ovulation) and one without luteal phase (spontaneous
Vaginal smears taken at the time of first intromission
presented only cornified superficial cells in all individuals. This
cellular constitution did not change in any group of individuals
when it was taken again at the end of the group's time period.
ovulation) when the rats receive some certain stimuli.
Nalbandov (2, 9) also reported that in spontaneous ovulators,
LH is cyclic, independent of copulation but provoked by the
interplay of the neuroendocrine system.
Induced ovulation, on the other hand, occurs only when the
cervix or part of the vagina is appropriately stimulated (10, 11).
In this case, our oestrus rats that did not present functional
structures could be grouped under the spontaneous ovulators
producing GFs, ovulating but not developing CL (12). But
these groups of rats involve LH action; therefore, the inability
to produce CL is not quite clear. If the reason for lack of CL
development is inappropriate induction, at least, an ovulated
follicle could have been observed. There is a need for a better
understanding of the physiology of the ovarian cycle in these
groups of rats.
About 67% of the obviously active rats showed signs of
activity only on one ovary. It is not clear whether it can also
be attributed to incomplete induction as they all carried
primary and growing follicles but no GFs on the affected
ovaries. Furthermore, it is reported that the left ovary is more
active in mammals than the right ovary (6), observation in this
study cannot quite stand with that bias because while
correctly, there were higher follicular, activities on the LOs
Functional Structures of the Ovaries
The data on Graafian follicles (GF) and corpora lutea (CL)
counted from the sectioned ovaries of 50 rats are presented in
Tables' 1 - 3 and Figure 1. Contrary to expectation, some
individuals did not present any functional structures at all on
sectioning. Only group one had all the members presenting
functional structures with two only on the right ovaries. One
member each in group 2, 6, 9 and 10, and two each from 3, 4,
, 7 and 8, making a total of 14 rats out of 50, did not present
any GF or CL in both right ovaries (RO) and left ovaries (LO).
Out of 36 rats with apparent ovarian activity, 11 (30.6 %),
3 (36 %) and 12 (33.3 %) were active only on the RO, LO and
both ovaries respectively. However, out of the 36, only 26 (72
) showed any signs of ovulation in their ovary sections with
rates ranging from 33 % to 100 % in different groups: All the
members of groups six and seven and a couple of individual
members in groups 1, 2, 3, 4, 5, 8 and 10, making 15 individuals
presented 100 % ovulations of all GFs developed. On the other
hand, the entire number of rats in group nine and another couple
of individuals from groups 1, 3, 4 and 5 making a total of nine
rats presented zero ovulations of all the GFs observed. Both
extremes included all categories of active rats.
42:36), ovulatory activities, on the contrary were higher on
the ROs (58 %:54 %).
This work also has no explanations for complete
ovulations in one quarter of the active rats.
A total of 78 GFs (36 RO, 42 LO) with 47 (60 %; 23 RO
and 24 LO) becoming CLs were obtained from a pool of all the
rats. Ovulation rates pooled for active only on the ROs or LOs
were 58 % (11 CLs of 19 GFs) and 54 % (13 CLs of 24 GFs)
respectively. Of the 12 rats active on both ovaries, ovulation
rates were spread as 0 %, 33 %, 43 % 50 %, 67 %, 80 % and
It can be concluded from this work that what triggers the
"stance" for mounting in the albino laboratory' rat is the
mechanism behind the ovulation process. It can also be
concluded that any follicle that did not ovulate at the time of
onset of mounting may not ovulate again. Therefore, we
conclude that the ovulation time in the laboratory albino rat in
our laboratory, and probably elsewhere, is the beginning of
oestrus, peaking maximally at the sixth and seventh hour of
oestrous. However, every observation needs validation in
00 % in 1, 2, 1, 3, 2, 1 and 2 rats respectively. Total average
for ovulation rate in this study is 60 %.
This study was aimed at pegging the ovulation time for
rats in our local laboratory for greater usefulness as
reproductive research models. Ovulation occurs within the
oestrus period in most mammals (6) and has been reported to
occur at 8-10 hours into oestrus in rats with a heat period of
2-15 hours (2, 5). Though no data was collected on the heat
period of the laboratory rat in this study, it appeared to be
much shorter than 12 hours. This is in contrast with Paccola et
al., (1) that found out that the ovulation was 12 hours with an
average duration of estrous cycle ranges from 3.5 to 5.5 days.
This substantiates the need for knowledge of the exact
ovulation time in the laboratory rats used in our own
environment. From the tables and the figure presented, it is
clear that ovulation time is not pegged in the laboratory albino
rat, at least in the laboratory where the work was carried out.
It is more or less a spread from the onset of oestrus (7). From
the high ovulation rate observed in the first hour (72%), it is
possible that the rest of the groups could have ovulated in the
first hour also.
It is also confusing, that about 33% of the rats that
participated in vigorous sexual activity and were confirmed to
be in oestrus by vaginal smear examination could neither
present CLs nor GFs . Well, Krinke (8) reported two types of
cycle in the rats such as a cycle with a luteal phase (induced
Figure 1. Ovulation rate in different groups of rats
corresponding to different hours of oestrus periods