EMBRYONIC HEMOGLOBIN SYNTHESIS
103
and HbF may be synthesized by the cell population from a given organ.
Second, it is proposed that on the molecular level, some kind of repression or derepression of the opérons for ß- and γ-chain synthesis are
involved, along the lines proposed by Jacob and Monod (1961). While
this may be so, it only identifies the level of molecular control (DNA)
and not the precise mechanism. Since a given erythrocyte can synthesize
both HbF and HbA, the "switch," if it exists, is not a mutually exclusive
one (cf. Baglioni, 1963). These proposals point the way for some of the
types of questions we should like to ask of the developing chick and
amphibian, although most of our present information is solely on the
descriptive level.
1. Chickens
Because of the extensive heterogeneity of chicken Hb, even the descriptive basis of the change in Hb types is quite complex. One must deal
with the cessation of synthesis of several types of embryonic Hb and
initiation of synthesis of several types of adult Hb, and the changes may
not be simultaneous, i.e., not all embryonic Hb's may recede at the
same time, nor all adult Hb appear together. Certainly, the cellular and
hématologie description of embryonic red cell production is far from
simple. Lucas and Jamroz (1961) have admirably described and reviewed
the status of chick embryo hematology. We will only record here that a
primitive erythrocyte line appears, and is later supplanted by waves of
production of the definitive red cell lines. Several organs are involved
in hematopoiesis at different stages of embryogeny (yolk sac, spleen,
bone marrow), though this may not be a sequence of strictly exclusive
production sites (Romanoff, 1960). The primitive cell line begins to
disappear by the fifth or sixth day of incubation, and the immature and
mature representatives of the definitive cell line begin to predominate.
By 13 or 14 days, only definitive erythrocytes are found (Lucas and
Jamroz, 1961; Fraser, 1963b).
The change from a primitive to a definitive cell line is correlated to
some extent with changes in the types of Hb, i.e., gross changes in the
types of Hb present occur between 4 to 5 days and 11 to 13 days of
incubation, by which time the adult Hb picture obtains. Many of the
differences in details observed depend, of course, on the extent and
nature of Hb heterogeneity revealed by the methods employed by a given
investigator. Table II summarizes some studies on the changes in Hb
during chick embryogenesis. This also recapitulates some of the facts on
chicken Hb heterogeneity described by various authors. It should be
added that Borgese and Bertles (1965) have studied Hb of ducks by
starch-gel electrophoresis. They find the 7-day embryo contains two
Hb's, both of which are distinct from the two Hb's found in the adults.
103
and HbF may be synthesized by the cell population from a given organ.
Second, it is proposed that on the molecular level, some kind of repression or derepression of the opérons for ß- and γ-chain synthesis are
involved, along the lines proposed by Jacob and Monod (1961). While
this may be so, it only identifies the level of molecular control (DNA)
and not the precise mechanism. Since a given erythrocyte can synthesize
both HbF and HbA, the "switch," if it exists, is not a mutually exclusive
one (cf. Baglioni, 1963). These proposals point the way for some of the
types of questions we should like to ask of the developing chick and
amphibian, although most of our present information is solely on the
descriptive level.
1. Chickens
Because of the extensive heterogeneity of chicken Hb, even the descriptive basis of the change in Hb types is quite complex. One must deal
with the cessation of synthesis of several types of embryonic Hb and
initiation of synthesis of several types of adult Hb, and the changes may
not be simultaneous, i.e., not all embryonic Hb's may recede at the
same time, nor all adult Hb appear together. Certainly, the cellular and
hématologie description of embryonic red cell production is far from
simple. Lucas and Jamroz (1961) have admirably described and reviewed
the status of chick embryo hematology. We will only record here that a
primitive erythrocyte line appears, and is later supplanted by waves of
production of the definitive red cell lines. Several organs are involved
in hematopoiesis at different stages of embryogeny (yolk sac, spleen,
bone marrow), though this may not be a sequence of strictly exclusive
production sites (Romanoff, 1960). The primitive cell line begins to
disappear by the fifth or sixth day of incubation, and the immature and
mature representatives of the definitive cell line begin to predominate.
By 13 or 14 days, only definitive erythrocytes are found (Lucas and
Jamroz, 1961; Fraser, 1963b).
The change from a primitive to a definitive cell line is correlated to
some extent with changes in the types of Hb, i.e., gross changes in the
types of Hb present occur between 4 to 5 days and 11 to 13 days of
incubation, by which time the adult Hb picture obtains. Many of the
differences in details observed depend, of course, on the extent and
nature of Hb heterogeneity revealed by the methods employed by a given
investigator. Table II summarizes some studies on the changes in Hb
during chick embryogenesis. This also recapitulates some of the facts on
chicken Hb heterogeneity described by various authors. It should be
added that Borgese and Bertles (1965) have studied Hb of ducks by
starch-gel electrophoresis. They find the 7-day embryo contains two
Hb's, both of which are distinct from the two Hb's found in the adults.