114
FRED H. WILT
(1964a) found a mitotic index of 7 in 7- to 10-somite blastoderms, and
this fell to 4 or 5 after 24 hr of incubation. Hell examined cell division
in blood island cells of deembryonated blastoderms cultivated, in vitro,
by autoradiographic inspection of thymidine-H
3
incorporation. She calculates that blood island cells have a cell division cycle of about 11 hr.
About 60% of the blood island nuclei are synthesizing DNA at any
given moment. The cells of the ectoderm and endoderm seem to divide
very little during the 24-hr period in culture. A knowledge of the rate
and variation of cell division time in cells of blastoderms prior to somite
formation would be of great interest.
While much remains to be done on these early descriptive phases of
Hb formation, especially in the introduction of less tedious specific
and sensitive methods of determining the rate of Hb synthesis, it is
probable that the first substantial synthesis of Hb takes place in the
blood islands of the area opaca vasculosa at about the 7-somite stage.
IV. Control of Embryonic Hemoglobin Formation
A. Studies with Metabolic Inhibitors
The application of metabolic inhibitors has been a powerful tool in
gaining understanding of biosynthetic control mechanisms. O'Brien initiated studies along this line for embryonic Hb synthesis by studying the
effects of 8-azaguanine on the development of the hemoglobin in the
chick embryo. These studies and their successors form a basis for understanding the molecular control mechanisms operating in the initiation
of embryonic hemoglobin synthesis. A number of inhibitors have now
been studied, and it is convenient to discuss them by their supposed
mode of action, although it must be remembered that many inhibitors
affect several kinds of metabolism and their primary mode of action
may still be in dispute. A summary of sensitivity of the chick blastoderm
to some inhibitors is shown in Fig. 5.
1. Nucleic Acid Metabolism
Actinomycin D is an antibiotic which is commonly used to inhibit
RNA synthesis, which it does by virtue of its ability to bind to deoxyguanine of native DNA. Actinomycin D can also cause inhibition of
DNA synthesis, especially at higher doses, and it may have other important side effects on cell metabolism. The degree of inhibition of RNA
synthesis varies with the dose employed. It is clear that the interpretation of results from experiments using actinomycin D are difficult,
especially when a particular biochemical event does not occur in its
presence. Nevertheless, it has been and will continue to be useful in
many studies on cell differentiation, especially when a given biosynthetic
FRED H. WILT
(1964a) found a mitotic index of 7 in 7- to 10-somite blastoderms, and
this fell to 4 or 5 after 24 hr of incubation. Hell examined cell division
in blood island cells of deembryonated blastoderms cultivated, in vitro,
by autoradiographic inspection of thymidine-H
3
incorporation. She calculates that blood island cells have a cell division cycle of about 11 hr.
About 60% of the blood island nuclei are synthesizing DNA at any
given moment. The cells of the ectoderm and endoderm seem to divide
very little during the 24-hr period in culture. A knowledge of the rate
and variation of cell division time in cells of blastoderms prior to somite
formation would be of great interest.
While much remains to be done on these early descriptive phases of
Hb formation, especially in the introduction of less tedious specific
and sensitive methods of determining the rate of Hb synthesis, it is
probable that the first substantial synthesis of Hb takes place in the
blood islands of the area opaca vasculosa at about the 7-somite stage.
IV. Control of Embryonic Hemoglobin Formation
A. Studies with Metabolic Inhibitors
The application of metabolic inhibitors has been a powerful tool in
gaining understanding of biosynthetic control mechanisms. O'Brien initiated studies along this line for embryonic Hb synthesis by studying the
effects of 8-azaguanine on the development of the hemoglobin in the
chick embryo. These studies and their successors form a basis for understanding the molecular control mechanisms operating in the initiation
of embryonic hemoglobin synthesis. A number of inhibitors have now
been studied, and it is convenient to discuss them by their supposed
mode of action, although it must be remembered that many inhibitors
affect several kinds of metabolism and their primary mode of action
may still be in dispute. A summary of sensitivity of the chick blastoderm
to some inhibitors is shown in Fig. 5.
1. Nucleic Acid Metabolism
Actinomycin D is an antibiotic which is commonly used to inhibit
RNA synthesis, which it does by virtue of its ability to bind to deoxyguanine of native DNA. Actinomycin D can also cause inhibition of
DNA synthesis, especially at higher doses, and it may have other important side effects on cell metabolism. The degree of inhibition of RNA
synthesis varies with the dose employed. It is clear that the interpretation of results from experiments using actinomycin D are difficult,
especially when a particular biochemical event does not occur in its
presence. Nevertheless, it has been and will continue to be useful in
many studies on cell differentiation, especially when a given biosynthetic