Biology Reference
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addition to the large cell population, another major population of smaller
cells appears. This population is apparently composed of immature adult
type erythrocytes. The relative number of large larval erythrocytes markedly
decreases in the post climax, and these larval erythrocytes are totally
replaced by small, adult type erythrocytes in juveniles (Fig. 10).
In rainbow trout, hemoglobin subunit components of larval type
erythrocytes are different from those of adult type erythorcytes (Iuchi
and Yamagami, 1969; Iuchi, 1973b, 1985) and larval type hemoglobin has
higher oxygen affi nity and less Bohr effect compared to adult hemoglobins
(Iuchi, 1973b). Miwa and Inui (1991) also found that migrating patterns of
hemoglobins of larval and adult type erythrocytes of the Japanese fl ounder
were different in polyacrylamide gel electrophoresis. Flatfi sh change
behavior and living habitat during metamorphosis, and hence, availability
of oxygen as well as the mode of oxygen needs by the animal may change.
Therefore, it is meaningful to further clarify the difference of biochemical
and physiological nature of larval and adult type hemoglobins.
Thyroid hormone is involved in erythropoiesis in a variety of animals
such as mammals (Popovic et al., 1977; Daniak et al., 1978), reptiles (Thapliyal
and Kaur, 1976). In amphibians, the shift from tadpole erythrocytes to frog
erythrocytes is also stimulated by thyroid hormone (Hollyfi eld, 1966;
Moss and Ingram, 1965; Meints and Carver, 1973; Thomas, 1974). Miwa
and Inui (1991) found that administration of T4 to prometamorphic larvae
of the fl ounder induced precocious shift of the erythrocyte population
from larval to adult type erythrocytes (Fig. 11). Thus, it is evident that the
shift of erythrocytes during fl ounder metamorphosis is also controlled by
thyroid hormone.
Sullivan et al. (1987) reported the presence of nuclear thyroid hormone
receptors in adult type erythrocytes of trout. They also observed that the
number of the receptors diminished during the cell development from
immature erythrocytes to mature erythrocytes. These fi ndings suggest
that thyroid hormone acts directly on the erythrocyte in trout. In anuran
amphibians, it is known that the number of thyroid hormone receptors in
an erythrocyte nucleus increases during prometamorphosis and reaches
maximum just prior to metamorphic climax, and that thyroid hormone
treatment increases the number of thyroid hormone receptors in erythrocytes
(Galton et al., 1994). It is plausible that a similar change in thyroid hormone
receptors also occurs in fl atfi sh metamorphosis.
In higher vertebrates, embryonic erythrocytes originate from yolk sac,
while the fetal erythropoiesis is replaced by the liver. The spleen, thymus and
other connective tissue organs may also produce erythroid during this time.
The location of erythropoiesis is then transferred to bone marrow (Wood
et al., 1979). In rainbow trout, larval erythropoiesis takes place in both the
