A kangaroo’s pouch is an entire network of structures dedicated
to keeping the offspring alive, known as a joey. Its main components are the mammary glands and teats, which
is where the milk is excreted and the joey is attached, providing nourishment
for growth of the young kangaroo.
The development of these structures are still under much
study, and are thought to be related to pubertal development.
The paper that I chose to
critique is;
“Pubertal
Development of the pouch and teats in the marsupial Macropus eugenii”
Authors: S.C Nurse and M.B Renfree
Journals of Reproduction and Fertility Ltd. 1994
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| Joey on mothers everted teat |
Purpose of the study:
The purpose of this paper is to determine the changes that
occur in the pouch and teats (mammary glands) during the kangaroo’s life in
relation to the stage of puberty and the effects of gonadal steroids in this
developmental process. (Nurse, 1994)
Materials and Methods:
In order to obtain data of so many different results, there
had to be a number of smaller studies done for this paper. It is important to
understand that when puberty is referred to it is describing the animal’s first
ovulation where a single corpus leteum is present in the ovary and therefore
will have an increase in progesterone. (Nurse, 1994)
All the kangaroo’s that were used were from a small breeding
colony where they were studied/examined three times a week from the time they
became developed enough and left their own mother’s pouch until the time where
they would give birth to an offspring themselves. (Nurse, 1994) This time frame
entails the 10 weeks before puberty to the 4 weeks afterwards. The kangaroos
underwent ovariectomies (removal of the ovaries) afterwards and were studied
individually.
The teats development was measured in the degree of
inversion of the teat, in which there are three stages; inverted –where the
teat is completely internal, everting - intermediate stage where the teat is
raised but not fully everted and everted – mature stage where the teat is
everted from the mammary gland.
(Nurse, 1994) The size of the pouch opening is also measured to show
that the size of the pouch changes as the kangaroo reaches a latter part of
puberty, and these data sets were further analysed by statistical analysis.
To determine the effects of the gonadal steroids in this
process the researches conducted steroid replacement treatment of oestradiol
benzoate and progesterone and results were gathered over several weeks.
Note:
While an animal undergoing an ovariectomy, this will stop the normal changes
and growth of the teats and the pouch, but the injection of oestradiol can
counteract and restore this.
Main findings of the study:
As expected, the degree of teat eversion in the kangaroo’s
before and after puberty was significantly different, as 74% of the animals
before they reached puberty had inverted teats whereas only 4.3% of the animals
had inverted teats after they went through puberty, meaning puberty is the
independent variable as it is the determining factor of the development of the
teats. (Nurse, 1994) While the
animal first undergoes puberty/ovulation it therefore has an increase in
oestradiol concentrations which is why only four percent have inverted teats
after puberty occurs. Also, the pre-puberty animals had less inverted teats if
any while half of the post-pubertal animals had four fully everted teats.
After ovulation and mating originally began, the teats
appeared as round lumps only but after just a few weeks the development of the
teats was more apparent as they began to protrude from the mammary region.
(Nurse, 1994) All teats were
and should be fully everted by a month after puberty, even if they had not
given birth already.
An interesting data set is that the ovariectomized young,
even after puberty, did not have everted teats until the steroid treatment was
introduced to them. When the
oestradiol benzoate was injected, there was no immediate response however after
just a few weeks the animals showed development and protrusion of their teats. The injection of progesterone had no
further effect, as most teats were already inverted. However, swelling of the
mammary glands was apparent. (Nurse, 1994)
Pouch size has much the same pattern of the teat development
– as the kangaroo becomes more mature, development and size of the structures
increases in the pouch and the pouch itself does the same. When the joey was undergoing puberty,
the pouch became larger in size and the rate at which it does so increases
after the initial stages of puberty. This is because oestradiol is being released from the
Graafian follicle of the ovaries, as the kangaroo has increase levels of this
steroid during puberty. When progesterone was injected, there was simply no
change in the size of pouch openeing. (Nurse, 1994) Therefore, this data shows
that it is in fact the onset of puberty and not pregnancy of the kangaroo that
causes the increase in pouch size.
We are able to further conclude that there is no difference
in the development of mammary glands teats during the oestrous cycle and
pregnancy. (Nurse, 1994) This is
further proved from the results of the comparison of kangaroos with and without
young, where having young makes no difference in pouch size.
Also, with regards to the impact of specific steroids and
pouch growth, the intact young had a higher growth rate of pouch size before
and after puberty or at the start of the treatment with oestradiol compared to the
ovariectomized young. Also,
progesterone had no noticeable effects on the size of the pouch opening. (Nurse, 1994)
To summarize, the ovarian oestradiol concentrations ultimately
determines the development progress of the teats and pouch size of the kangaroo,
which increases when or just before puberty first occurs. (Nurse, 1994)
My thoughts on the study:
Overall, the
paper was well written and conducted. This is because the experiments were very well
organized and presented. There
were many trials conducted and data collected, as the animals were studied
quite frequently over a suitable period of time. For example, the kangaroo’s were studied three times a
week for teat eversion progress, so there would be no time frame where the
researchers would miss recording any data, increasing the accuracy of the
study.
Another strong point of the study is the number of factors
they studied to determine the specific cause of mammary gland and pouch size
development. For example, not only
did they set-up studies to determine the effects of the steroids on teat
development, but they did the same for pouch growth. Plus, they not only used oestradiol, but progesterone too.
The paper was clearly explained, as the figures depicted the
information gathered very well.
For instance, in the paper you may see Figure 4, which compares the
growth of the pouch opening growth in a young intact at puberty and a young
which was ovarietomized (removed).
The figure clearly indicates the results as they are presented by a
relatively straight line, making it easy to read.
A weak point in the paper is the explanation of the anatomy
and physiology aspects of the mammary glands and ovarian steroids. Although the paper overviews the basic
information necessary to understand the results of the study, it doesn’t go
into detail which could definitely provide the readers with a better
understanding of what is actually happening in the kangaroo.
Again, overall the paper was clearly presented, and the
results were as the author had predicted. There were a few minor changes from the expected
results, but as the purpose was to determine the cause of the development of
teats of the mammary gland and pouch size of a kangaroo, in relation to puberty
and the gonadal steroids secreted at that time, the conclusion was clearly
presented in the discussion and proved from the results obtained. I really liked this paper, it was
interesting and very informative!
