Age Mitotic Mus Musculus

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AGE AND MITOTIC ACTIVITY IN THE MALE
MOUSE, MUS MUSCULUS L.
BY W. S. BULLOUGH, University of Sheffield
(Received 18 February 1949)

(With Six Text-figures)
I. INTRODUCTION

In earlier analyses of mitotic activity in the male mouse (Bullough, 19480, b) it was
shown that the routine of waking and sleeping determines the form of the diurnal
mitosis cycle, and that changes from thenormal in this routine result immediately in
changes from the normal in the mitosis cycle. The opinion was then expressed that
factors such as the age and sex of the animals, and the habits of the laboratory staff,
might be expected to influence the daily round of exercise and rest, and so to affect
the form of the mitosis cycle.
Opportunity has now been found to discover the effect of age onthe daily routine
and mitotic activity of the mouse, and thefollowingpaper is a review of conditions in
the male.
II. MATERIAL AND METHODS
(1) The mice
As in previous work, the observations were all made on mice of two pure line strains,
Kreyberg's white label albinos and Strong's CBA agoutis. In the younger mice no
differences were found between the strains, but with increasing age markeddifferences
developed in both habits and mitotic activity. It is therefore unfortunate that so
many of the mice of the older age groups were of one strain, the Strong's CBA agouti.
This was due to the fact that these males rarely fight, and are therefore far more
easily kept for long periods than are the more irritable Kreyberg's mice.
The results took about 12 months to collect, and thusrepresent mice examined at
all seasons of the year. However, apart from unavoidable variations in the length of
day, conditions were kept as uniform as possible. The room temperature was
maintained at 200 C , and the mice received a regular diet of rat cake, dog biscuit,
oats or flaked maize, and chopped carrots. Invariably they were given their food
between 09.00 and 10.00 hr., and always itwas given in excess so that at no time were
they without something to eat.
(2) The times of day
The timqs of day recorded in the various experiments sometimes represent
Greenwich mean time, sometimes British summer time, and occasionally double
British summer time. In practice it was found unnecessary to record which of these
systems was in operation when an experiment was performed, sincethe animals
quickly adapted themselves to a change of the clock. It thus became abundantly

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clear that the daily habits of the animals were adjusted to the time of feeding, and
they took only a few days to become accustomed to an hour's change either way.
(3) The earclip technique

This has already been described in detail by Bullough (1948 a). Small pieces ofear were removed at intervals by means of a conchotome, and were fixed in Bouin's
alcoholic fluid. After sectioning at a thickness of jfi, the mitoses were counted in
section lengths of 1 cm. From each earclip ten such counts were made, and from
these an average figure was obtained. As each experimental group usually consisted
of clips from ten mice, ten average figures were available from whichto derive the
mean and standard error. The latter was calculated by the method for small samples
recommended by Simpson & Roe (1939).
(4) The colchicme technique

As a check on the results obtained by the earclip technique, mice were killed and
examined after their mitoses had been arrested by means of colchicine. T o each adult
animal, weighing about 25 g., o-i mg. of colchicine dissolvedin 0-25 c.c. of water was
injected subcutaneously, but juvenile animals received proportionately less according
to their weight. After 12 hr. the mice were chloroformed, dissected widely open, and
fixed whole in Bouin's alcoholic fluid.
Half of each group of animals was injected at 09.00 hr. and killed at 21.00 hr.,
while half was injected at 21.00 hr. and killed at 09.00 hr. Thus it was...