Part III -Long distance-cell to cell communication in Xenopus Laevis.

For the last part of this blog, I will investigate long distance cell to cell communication between myself, a muscle cell and 2 other interrelated cells within myself.  Long distance signaling are cellular signals which are carried over long distances in the body called endocrine signaling. Endocrine signals employ hormones which are produced by endocrine cells. They travel through the blood to reach all parts of the body. Most hormones initiate a cellular response by initially combining with either a specific intracellular or cell membrane associated receptor protein. Specificity of signaling can be controlled if only some cells can respond to a particular hormone. A cell may have several different receptors that recognize the same hormone and activate different signal transduction pathways, or a cell may have several different receptors that recognize different hormones and activate the same biochemical pathway. For many hormones, including most protein hormones, the receptor is membrane-associated and embedded in the plasma membrane at the surface of the cell.

My 1^st hit is with the cell of the hypothalamus. When an organism is threatened physically or emotionally, the hypothalamus readies the body to “fight” or “take flight” by sending impulses to the adrenal medulla. In response, the medulla secretes norepinephrine (in small amounts) and epinephrine (in larger amounts). Norepinephrine causes blood vessels in the skin and skeletal muscles to constrict, raising blood pressure. It is then that my hit is undertaken. Epinephrine causes an increase in heart rate and contraction, stimulates the liver to change glycogen to glucose for use as energy by the cells, and stimulates fatty tissue to break down and release stored fats for use as energy by the cells as well. The actions of both hormones bring about increased levels of oxygen and glucose in the blood and a faster circulation of blood to the body organs, especially the brain, muscles, and heart. Reflexes and body movements quicken and the body is better able to handle a short-term emergency situation.

The contraction and expansion of the muscle.

http://cnx.org/content/m46635/latest/2114_Skeletal_Muscle_Vein_Pump.jpg

My 2nd hit is with Endothelial cells (ECs). These cells promote muscle relaxation through the release of prostaglandins, and endothelium-derived hyperpolarizing factors (EDHFs). These paracrine factors promote K⁺ efflux from muscle cells, with ensuing relaxation. Recent evidence suggests that ECs may also effect muscle relaxation via direct myoendothelial coupling. Regions of close apposition between endothelium and muscle cells are believed to contain myoendothelial gap junctions that promote the direct transfer of electrical and chemical signals between endothelial cells and muscle cells. Membrane potential has typically been measured in a single cell, with electrical coupling inferred between these respective cell types.

My communication with nerve cells.

Not only do I communicate with my fellow muscle cells but I can communicate with other cells through autocrine signalling, such as nerve cells. This occurs when a cell secretes signal molecules that can bind back to its own receptors. During my development, for example, once I am directed along a particular pathway of differentiation, I can secrete autocrine signals to myself that will reinforce this decision. 

Cell producing a signaling molecule to which it responds.

Autocrine signaling is most effective when it is performed simultaneously by neighboring cells of the same type, and it is likely to be used to encourage groups of identical cells to make the same developmental decisions. As a result, the autocrine signaling mechanism is known as the “community effect”. Being that I am in close range to my fellow nerve cell, I am able to respond to a differentiation inducing signal which allows me to actively communicate with it, rather if I were alone and isolated. A group of us cells will produce a higher concentration of a secreted signal than if we were to work alone. When this signal binds back to a receptor on the same cell type, it encourages us, the cells to respond coordinately as a group.

http://www.ncbi.nlm.nih.gov/books/NBK26813/figure/A2751/?report=objectonly

Autocrine signalling.

Cell to cell communication within the Xenopus Laevis.

We cells have several different ways of communicating with each other, which depend on which type of cell we’re communicating with and also how far away that cell is. We can communicate by direct contact with each other, by using short range signals, by using long range signals or even by complex message in the form of electrical signals. All cells receive signals via signal receptor proteins, which may be inside the cell or on the cell membrane and they have a high affinity for binding with signal molecules. Once these signal molecules bind to their specific receptors, a signal cascade is usually set off and the cell reacts to this. Muscle cells, such as myself, are immediately surrounded by other muscle cells and together we make up muscle tissue. In the neighbouring areas, a variety of of types of cells exist, such as blood cells and nerve cells and I communicate with them very often.

As a muscle cell, I am surrounded by cells of the same type and we communicate with each other via direct cell-cell interactions using a mechanism called Gap Junctions. Gap junctions are specialized tube-like junctions, filled with water, that can form between cells in direct contact with each other, connecting the cytoplasm of the cells. Using these gap junctions, I can communicate with other muscle cells around me by sending, or receiving, small signal molecules and/or ions that help in determining the process I, or others, have to perform, whether it is growth or work.

As a mature muscle cell, I can send signals directly to neighbouring cells thereby causing an effect or change in the internal composition in relation to the signals I send. For example, as muscle cells in the leg of Xenopus laevis, my neighbours and I must be able to utilize energy stores quickly and efficiently for the muscle to perform its function, which relates to movement in the organism. Therefore, we have many mitochondria in us to quickly produce energy for us to work. The mitochondria that we contain are signalled when they are needed to perform their function, mainly by an increase in Ca2+ levels inside of our cytoplasm. Using gap junctions, Ca2+ can be transferred from cell to cell easily and quickly and so can signal the start of work in many cells in a relatively short time.

Skeletal muscle cell

How did I originate?

In the first part of my journey, I form through the Primordial Oogonium. This then leads to the process of Oocytogenesis, in which my primary self (Oocyte) is formed. Meiosis I then occurs forming my first polar body and my secondary Oocyte self.

My purpose within the Xenopus laevis is to first divide meiotically. After my 2^nd meiotic division, I will stop developing and completion of my future self (the ovum) will be paused (in Metaphase II called Dictyate) until I am to be fertilized. This process occurs as follows (highlighted):

Oogonium —> (Oocytogenesis) —> Primary Oocyte —> (Meiosis I) —> First Polar Body (Discarded afterward) + Secondary oocyte —>  (Meiosis II) —> Second Polar Body (Discarded afterward) + Ovum.

When I get older, my main goal is to become fertilized and ultimately grow into a fully functioning organism, such as the one I currently reside within.

I am an active site for RNA and protein synthesis. My structure comprises of:

• Cytoplasm.

I am rich in cytoplasm which contains yolk granules to nourish myself, early in development.

• A Nucleus.

During my stage of oogenesis, my nucleus is called a germinal vesicle, which stores my genetic material.

• A Nest.

The space wherein I am located in my immature state is the cell-nest.

• Zona pellucida.

The zona pellucida protects me during my development.

Structure of an Oocyte

References:

1) “A summary of oogenesis in Xenopus laevis.” Dept. of Biology, University of Utah. The University of Utah, n.d. Web. 22 Sept. 2013. http://biologylabs.utah.edu/gard/html/Oogenesis/Oogenesis_body.htm

2) http://www.dartmouth.edu/~anatomy/Histo/lab_6/female/DMS174/29.gif

3) http://php.med.unsw.edu.au/embryology/images/6/65/Ovary_histology_061.jpg

My identity revealed.

This is my life! I am an oocyte or egg cell. I am a female gametocyte involved in sexual reproduction. I am contained within the ovarian follicle which is enclosed within the ovary.

A secondary oocyte

Reference:

1) http://www.clipart.dk.co.uk/DKImages/exp_humanbody/exp_human002.jpg

My Model Organism.

My model organism is the Xenopus laevis –The African clawed frog. The Xenopus is also commonly referred to as the African clawed toad, African claw-toed frog or the platanna. The genus Xenopus is the only frog with clawed toes. The African clawed frog has a flat body with a relatively small head.  Its skin is smooth, with dorsal surfaces usually colored in mottled hues of olive-brown or gray with darker marks and ventral surfaces a creamy white color.  This frog has no tongue, no teeth, no eyelids, and no external eardrums.  Its forelimbs have four unwebbed fingers and its hind limbs have five long, webbed toes with dark claws on the three outer toes.

A close up of the Xenopus laevis

The Xenopus laevis’ forelimbs (left) and hind limbs (right)

References:

1) Willigan, Erin. “Introduced Species Summary Project – Xenopus laevis.”Columbia University in the City of New York. N.p., 20 Oct. 2001. Web. 26 Sept. 2013. http://www.columbia.edu/itc/cerc/danoff-burg/invasion_bio/inv_spp_summ/xenopus_laevis.htm

2) http://www.critterzone.com/animal-pictures-nature/stock-photos/Head-face-closeup-African-clawed-frog-AWAM080508-47.jpg

3) http://www.critterzone.com/animal-pictures-nature/stock-photos/Claw-foot-African-clawed-frog-Xenopus-laevis-AWAM080508-12.jpg

4) http://wwwdelivery.superstock.com/WI/223/4201/PreviewComp/SuperStock_4201-23296.jpg

What is a Model Organism?

Perhaps?

A model organism can be defined as a species that has been widely studied, usually because it is easy to maintain and breed in a laboratory setting and has particular experimental advantages. Model organisms are used to obtain information about other species, including humans that are more difficult to study directly.

Why use a model organism?

Many of the things we study in Biology, such as disease, development and genetics needs to be studied in vivo, to see how pathways and signals, for example, really work. Realistically always studying these in humans would be extremely expensive as well as potentially unsafe and unethical. We use model organisms instead as they provide amazing insight that cannot be gained from lab equipment. What we learn about that animal can then be used to predict what happens in other animals. This is because all living organisms have evolved from the same ancestor and many pathways are the same across different species. In addition, model organisms are readily available, can be easily manipulated and display rapid development with their short life cycles.

Model organisms and their link to humans

References:

1) Twyman, Richard. “What are model organisms? | The Human Genome .” In the genome | The Human Genome . N.p., 28 Aug. 2002. Web. 21 Sept. 2013. http://genome.wellcome.ac.uk/doc_WTD020803.html

2)  “Using Model Organisms to Study Health and Disease – National Institute of General Medical Sciences.” NIGMS Home – National Institute of General Medical Sciences. N.p., 12 June 2013. Web. 21 Sept. 2013. http://www.nigms.nih.gov/Education/modelorg_factsheet.htm

3) http://www.prokop.co.uk/Research/LAYMAN/model-animals.gif