436
T .
C .
H S U
shows unilateral positive heteropyknosis (allocycly) while the other
element is isopyknotic. Possibly in man, too, one of the X chromosomes
of female cells may be heteropyknotic (or allocyclic), since sex chromatin
could be detected in female cells. Indeed, Ohno and Makino (1961)
found this to be true. However, these authors also found a small but
distinct chromatin mass in some male fetal cells. They also found the
heteropyknotic Y in metaphase. It should be mentioned that in regular
nuclear sexing, male cells may also show an occasional heterochromatic
mass, though not as large nor as frequent as in the female cells. It is
quite possible that the Y chromosome may form a small but conspicuous
chromatin body in the somatic nucleus. Fig. 9a represents a cell from a
primary culture of a male mouse in which the Y chromosome is clearly
heteropyknotic; and Fig. 9b depicts a cell in prophase taken from strain
H-4c established by Billen and Debrunner (1960) from a female mouse,
in which the X element was strongly heteropyknotic in prophase. It is of
interest to note that after almost two years of continuous cultivation
in vitro, this property was retained.
During recent years the Chinese hamster, Cricetulus griseus, has been
a popular material for cell research because its diploid number is only
22 (Fig. 10), a number of the chromosome pairs are identifiable, and
most of the cell lines from this species have a short generation time and
a near-diploid constitution (Yerganian and Leonard, 1961). O f special
interest among the chromosomes of this species are the sex chromosomes. The X chromosome is a medium-sized submetacentric; the Y
is only slightly smaller than the X . This is somewhat different from the
majority of mammalian species in which the Y chromosome is usually
one of the smallest elements of each complement. The most interesting
phenomena about the sex chromosomes of the Chinese hamster are
(1) the two X chromosomes are somewhat morphologically different;
and (2) they differ in the thymidine labeling pattern (Fig. 11). When
the cells were pulse-labeled with tritiated thymidine and fixed at
various intervals thereafter, Taylor (1960a,b) found the sex chromosomes may, among some cells, show heavy labeling while the autosomes were free from label (Fig. l i b ) Conversely, among other cells the
autosomes may show heavy labeling while the sex chromosomes were
free from label (Fig. 11a). Generally, samples fixed soon after labeling
tend to show the label in the sex chromosomes, and those fixed later,
in the autosomes. These data suggest that the chromosomes of a cell
do not synthesize DNA at the same time, some lagging after others. The
delayed D N A synthesis appears to be characteristic of the sex chromosomes in the Chinese hamsters. More recently, it has been shown that
in the human, too, one X chromosome exhibits asynchronous D N A
synthesis (Morishima, Grumbach and Taylor, 1962). The difference in
T .
C .
H S U
shows unilateral positive heteropyknosis (allocycly) while the other
element is isopyknotic. Possibly in man, too, one of the X chromosomes
of female cells may be heteropyknotic (or allocyclic), since sex chromatin
could be detected in female cells. Indeed, Ohno and Makino (1961)
found this to be true. However, these authors also found a small but
distinct chromatin mass in some male fetal cells. They also found the
heteropyknotic Y in metaphase. It should be mentioned that in regular
nuclear sexing, male cells may also show an occasional heterochromatic
mass, though not as large nor as frequent as in the female cells. It is
quite possible that the Y chromosome may form a small but conspicuous
chromatin body in the somatic nucleus. Fig. 9a represents a cell from a
primary culture of a male mouse in which the Y chromosome is clearly
heteropyknotic; and Fig. 9b depicts a cell in prophase taken from strain
H-4c established by Billen and Debrunner (1960) from a female mouse,
in which the X element was strongly heteropyknotic in prophase. It is of
interest to note that after almost two years of continuous cultivation
in vitro, this property was retained.
During recent years the Chinese hamster, Cricetulus griseus, has been
a popular material for cell research because its diploid number is only
22 (Fig. 10), a number of the chromosome pairs are identifiable, and
most of the cell lines from this species have a short generation time and
a near-diploid constitution (Yerganian and Leonard, 1961). O f special
interest among the chromosomes of this species are the sex chromosomes. The X chromosome is a medium-sized submetacentric; the Y
is only slightly smaller than the X . This is somewhat different from the
majority of mammalian species in which the Y chromosome is usually
one of the smallest elements of each complement. The most interesting
phenomena about the sex chromosomes of the Chinese hamster are
(1) the two X chromosomes are somewhat morphologically different;
and (2) they differ in the thymidine labeling pattern (Fig. 11). When
the cells were pulse-labeled with tritiated thymidine and fixed at
various intervals thereafter, Taylor (1960a,b) found the sex chromosomes may, among some cells, show heavy labeling while the autosomes were free from label (Fig. l i b ) Conversely, among other cells the
autosomes may show heavy labeling while the sex chromosomes were
free from label (Fig. 11a). Generally, samples fixed soon after labeling
tend to show the label in the sex chromosomes, and those fixed later,
in the autosomes. These data suggest that the chromosomes of a cell
do not synthesize DNA at the same time, some lagging after others. The
delayed D N A synthesis appears to be characteristic of the sex chromosomes in the Chinese hamsters. More recently, it has been shown that
in the human, too, one X chromosome exhibits asynchronous D N A
synthesis (Morishima, Grumbach and Taylor, 1962). The difference in