206
H . F I R K E T
As shown in the table, individual variations of duration and aspects
of normal divisions are quite large even within the same culture, and
should be kept in mind when experimental modifications are studied.
The overall duration of mitosis in higher vertebrates at 37-38°C* is
usually between 40 and 90 min. This is somewhat longer than often
quoted older figures: careful phase-contrast examination allows an
identification of early and late stages beyond the limits recognized
previously on living material.
The type of cell, the temperature (an important factor) but also the
conditions of culture may affect these durations. W e cannot be certain
that division in the same tissues proceeds at the same pace in vivo. This
is unfortunate because the duration of mitosis is an essential datum in
most calculations about growth or renewal of cell populations (see
p. 219). In the animal, we have to fall back on indirect methods for the
measurement of the duration of mitosis. These (colchicine accumulation, X-ray inhibition of divisions, see review in Lusbaugh, 1956, or
radioactive precursor accumulation, Fry, Lesher and Kohn, 1961)
often give divergent results and their reliability is not proven. It would
be useful to test them on tissue cultures: the results obtained with these
indirect methods could then be compared with direct measurements on
film records of the same material.
B. S P E C I A L ASPECTS
The use of mitotic poisons and other experimental influences have
shown that cell division is not a unique and integrated phenomenon
but rather a combination of several parallel mechanisms. It is possible
to interfere with one or the other of these mechanisms without affecting
the others. Thus it is not artificial to examine the evolution of each of
the various parts and appreciate its relationship to the others.
1. Chromosomes
The number, size and general shape of metaphase chromosomes can
now be very clearly analysed in tissue culture by spreading techniques
devised by Hsu (see Chapter 12). Apart from exceptional cases ("lampbrush" chromosomes, etc.), vertebrate chomosomes do not constitute
very favourable objects for the study of their inner structure. It is not
easy to separate eu- and heterochromatic regions or to visualize their
coiling (Fig. 1). The coiling cycle is rather inferred from comparison
with plant chromosomes and the fact that long thin beaded prophasic
*The duration of mitosis is very sensitive to temperature (see reviews in Hughes (1952) and
Mazia(1961)). Cells of higher vertebrates do not initiate divisions below 21 ° C, nor above 44° C.
Some mechanisms such as D N A synthesis, still go on at 45°C (Firket, 1958a).
H . F I R K E T
As shown in the table, individual variations of duration and aspects
of normal divisions are quite large even within the same culture, and
should be kept in mind when experimental modifications are studied.
The overall duration of mitosis in higher vertebrates at 37-38°C* is
usually between 40 and 90 min. This is somewhat longer than often
quoted older figures: careful phase-contrast examination allows an
identification of early and late stages beyond the limits recognized
previously on living material.
The type of cell, the temperature (an important factor) but also the
conditions of culture may affect these durations. W e cannot be certain
that division in the same tissues proceeds at the same pace in vivo. This
is unfortunate because the duration of mitosis is an essential datum in
most calculations about growth or renewal of cell populations (see
p. 219). In the animal, we have to fall back on indirect methods for the
measurement of the duration of mitosis. These (colchicine accumulation, X-ray inhibition of divisions, see review in Lusbaugh, 1956, or
radioactive precursor accumulation, Fry, Lesher and Kohn, 1961)
often give divergent results and their reliability is not proven. It would
be useful to test them on tissue cultures: the results obtained with these
indirect methods could then be compared with direct measurements on
film records of the same material.
B. S P E C I A L ASPECTS
The use of mitotic poisons and other experimental influences have
shown that cell division is not a unique and integrated phenomenon
but rather a combination of several parallel mechanisms. It is possible
to interfere with one or the other of these mechanisms without affecting
the others. Thus it is not artificial to examine the evolution of each of
the various parts and appreciate its relationship to the others.
1. Chromosomes
The number, size and general shape of metaphase chromosomes can
now be very clearly analysed in tissue culture by spreading techniques
devised by Hsu (see Chapter 12). Apart from exceptional cases ("lampbrush" chromosomes, etc.), vertebrate chomosomes do not constitute
very favourable objects for the study of their inner structure. It is not
easy to separate eu- and heterochromatic regions or to visualize their
coiling (Fig. 1). The coiling cycle is rather inferred from comparison
with plant chromosomes and the fact that long thin beaded prophasic
*The duration of mitosis is very sensitive to temperature (see reviews in Hughes (1952) and
Mazia(1961)). Cells of higher vertebrates do not initiate divisions below 21 ° C, nor above 44° C.
Some mechanisms such as D N A synthesis, still go on at 45°C (Firket, 1958a).