434
T . G. HSU
contemporary cytologists is supposedly, to a large extent, genetically
inactive. Is the X chromosome of the grasshoppers then largely inert?
This is rather difficult to conceive. Perhaps the phenomenon of heteropyknosis is not a property limited to the heterochromatin of the Heitz
type. Heteropyknosis may merely represent an out-of-cycle condensation system of a certain chromosome or a chromosome segment
exhibited at certain stages of certain tissues. Thus euchromatin may
also exhibit heteropyknosis at one time or another. After a detailed
presentation of the phenomenon of heteropyknosis, White (1954)
conservatively claims: " W e are thus forced to the conclusion that there
are at least two kinds of heterochromatin. There may, of course, be
many more. In fact it seems probable that the distinction between
heterochromatin and euchromatin is not an absolute one, but, rather,
that these terms have been applied to the extremes of a continuous
series between which a great many intermediates exist."
This brief account of heterochromatin and heteropyknosis is introduced to show the complexity of the phenomenon and the uncertainty
of our knowledge concerning its nature and its function. There is a
feeling that in this area of research cytogeneticists have been beating
around the bush. Since the phenomenon of heteropyknosis is common,
it must have some important physiological implications. Yet all
suggestions concerning the functions of heterochromatin appear to be
far from being concrete. Before presenting more speculations, I would
like to summarize the recent progress in the studies on the phenomenon
of heteropyknosis in mammalian cells.
In studies on the karyology ofthe mammary carcinoma of the mouse,
Tjio and Ostergren (1958) discovered that one of the chromosomes
was totally heteropyknotic. Ohno, Kaplan and Kinosita (1959a)
observed from the prophase of regenerating rat liver cells that in females
one X chromosome is positively heteropyknotic while the homologue
is isopyknotic. In tetraploid cells, there are two sex chromatin masses
in interphase and two heteropyknotic X chromosomes in prophase.
Indication is thus strong that the sex chromatin seen in the interphase
nuclei represents one X chromosome, the one that is heteropyknotic in
prophase. In the opossum, Didelphis virginiana, however, cells of both
sexes show a sex chromatin, being a single X for the female and the Y
for the male (Ohno, Kaplan and Kinosita, 1959b). Subsequent works
on normal and neoplastic mouse cells by Ohno and Hauschka (1960)
established beyond doubt that the heteropyknotic chromosome observed by Tjio and Ostergren was one of the X chromosomes of the
female mouse, the same phenomenon observed in rat tissues, and
that the situation is not confined to the tumor cells.
It appears, therefore, that in female mammals one X chromosome
T . G. HSU
contemporary cytologists is supposedly, to a large extent, genetically
inactive. Is the X chromosome of the grasshoppers then largely inert?
This is rather difficult to conceive. Perhaps the phenomenon of heteropyknosis is not a property limited to the heterochromatin of the Heitz
type. Heteropyknosis may merely represent an out-of-cycle condensation system of a certain chromosome or a chromosome segment
exhibited at certain stages of certain tissues. Thus euchromatin may
also exhibit heteropyknosis at one time or another. After a detailed
presentation of the phenomenon of heteropyknosis, White (1954)
conservatively claims: " W e are thus forced to the conclusion that there
are at least two kinds of heterochromatin. There may, of course, be
many more. In fact it seems probable that the distinction between
heterochromatin and euchromatin is not an absolute one, but, rather,
that these terms have been applied to the extremes of a continuous
series between which a great many intermediates exist."
This brief account of heterochromatin and heteropyknosis is introduced to show the complexity of the phenomenon and the uncertainty
of our knowledge concerning its nature and its function. There is a
feeling that in this area of research cytogeneticists have been beating
around the bush. Since the phenomenon of heteropyknosis is common,
it must have some important physiological implications. Yet all
suggestions concerning the functions of heterochromatin appear to be
far from being concrete. Before presenting more speculations, I would
like to summarize the recent progress in the studies on the phenomenon
of heteropyknosis in mammalian cells.
In studies on the karyology ofthe mammary carcinoma of the mouse,
Tjio and Ostergren (1958) discovered that one of the chromosomes
was totally heteropyknotic. Ohno, Kaplan and Kinosita (1959a)
observed from the prophase of regenerating rat liver cells that in females
one X chromosome is positively heteropyknotic while the homologue
is isopyknotic. In tetraploid cells, there are two sex chromatin masses
in interphase and two heteropyknotic X chromosomes in prophase.
Indication is thus strong that the sex chromatin seen in the interphase
nuclei represents one X chromosome, the one that is heteropyknotic in
prophase. In the opossum, Didelphis virginiana, however, cells of both
sexes show a sex chromatin, being a single X for the female and the Y
for the male (Ohno, Kaplan and Kinosita, 1959b). Subsequent works
on normal and neoplastic mouse cells by Ohno and Hauschka (1960)
established beyond doubt that the heteropyknotic chromosome observed by Tjio and Ostergren was one of the X chromosomes of the
female mouse, the same phenomenon observed in rat tissues, and
that the situation is not confined to the tumor cells.
It appears, therefore, that in female mammals one X chromosome