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”
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!
First, let's learn a little about kangaroos:
Kangaroo's are a special marsupial species which make their home in different parts of Australia and Tasmania. They are widely known for their choice of locomotion - hopping! They have powerful hindlimb's and use their tails for balancing to do so. There are many, many species of kangaroos, but there are three major; the red kangaroos, east and west grey kangaroos.
You will usually see kangaroo's in groups called mobs, and each mob has a boomer- an alpha male. The alpha male has it's choice of a female mate and if the female raises it's tail in response to the male, mating can and will take place. They are generally very gentle animals, but if they feel threatened they will stand on their powerful hind legs and show their boxing position, or kicks the threat (9).
This brings us to the most interesting feature! Not only is the Kangaroo among the most adorable animals, it has a number of characteristics that make is especially unique. The most obvious of course - it's pouch! The kangaroo is a marsupial mammal, which simply means that once they give birth to their young, they carry their offspring in their pouch. Their pouch is therefore their specialized adaptation to be able to do so. Once the joey is big enough to venture out into the world, it often returns to it's mothers pouch for nourishment and protection. How does it get in the pouch? By somersaulting into it's mother! (9)
The kangaroo also is the only bipedal marsupial mammal and has special adaptations that allow for it to keep the embryo on "pause" from developing until suitable conditions are met - pretty neat!
Marsupium in Latin = "pouch"
A newborn joey in it's mama's pouch (8)
A kangaroo only carries their young for about one month (therefore one month of gestation) before they give birth to them. The offspring is referred to as a "joey" which through miraculous efforts finds it's way from the mother's birth canal to the mothers pouch by crawling through the mothers hair. This is rather amazing for the reason being that they are blind and extremely undeveloped and they are able to do all of this in under 3 minutes! Once the joey reaches it's mothers pouch, it nestles in and attaches to a teat so they can be fed. This help explains the main purpose of the kangaroo's pouch - for nursing and further development of the joey. (2)
The first few moments of a joeys life illustrating the importance of the pouch:
The young kangaroos live within their mothers pouch from anywhere to 120 to 400 days. They need to stay there this long as they are extremely underdeveloped when they first enter the mother's pouch (even so small as a jellybean or honeybee).
Anatomy of the Pouch
As the purpose of the pouch is to provide a safe place for the young joey to develop and be nursed by it's mother, it is important to understand the anatomy of the mammary gland. All mammalian mothers have a mammy gland, as this is one of it's defying features. The mammary gland is simply a modified sweat gland, which consists of a mamma and a teat, with associated duct systems used to secrete the milk produced. (4)
wikki
The mammae are glandular structures with lobes separated by adipose tissues (which is required and derived from mesoderm but absent in males) with the main purpose of milk production. Inside the lobes are connective tissue and aveoli with milk secreting cells. The alveoli's clusters of cells are all interconnected and further connected to the lactiferous sinus (milk sinus) by the lactiferous ducts. The milk sinus is where the milk is stored.(4)
(24)
You can see from both images on the right how the canals are linked to the secreting cells of the lobes and therefore the milk sinus. When an organism becomes pregnant, the number and complexity of the secreting cells of the alveolus increase and all the ducts mature. The cells are now ready to be stimulated (described below in the blog) by specific hormones to produce milk. The milk fills the alveolar lumens while the myoepithelial cells contract forcing the milk out into the lactiferous ducts/intercalary ducts.
Below is an image that sums up the organization of the mammae referred to as a Terminal Duct Lobular Unit (TDLU) which has a cluster of aveoli with milk producing cells forming a lobule.
A cross section of the TDLU is also shown, futher showing the duct and lobule layout.
terminal ductules
Interlobular stroma (Adipose tissue and connective tissue)
Intralobular Stroma
(21)
Note: This is of a human nipple, as there hasn't been much direct research in this area for kangaroos. The main idea from this figure is to see the spaces of lactiferous ducts, and how close they are to the exit area of the gland. Also, like the gland of a marsupial mammal, there is much connective tissue and adipose tissue.
The projection of the milk sinus brings us to the second part of the mammary gland - the teat. The teat is the end of the mammary gland to which becomes in contact with the young and the recipient of the milk via the teat canal or the papillary duct. The opening of a teat is referred to as a ostium, which is where the milk exits the gland. This mechanism of suckling of only small amounts of milk at a time and the prevention of back flow of milk back into the teat is due to sphincter.
Teat and Mammry Gland Anatomy (25)
The teat is associated with one mammary gland, and because a typical kangaroo has four it would therefore have four mammary glands. Each of these four glands are independently developed.
As shown in the image below, there are two unactivated teats (indicated with the red arrows). These are quite evident as they are not enlarged yet, due to lack of suckling from a joey, and therefore no milk storage, also known as cistern. (5)
The green arrow shows the joey attached to the teat. Young kangaroos have a few special adaptations that allow them to do so; their mouth has a hard palate and tongue used for attachment, ridges will form on either side which causes the lips/mouth to be fused around the teat. This also makes it very hard to rip or remove the teat from the joey, but in return it secures it's nourishment for the next number of days. (5)
The blue arrows are pointing to a teat that was previously occupied by a joey who developed fully and left the mothers womb, only returning at first to continue sucking when needed, and therefore it stores milk until the it is no longer needed. (5) In fact, just before the joey is about to start leaving the pouch frequently the teat it suckled on grows in size as it storing plenty of milk for the new, energetic joey.
Developed/Undeveloped Teats (5)
Joey attached to it's mother's teat:
Production of Milk:
Unlike humans and other eutherian mammals, the marsupials (ie - kangaroos) put their reproductive emphasis on the process of the lactation of milk for their young, instead of the gestation period with the young still within them. The milk secreting cells are activated by the hormones: prolactin and oxytocin indirectly. (22) These hormones are released from the pituitary gland stimulated not by suckling on the teats directly but because of the hormone level change that occurs because of it. Meaning, unlike eutherian mammals, marsupials the direct causation of prolactin release is not due to suckling of the teat but by suckling, it does make the mammary gland more sensitive to prolactin by causing the formation/activation of prolactin receptors of the mammary gland. Therefore, the mother would secrete prolactin as it detects low levels as it had no reason for the hormone to be plentiful before in it's lifecycle. The prolactin then stimulates the secretory cells to release the milk into the lumen of the alveolus only to be excreted out into the milk ducts to make it's way into the teat to be delivered to the newborn. (23)
Prolactin is not the only hormone involved, however. Oxytocin is widely known as the hormone that helps stimulate contractions during labour in humans. In marsupials, like the kangaroo, still provides the function of releasing milk to newborns (as it does in eutherians) it plays a bit of a different role. For instance, when the joey is attached to the teat with milk be ejected constantly the mammary gland is being very responsive to the levels of oxytocin by secreting more and more milk. The mammary gland, however, becomes less and less responsive to the oxytocin overtime (differing to eutherians) meaning that there will be less milk at a slower rate. You would then wonder how does the new, young joey get all the proper nourishment it requires? This is where the oxytocin and mammary gland relationship keeps on getting more unusual - because the joey only comes back into it's mothers pouch occasionally, but yet "sucks" very hard and quickly, there is a lot of milk secreted from the gland.(23)
Effects of hormones on milk secretion (22)
The content of the milk is also unique. This is due to the fact that while all mammals milk changes in when you go from one phase of lactation to another, it is very extreme in marsupials like kangaroos. During the early development of the joey's milk will have a higher concentration of carbohydrates while the concentration of lipids is low - as they don't need so much fat content because they are essentially non-mobile. This however is reversed near the end of the joey's development stage in the pouch.(22)
Did you know? Years ago, it was believed that kangaroo's developed their young by the offspring budding off from a teat itself.
What exactly is a pouch? The pouch is essentially a fold of skin with an opening, into which a joey can go. If it is a fold of skin, then we know exactly what the pouch consists of. Unfortunately, there isn't much research done with specific mammals and their skin, but the basic concepts are the same for almost all. Firstly, the skin is part of the integumentary system which, for example, functions as a protective barrier to humans internal organs, and also important for the prevention of water loss. (1) The skin has three layers:
Model of skin layers (1)
1. Epidermis - the outermost layer of the skin. Consists of the stratum corneum which prevents the entry of foreign and potentially harmful substances and prevents the loss of body fluids therefore preventing desiccation of the skin. This layer also contains keratinocytes and melanocytes. It also contains the basal layer which of course contain basal cells. The basal cells constantly replace the old ones as the this layer, and therefore "the skin" is constantly shedding. 2. Dermis - This layer is important as it contains all the blood vessels, lymphatic vessels, hair follicles, sweat glands, collagen
bundles, fibroblasts and nerves. The collagen present gives this layer much strength, flexibility and holds the layer together. It also contains rough connective tissue therefore giving it it's strength. 3. Hypodermis/Subcutaneous - This is the deepest layer of the skin, and therefore the closest to the internal organs. Because of this, it consists of fat and connective tissue mostly, creating a network. Therefore it makes sense that it acts as a cushion for organs and conserves body heat. (1) The hair is an important characteristic for all mammals, as it is found on them an no other organisms. It's main functions include insulation, camouflage, sex recognition, and aggressive displays.(11) Of course, every mammal is different, as uses it's hair for different purposes. Hair consists of the shaft, the root and the follicle which is all made up of proteins chains of amino acids) and is often keratinized making it strong and durable. The root holds the hair in place attached to a shaft and all covered by the hair follicle. The sebaceous glands are present to secrete oil which helps trap moisture in the skin. There is a muscle called the Arrector pili, and it is this muscle that controls the action of the animals hair to "stand up" as a mechanism to regulate body temperature by keeping heat in because it is connected to the connective tissue around the hair follicle. This muscle is derived from the embryonic mesoderm. (11) The hair bulb is where the new cells are formed to develop the hair. The innermost layer of the hair shaft has layers of cells referred to as a pith ( the cells are most often dried epithelial cells and soft keratin). The middle layer of the hair shaft is called the cortex and basically is just composed of the harder keratin. The outermost layer of the hair shaft is called the cuticle, and is made of colourless keratinized cells (1) . As you can see in the cross section picture below, the hair shaft consists of many different layers of cells.
Hair Follicle (10)
Microscopic view of the hair structure (11)
There are also many different types of hair, fur being one of them. And of course mammals like the kangaroo have fur for the most part, with the exception of a few species with shorter hair. Their fur, again, acts as a heat regulator keeping the heat in for the most part. You will also notice in pictures that there is not only hair on the outer portion of the pouch, but hair inside of the kangaroo's pouch, as there are not many outwardly structures of the body that doesn't have some type of hair - it's everywhere! Fur normally is comprised of guard hair and ground hair - ground hair is the undercoat, and keeps the mammal warm where guard hair used for mechanisms such as camouflage and protection of the skin and undercoat. (12) They joey even grows it's own hair and eventually fur while inside the mothers pouch not long after arriving! Did you know?? Of all the marsupials the kangaroos have the biggest pouch!
Early Histological Development: An early embryo of any sort has three basic layers, each of which develops into various elements to create a living organism; the ectoderm giving rise to the skin and nervous system, the mesoderm giving rise to the muscle, blood vascular system and sex organs and the endoderm develops into the alimentary canal and digestive glands. (15) The pouch and the mammary gland are formed in early development of a joey. The mammary gland develop involves the mesoderm and ectoderm forming the mamma, the projections and folds of the ectoderm invades the mesoderm and soon the ducts are apparent. The teat is developed from mesenchymal cells around the mammary bud which is composed of ectodermal cells, but before the teat is apparent, the cistern develops. (15) As the teat develops the outside becomes skin-like due to the surface cells. As the pouch is a fold of skin with an opening, it is derived from the ectoderm. However, once the pouch is developed it can grow and change throughout the life cycle of the kangaroo, for example during puberty of the kangaroos, their pouch fully forms with teats appearing on the inside, and once it grows to this size it doesn't return. Pathology of Kangaroo pouch:
The joey while only a developing kangaroo in the pouch can be ver sensitive to heat and cold, and can therefore die of a heatstroke, as they are considered poikilothermic. (16)
If joey's are hand raised because they are orphans, then they may get too much or too little lactose, resulting in possibly cataracts or diarrhoea. This could be due to the lack of galactokinase and transferase. (16)However, from the studies conduction it seems as though joeys fed milk with more lactose get cataracts- which means that the lens of the eye is clouded affecting vision. (17)
In the red kangaroo, genital abnormalities due to genetics could include aplasia of the pouch and teats.(16)
The pouch is a continuous home:
Kangaroos have a unique adaptation that allows them to constantly be fertilized. Mating can occur just a few days after the first joey is expelled from the birth canal and finds its way to the mothers pouch. However, the newly concieved embryo will be in a paused or dormant state until the first joey is about to leave the mothers pouch. The mothers body knows when to resume development of the new fetus because the lessening of suckling on the teat in the pouch "upsets" the hormones again. (2)
But they don't stop here! After the second offspring is developed and emerges from the birth canal the mother can mate once again and repeat the entire process, waiting for the second joey to develop enough so that it can make room for the third joey - but don't worry, the first joey is completely independent of the mothers pouch by now and no longer needs it's milk and nourishment.
Not only can the kangaroo maintain having three different joeys consecutively, but it can produce specialized milk for the different joeys, as they are in different developmental stages and therefore have different nourishment requirements. Each teat produces milk with different composition as the joey who is less developed has less fat in their milk compared to the one who joey who needs it as they are bigger, more functional and burn more energy. (22)
Did you know?? Kangaroos can have up to 25 offspring in it's lifetime, but they live no more than 15 years.
Caring of the pouch:
A Mother cleaning it's pouch (18)
The mother kangaroo will constantly lick and clean it's pouch to make sure that it a sanitary environment for it's new offspring. (7)
This is actually one of the first signs of pregnancy for a kangaroo - licking it's pouch in preparation for the arrival of it's baby. In fact, the closer the moment of birth gets, the more intense the licking and biting becomes. (7)
Just for fun - What can you do? Become a surrogate pouch!
The department for environment and conservation
Joey's in their Eskys (14)
in Austrailia have stepped in and created animal hospitals for all animlas, including joeys who are motherless and still need help and nourishment. Normally, joeys wouldn't be able to survive on their own outside of their mother's pouch, but by creating a similar environment to the pouch with warmth and nourishment, we can save the orphaned joeys! You can adopt these joeys, and provide them constant care until they are able to do so on their own. One of the most important rules is that you have to supply them with constant warmth. As you can see in the picture on the right, people developed the idea of building them little bed boxes/eskys. To keep these a pouch-like as possible they are all lined with fleece or other soft materials, and the size of the box will grow as the joey does. This is a very specific process, even down to the details of the shape of the box, and it needs to be round on the bottom especially. When the joey is almost fully developed you can use a hanging pouch contraption. The joeys also need to be fed of course, as seen on the right and this is done on your lap in a particular manner. For more information, see the link below! (14)
