12. G E N E T I C C Y T O L O G Y
399
other phases of research germaine to genetic cytology not included here
have had no progress during recent years. Studies on the ultrastructure
of chromosomes, nucleolus, and mitotic apparatus have begun, and
rapid development is anticipated in the molecular approach to basic
cytology.
I I . C Y T O G E N E T I C S OF M A N
In studies on the effects of electrical stimulation of nerve cells in the
cat, Barr and Bertram (1949) noted in some preparations the existence
of a small mass of chromatin in each nucleus. This chromatin mass was
a distinct body, Feulgen positive; it varied in position from the nucleolus
to the nuclear membrane, and was present in the nuclei of some cats
but absent in others. No correlation could be made between the experimental procedures and the presence or absence of this nuclear chromatin,
termed nucleolar satellite by these authors. It was finally discovered
that this accessory body of the nucleus was associated with the sex of
the animal, viz. it was present in the nuclei of females but absent in
those of the males. Fig. la and l b represent two normal neurons,
female and male respectively, showing the presence of this chromatin
in the female cell.
This classic discovery led to a survey of tissues other than nerve cells
and in animals other than the cat. It was found that the same phenomenon existed in man and that the
4
'sex chromatin" is not a unique
property of the brain cells, although it is particularly conspicuous in the
nuclei of neurons. In determining sex from cell nuclei, skin biopsy
(Moore, Graham and Barr, 1953), buccal smear (Moore and Barr, 1955),
or even leukocytes from peripheral blood (Davidson and Smith, 1954)
can be used. More recently, sex chromatin also has been conclusively
demonstrated in cells in culture (Miles, 1959; Hayflick and Moorhead,
1961). Fig. 2 depicts cells from a fibroblast culture of a female human
embryo, many of which clearly show sex chromatin, also known as
the Barr body. When present (female), it is conventionally termed
"positive"; when absent (male), it is termed "negative".
The nature of sex chromatin has been a subject of speculation. Since
it is Feulgen positive, it must contain D N A . Therefore, it must represent
chromosomal material. T w o questions thus arose: (1) does it represent
more than one chromosome, a single chromosome, or only a part of
a chromosome, and (2) which chromosome or chromosomes contribute to the sex chromatin? Since, karyologically, females differ from
males in the sex chromosomes, i.e. two X chromosomes in the female
and a single X plus a Y chromosome in the male (at least in mammals),
399
other phases of research germaine to genetic cytology not included here
have had no progress during recent years. Studies on the ultrastructure
of chromosomes, nucleolus, and mitotic apparatus have begun, and
rapid development is anticipated in the molecular approach to basic
cytology.
I I . C Y T O G E N E T I C S OF M A N
In studies on the effects of electrical stimulation of nerve cells in the
cat, Barr and Bertram (1949) noted in some preparations the existence
of a small mass of chromatin in each nucleus. This chromatin mass was
a distinct body, Feulgen positive; it varied in position from the nucleolus
to the nuclear membrane, and was present in the nuclei of some cats
but absent in others. No correlation could be made between the experimental procedures and the presence or absence of this nuclear chromatin,
termed nucleolar satellite by these authors. It was finally discovered
that this accessory body of the nucleus was associated with the sex of
the animal, viz. it was present in the nuclei of females but absent in
those of the males. Fig. la and l b represent two normal neurons,
female and male respectively, showing the presence of this chromatin
in the female cell.
This classic discovery led to a survey of tissues other than nerve cells
and in animals other than the cat. It was found that the same phenomenon existed in man and that the
4
'sex chromatin" is not a unique
property of the brain cells, although it is particularly conspicuous in the
nuclei of neurons. In determining sex from cell nuclei, skin biopsy
(Moore, Graham and Barr, 1953), buccal smear (Moore and Barr, 1955),
or even leukocytes from peripheral blood (Davidson and Smith, 1954)
can be used. More recently, sex chromatin also has been conclusively
demonstrated in cells in culture (Miles, 1959; Hayflick and Moorhead,
1961). Fig. 2 depicts cells from a fibroblast culture of a female human
embryo, many of which clearly show sex chromatin, also known as
the Barr body. When present (female), it is conventionally termed
"positive"; when absent (male), it is termed "negative".
The nature of sex chromatin has been a subject of speculation. Since
it is Feulgen positive, it must contain D N A . Therefore, it must represent
chromosomal material. T w o questions thus arose: (1) does it represent
more than one chromosome, a single chromosome, or only a part of
a chromosome, and (2) which chromosome or chromosomes contribute to the sex chromatin? Since, karyologically, females differ from
males in the sex chromosomes, i.e. two X chromosomes in the female
and a single X plus a Y chromosome in the male (at least in mammals),