Tag Archives: reproduction

SYR: How sheep can help us understand why girls are reaching puberty at younger ages

michelle-bedenbaughThis guest post is the second written by Michelle Bedenbaugh, a Ph.D. student in the Physiology and Pharmacology Department at West Virginia University. Check out her first post on the benefits of using large animal models to study reproduction. It is also part of our Speaking of Your Research series of posts where scientists discuss their own research. In this post, Michelle discusses some of the cells and signaling pathways that are important for controlling the timing of puberty and how the use of sheep as a model is beneficial for this type of research. If you would be willing to write a guest article for Speaking of Research, please contact us here.

For those of you who have been watching the news in the United States over the past 5-10 years, you have probably heard a few discussions about the fact that girls are reaching puberty at younger ages.  In the 1980s, girls normally reached puberty around the age of 13.  In 2010, the average age of girls reaching puberty had dropped to 11 and has since continued to decline.  Reaching puberty at earlier ages is associated with several adverse health outcomes, including polycystic ovary syndrome (PCOS), metabolic syndrome, obesity, osteoporosis, several reproductive cancers and psychosocial distress.  The public and researchers have pointed fingers at several potential culprits, including an unhealthy diet, chemicals that disrupt the body’s normal hormonal environment, and an individual’s genetic predisposition to disease.  In reality, a combination of factors have probably led to the decrease in the age at which girls reach puberty, but I don’t want get into a discussion about these factors today.  Instead, I want to talk about some of the signaling molecules in the body that these factors may be influencing to affect the initiation of puberty.

As with many processes in the human body, the brain plays a critical role in the control of reproduction and the timing of puberty.  Within a specific area of the brain called the hypothalamus, several populations of neurons (specialized cells in the brain) exist that control reproduction.  The activity of these neurons is influenced by various factors that are communicated from other parts of the body and outside environment to the brain, including nutritional status, concentrations of sex steroids (like estrogen and testosterone), genetics, and many other external factors.  All of these factors tell the brain when an individual has obtained the qualities necessary to successfully reproduce and therefore undergo pubertal maturation.  Gonadotropin-releasing hormone (GnRH) neurons found in the hypothalamus are the final step in this chain of communication and are essential for the initiation of puberty.

A GnRH neuron present in the hypothalamus.

A GnRH neuron present in the hypothalamus.

Most of these nutritional, hormonal, genetic and environmental signals are not directly communicated to GnRH neurons.  Instead, they are conveyed through other types of neurons that then relay this information to GnRH neurons which either stimulates or inhibits the release of GnRH.  Because GnRH is a signaling molecule that ultimately stimulates the maturation of male (sperm) and female (egg) gametes, stimulating GnRH in turn stimulates reproductive processes while inhibiting GnRH inhibits reproductive processes.  The perfect balance of stimulatory and inhibitory inputs is needed for GnRH to be released and for puberty to be initiated.  Consequently, if stimulatory inputs signal to increase GnRH prematurely, puberty will occur earlier, which may result in several of the health concerns that were mentioned above later in life, including reproductive cancers and psychosocial distress.  In contrast, if inhibitory inputs block the release of GnRH, puberty will never occur and result in infertility.

My research looks at some of these stimulatory and inhibitory inputs and how they communicate with each other, as well as with GnRH neurons.  Two of the stimulatory signaling molecules that we research are kisspeptin and neurokinin B (funny names, I know).  We also study dynorphin (another funny name), a molecule that inhibits GnRH release.  These three molecules can all individually affect GnRH release and therefore reproduction.  However, the really cool thing about these three molecules are that they are actually present together in a special type of neuron that is only found in one small and highly specific area of the hypothalamus.  Because these neurons contain kisspeptin, neurokinin B, and dynorphin, they are often called KNDy (pronounced “candy”) neurons.  The fact that kisspeptin, neurokinin B, and dynorphin are all present in these KNDy neurons together allows for them to communicate directly and affect each other’s release.  This communication then ultimately affects the release of GnRH.  Before puberty, inhibitory inputs, like dynorphin, dominate and don’t allow for adequate amounts of GnRH to be released to stimulate reproduction.  As an individual matures, stimulatory inputs, like kisspeptin and neurokinin B, begin to outweigh inhibitory inputs, and GnRH can be released in adequate amounts to support reproductive processes.  Below is a figure that summarizes how we think all of this works within the body.  However, there is still a lot that we don’t know about how kisspeptin, neurokinin B and dynorphin interact with each other that is waiting to be discovered!

Hypothesized model for the initiation of puberty. (1) Internal and external factors are communicated to the body. (2) Next, these factors are relayed through various signaling pathways to stimulatory and inhibitory molecules present in neurons located in the hypothalamus. (3) Stimulatory and inhibitory molecules travel to GnRH neurons and affect the release of GnRH. (4) GnRH stimulates reproductive processes that are critical for the initiation of puberty. (5) Once all of the proper conditions are met, reproductive maturity is attained.

Hypothesized model for the initiation of puberty. (1) Internal and external factors are communicated to the body. (2) Next, these factors are relayed through various signaling pathways to stimulatory and inhibitory molecules present in neurons located in the hypothalamus. (3) Stimulatory and inhibitory molecules travel to GnRH neurons and affect the release of GnRH. (4) GnRH stimulates reproductive processes that are critical for the initiation of puberty. (5) Once all of the proper conditions are met, reproductive maturity is attained.

To complete all of these studies, we use sheep as our model.  I know what some of you are thinking.  “How in the world would sheep serve as a good model for how puberty is initiated in humans?  I don’t think I am similar to a sheep at all!”  In fact, sheep are actually an excellent model in which to do this research.  The signaling pathways that affect the release of GnRH in sheep are very similar to the signaling pathways in humans, and in some cases, are even more similar to the human pathways than the pathways present in mice or rats.  In humans and sheep, neurokinin B has only been found to stimulate GnRH release.  However, in rodents, there have been reports of neurokinin B both stimulating and inhibiting GnRH release.  Since neurokinin B is one of the main signaling molecules that we study, using sheep instead of mice or rats is more beneficial for modeling what is occurring in humans.

sheep-in-reproduction-research

Because we have to collect several blood samples from the sheep in order to measure hormone concentrations, having an animal with a larger blood volume is also advantageous.  Several hormones in the body (including GnRH) are released in a pulsatile manner, meaning one minute GnRH concentrations are high and a few minutes later they are low.  Therefore, in order to appropriately measure GnRH, blood samples need to be taken every 10-12 minutes for several hours.  This is not feasible in rodents.  If you took blood samples as frequently in rodents as is possible in sheep, you would risk killing the animal.  Some scientists who use rodents as their research model attempt to get around this issue by taking blood samples less frequently.  However, this means their hormone measurements are less accurate.

These are just a few of the many reasons why we conduct our research in sheep (to learn more about the advantages of using sheep and other large animal models to conduct research involving reproduction, see my previous post).

While most people (including myself) do not look back fondly on our awkward pubertal years, I absolutely love studying the signaling pathways the body uses to determine when it is ready to successfully reproduce.  We have discovered quite a bit over the past few decades concerning how different internal and external factors affect pubertal maturation, but there are still so many unknowns left to be determined.  I look forward to hopefully discovering some of these unknowns and improving our understanding of how puberty is initiated in both humans and livestock species.

Michelle Bedenbaugh

SYR: The case for using large animal models to study reproduction

michelle-bedenbaughThis guest post is written by Michelle Bedenbaugh, a Ph.D. student in the Physiology and Pharmacology Department at West Virginia University. It is part of our Speaking of Your Research series of posts where scientists discuss their own research. Michelle’s research involves examining the brain’s role in the initiation of puberty.  In this post, Michelle discusses the benefits of using large animal models to study reproduction.  If you would be willing to write a guest article for Speaking of Research, please contact us here.

With the increasing pressure to publish papers and the decreasing amount of funds made available to conduct experiments, it has become more difficult for researchers to survive and thrive in an academic setting (see here, here, and here). Scientists have had to adapt, and in many situations, this has led to a significant amount of research that relies heavily on small animal models, including rodents and invertebrates.  In addition to being less expensive than large animal models (sheep, pigs, cows, horses, etc.) there are also more genetic tools and techniques available to use in small animal models.  For example, transgenic mice, where certain genes can be either deleted or overexpressed, are used commonly by researchers worldwide.  Other cutting edge techniques, like optogenetics, where light can be used to control the activity of cells in the brain, are also being used on a more routine basis in rodent models and currently don’t exist in large animal models.

Optogenetics involved using light to control genetically modified cells inside the body

Optogenetics involved using light to turn off or on cells in the brain

While it is most likely easier, cheaper, and faster to conduct experiments using small animal models, in certain situations they are not always the most comparable to humans.  When modeling certain diseases or understanding certain physiological processes, larger animals, like sheep, pigs, and cows, provide a better model for scientists.  This post aims to look at some areas where larger mammals can provide important knowledge or understanding.

A few of the more obvious benefits to using large animal models when compared to small animal models are that large animals are more analogous to humans in regards to body size, organ size, and lifespan.  In addition to these similarities, animals like sheep, cows, and pigs are much less inbred when compared to rodents.  Some would argue that it is advantageous to use animals that are highly inbred because this decreases the amount of variability in an experiment.  However, each human has a unique genetic makeup, and sometimes solutions for problems in inbred rodents cannot be translated for use in humans.  Therefore, in these instances, it is probably more beneficial to use a less inbred large animal model.  Most large animal models also have the added benefit of being an economically important species.  The majority of researchers who use large animal models are attempting to find solutions to health issues that are present in humans.  However, successful experiments in large animal models have the ability to affect both human and animal health.  For example, if a researcher made an important discovery about the way food intake is controlled in cows, it would have the possibility of improving human health, as well as increasing profitability for cattle producers.  Because cows are very similar to sheep, it may also benefit sheep production as well.  Rodents are not an economically important species that provides food, fiber, or other essential products used by the human population.  Consequently, discoveries made in rodents and other small animal models may only benefit humans if the results are translatable.

My particular research focuses on furthering our understanding of how puberty is initiated in girls, and we use sheep as our animal model.  I won’t get into the specific benefits of using sheep to conduct puberty research today because I will discuss this more in my next post.  However, I did want to touch briefly on some of the advantages of using large animals to perform research used to study reproduction in a broader sense.

The brain plays an essential role in controlling reproductive processes.  The brain structure of large animals is more closely related to humans than small animals because large species have a sulcated cortex (meaning the surface of the brain is wrinkly) as opposed to small animal species which have a smooth cortex.

Comparison between mouse (smooth cortex) and human (sulcated cortex) brain. [Credit: Elizabeth Atkinson, Washington University in St. Louis]

Comparison between mouse (smooth cortex) and human (sulcated cortex) brain. [Credit: Elizabeth Atkinson, Washington University in St. Louis]

Sheep also have the advantage of their brain and the cellular pathways present within it being similarly organized to what is observed in non-human primates.  Hormones serve a major role in relaying information from the brain to reproductive organs and vice versa.  The actions of several hormones that aid in controlling reproduction in female sheep (like estrogen and progesterone) parallel the actions of these hormones in humans.  Older sheep also have a similar response to estrogen replacement therapy when compared to post-menopausal women.  The development and function of several structures on the ovary of sheep is also similar to that which is observed in women.  These structures have a major influence on the reproductive cycle and are critical for the maturation of female gametes (sometimes referred to as eggs).  Assisted reproductive technologies, many of which are used for in vitro fertilization (IVF) protocols in women who are having trouble conceiving, have been adapted from procedures used in livestock species.  For example, artificial insemination, where semen is collected from a male and usually frozen so that it can be used to inseminate a female at a later time, is commonly used in cows, sheep, horses and pigs and is similar to procedures conducted in humans.

Credit: Livestock Breeding Services - http://www.livestockbreedingservices.com.au/images/servicesai.jpg

A laparoscopic procedure is used to artificially inseminate sheep

 

Embryo transfer, where embryos from one female are placed into the uterus of another female, are also used in livestock species and humans.  In addition, sheep are also an excellent animal model for studying pregnancy.  Sheep are used often to examine how stress, maternal nutrition, and exposure to excess hormones or toxins affect the development of a fetus.

These are just a few examples that display reproductive processes occurring in many large animal species are easily relatable to those same processes which also occur in humans.  I only touched on a few species today, but there are many more animal models that are underused in research and would serve as great models for humans.  In addition, I only discussed some of the ways these animals can be used to study reproduction when in fact they can be used to mimic many other biological processes that occur in humans.  Depending on the subject matter being researched, the use of some animal models is more appropriate than others.  Regardless of cost or time, researchers should always consider which animal model may be the most appropriate for their experiments.  I believe conducting research in a variety of species as opposed to just one or two species will always be more advantageous and will aid us in solving health issues in humans more quickly.

Michelle Bedenbaugh

SR at SSR

SSR Heading

On Monday July 20th Tom Holder gave a presentation as part of the President’s Sympozium at the Society for the Study of Reproduction‘s 42nd Annual Meeting. The presentation was attended by around 500 members of the society, ranging from undergraduates up to well established professors. The talk provided suggestions on how to talk about science to those without a science background, as well the importance of speaking out. After the talk a number of members of SSR have been in contact to ask how they can get involved with Speaking of Research’s activities – a positive result all round.

SR at SSR

The rest of the conference provided an interesting overview of some of the research going on to tackle illness such as infertility, cancer and endometriosis. Animal models varied from fish and rodents, through to cattle, pigs and primates, with scientists careful to explain why their particular choice of animal was the most suitable for their particular research. GnRH (Gonadotropin-releasing hormone) agonists are a good example of the crucial role that animals have played in fighting disease in reproductive biology. After significant work with monkeys, mice and sheep scientists were able to come up with some of the leading treatments for prostate cancer, breast cancer and endometriosis – such as Leuprolide (Market Name: Viadur/Lupron) and Goserelin

You can read the abstract for Holder’s speech here.

Regards

Dave