Mitotic activity was studied in the oesophageal epithelium of adult male rats and 20 day old foetuses with
'special reference to the diurnal variation and its relationship to the lighting conditions and the activity of the animals. The animals were kept in standard conditions and all
factors liable to cause variation in the results were eliminated as far as possible.
For the investigation of the problem, the animals
were divided into ¿. groups. CONTROL GROUP: out of 36 rats
of this group, 24 rats, of the first 2 series, were kept under
natural lighting conditions and the 12 rats, of the 3rd series,
were kept under artificial lighting conditions for a period of
90 days (light during the day and darkness during the night).
REVERSED GROUP: 18 rats of this group, 12 rats of the 4th
series and 6 rats of the 5th series, were exposed to light
during the night and to darkness during the day for a period
of 90 days. At the age of 120 to 130 days, the rats of all
the 5 series were sacrificed, one from each series at 2- hourly
intervals in the course of 24 hours, except series V where the
interval was L. hours. The mitotic activity was expressed as the number of mitoses per section of the oesophagus (10/¹).
INDIVIDUAL VARIATION GROUP: 12 rats, kept under natural
lighting conditions, and 6 rats, kept under artificial
lighting conditions, were sacrificed at a time when a high rate
of mitosis was expected and the individual variation in the
Mitotic activity was studied. FOETUS GROUP: 6 foetuses,
from one pregnant female rat, were examined at 3- hourly inter - vals in a 24 -hour period, making 48 foetuses in all and the
average mitotic activity was determined for each sample of 6
.foetuses. In addition, the mitotic activity was studied in
the oesophageal epithelium of the pregnant female rats corresponding in time with each sample of foetuses.
The important finding which emerged from the observations was that, in spite of considerable individual variation, there existed a clearly defined single cycle of mitotic
activity in the oesophageal epithelium of the adult male rat.
The observations indicated that, in the conditions in which
the animals were kept, high mitotic activity tended to occur
during the period of light when the body activity was low and
low mitotic activity tended to occur during the period of
,darkness when the body activity was high. The period of high
Mitotic activity varied from 10 to 12 hours and the period of
low mitotic activity varied from 12 to 14 hours. The other
important feature was that, with the reversal in the lighting
conditions, the single cycle of the mitotic activity was completely reversed but maintained its position relative to the
lighting conditions and the activity of the animals, similar
to that observed in the control groups of experiments. The
body activity was also reversed so that the period of high body
activity continued to fall in the period of darkness. With
regard to the exact nature of the relationship of the mitotic
activity to the lighting conditions or the activity of the
animals, two constant features were noticed in all series of
experiments. Firstly, the rise in the mitotic activity
:occurred 9 to 10 hours after the onset of darkness, secondly,
the rise in the mitotic activity was closely associated with
the primary fall in the body activity. It was believed that
,either one or both of these factors initiated the rise of the
mitotic activity which, once raised, became independent of the
lighting conditions and the activity of the animals and proceeded uninterruptedly for a constant period of 10 to 12 hours.
'In a general way, a high rate of mitosis was related to light
and low body activity and a low rate of mitosis was related to
darkness and high body activity. Such a relationship was more
distinct in series II, IV and V where the duration of darkness
was short and the period of high mitotic activity was confined
to the period of light and associated with low body activity.
As the duration of darkness increased, as in series I, a part
of the period of high mitotic activity fell in the period of
darkness and coincided with the secondary rise in the body
activity. With a further increase in the duration of darkness, as in series III, half of the period of high mitotic
activity fell in the period of darkness and coincided with a
protracted secondary interval of high body activity. In the
case of the foetus, there was no evidence of a diurnal variation in the mitotic activity. In contrast to this, the
characteristic single cycle of the mitotic activity, similar
to that in adult male rats, was present in the pregnant female
rats from which the foetuses were obtained.
The above observations were discussed with the
findings of other workers and it was found that there was a fair agreement with regard to the relationship of high mitotic
activity to the period of light and low body activity and of
low mitotic activity to the period of darkness and high body
activity. As regards the probable factor or factors responsible for the determination of the mitotic cycle, it was suggested that the onset of darkness or the primary fall in the
body activity or both acted as a stimulus for the rise in the
mitotic activity which, once raised, became independent of the
)lighting conditions and the activity of the animals and continued for a constant period of 10 to 12 hours. Whatever was
the factor which controlled the mitotic cycle in the oesophageal epithelium of the adult rat, it was ineffective in the
case of the foetus.
