erythrocytes of mammals.
The observations on the extensive differences between embryonic and
adult chicken Hb suggest that many new genes are activated in order to
initiate adult Hb synthesis. It is quite likely that both subunit classes
change during the transition, rather than just one. There is a suggestive
correlation between the change from the primitive to the definitive
erythrocyte series and the change from embryonic to adult Hb. Furthermore, the initiation of different types of adult Hb synthesis seems to be
asynchronous to some extent. These important observations deserve to
be amplified with methods of high resolving power, but the actual
forces at play in the shift from embryonic to adult Hb still remain
largely unknown.
2. Frogs
It is clear that the transition from tadpole to adult also results in
profound changes in the types of Hb present. We have already noted
the heterogeneity of the types of Hb of both the tadpole and adult. Most
workers agree that there is a gradual change in Hb type, regardless of
the nature and extent of heterogeneity revealed by different methods,
throughout the metamorphic transition. Frieden and his co-workers
(1961; Trader et al., 1963) have shown that the changes they observe
(decrease in electrophoretic mobility, appearance of Hb dimers) occur
both in spontaneous and triiodothyronine induced metamorphosis. The
red cells also become more elliptical, and hematocrit increases during
metamorphosis. There are changes in nonheme proteins of erythrocytes
which occur gradually throughout metamorphosis, and Frieden (1961)
suggests some of the nonheme protein may be globin.
The hématologie picture is not worked out in the same detail for
amphibian metamorphosis as it is for chick embryo development. There
are, however, changing predominant sites of erythropoiesis throughout
embryogeny. Jordan and Speidel (1923) state that the major site of
erythropoiesis in the tadpole is the kidney, while in the adult it is the
spleen.
105
red cell lines present in 7-day blood. Fraser (1963a) has shown erythrocytes of 5-day blood incorporate uridine-H
3
, and some cells incorporate
thymidine-H
3
. As the cells mature the ability to incorporate both these
nucleosides decreases, but some incorporation of thymidine is seen in
polychromatic cells as late as 13 days. Cameron and Prescott (1963)
found in an examination of the chick (2 days after hatching) that the
capacity to synthesize RNA dropped off rapidly between mid and late
polychromatic stages and was absent in mature cells. Protein synthesis
was retained through the reticulocyte stage, and decayed less rapidly
than RNA synthesis. These observations on nucleated red blood cells
parallel those found with enucleate erythrocytes of mammals.
The observations on the extensive differences between embryonic and
adult chicken Hb suggest that many new genes are activated in order to
initiate adult Hb synthesis. It is quite likely that both subunit classes
change during the transition, rather than just one. There is a suggestive
correlation between the change from the primitive to the definitive
erythrocyte series and the change from embryonic to adult Hb. Furthermore, the initiation of different types of adult Hb synthesis seems to be
asynchronous to some extent. These important observations deserve to
be amplified with methods of high resolving power, but the actual
forces at play in the shift from embryonic to adult Hb still remain
largely unknown.
2. Frogs
It is clear that the transition from tadpole to adult also results in
profound changes in the types of Hb present. We have already noted
the heterogeneity of the types of Hb of both the tadpole and adult. Most
workers agree that there is a gradual change in Hb type, regardless of
the nature and extent of heterogeneity revealed by different methods,
throughout the metamorphic transition. Frieden and his co-workers
(1961; Trader et al., 1963) have shown that the changes they observe
(decrease in electrophoretic mobility, appearance of Hb dimers) occur
both in spontaneous and triiodothyronine induced metamorphosis. The
red cells also become more elliptical, and hematocrit increases during
metamorphosis. There are changes in nonheme proteins of erythrocytes
which occur gradually throughout metamorphosis, and Frieden (1961)
suggests some of the nonheme protein may be globin.
The hématologie picture is not worked out in the same detail for
amphibian metamorphosis as it is for chick embryo development. There
are, however, changing predominant sites of erythropoiesis throughout
embryogeny. Jordan and Speidel (1923) state that the major site of
erythropoiesis in the tadpole is the kidney, while in the adult it is the
spleen.