Teratogenic+Effects+of+Caffeine+and+Mercury

= Teratogens: Caffeine and Mercury =

__Normal Development__
Normal development of a human begins with the fertilization of an egg by a sperm in the fallopian tube. After the initial fertilization, the egg travels down the fallopian tube and is facilitated by cilia located on the inner walls of the fallopian tube. The fertilized egg, now called a zygote, begins to divide and eventually forms two layers of cells, an inner layer of cells (inner cell mass) and an outer layer of cells (trophoblast). A cavity known as the blastocoel forms as a result of these two layers forming and the embryo is now known as a blastocyst. The blastocyst eventually reaches the uterus and implants into the uterine wall via the interaction of various adhesion proteins. Prior to implantation the inner cell mass has differentiated into epiblast and hypoblast cells that will give rise to the three cell layers associated with gastrulation (Gilbert, 2010).

At this point the embryo undergoes more cell divisions and eventually begins the process of gastrulation. Gastrulation results in the formation of three distinct layers from the inner cell mass: endoderm, mesoderm, and ectoderm. Also, generated is the amniotic cavity and several other extraembryonic membranes. Following gastrulation the cells begin the process of neurulation. The embryo is known as a neuroblast during this stage of development. Neuralation begins with the ectoderm cells forming the neural plate, subsequently leading to the formation of the neural tube. Ectoderm cells are pre-programmed to form neural cells, and therefore must receive signals to do otherwise. There are two types of neurulation: primary and secondary. Primary neurulation is characterized by the nerual plate folding to form the neural tube. Secondary neurulation is characterized by the formation of solid tube first, and then the subsequent hollowing out of the tube. The folding of primary neurulation eventually results in the formation of three distinct cell layers: epidermis, neural crest cells, and the neural tube. The epidermis gives rise to the integumentary system and various other external structures. The neural crest cells give rise to craniofacial cartilage and bone, as well as smooth muscle and melanocytes. The neural tube gives rise to the spinal cord and brain (Gilbert, 2010).

Brain development begins with the formation of three distinct regions: prosencephalon, mesencephalon, and rhombencephalon. The prosencephalon then gives rise to the telencephalon and diencephalon, while the rhombencephalon gives rise to the metencephalon and myelencephalon, and the mesencephalon gives rise to itself. The brain regions form by the movement of cells that cause bulges to form and give rise to the brain regions. These various brain regions form the adult structures such as the cerebrum, thalamus, hypothalamus, cerebellum, midbrain, and others. Spinal cord development is the result primarily of primary neurulation, with only the most caudal portions of the spinal cord being formed by secondary neurulation. The secondary neurulation proceeds with the formation of a medullary cord that is formed from mesenchymal cells. This solid tube begins to show cavitation and eventually results in the hollow neural tube being formed (Gilbert, 2010).

__Teratogenic Effects of Caffeine__
Caffeine is considered teratogenic as it can be taken into the embryo by the surrounding environment of the maternal system through the blood vessel network. The caffeine molecule is now known to accumulate in the brain and as recent studies show that 70-95% of pregnant women now drink at least two cups of coffee a day, more studies are investigating the negative developmental mechanisms caffeine induces in the fetus. The process of neurodevelopment occurs early in development (within the first 72 hours) and because of the ability of caffeine to be absorbed by the fetus, caffeine is suggested to be consumed in limited amounts in expecting mothers. The exact effects of caffeine on neurodevelopment have been studied on chick embryos, showing the inhibition of neural tube closure and neural crest cell migration differences when exposed to high amounts of caffeine (Ma et al., 2012). Accumulation of caffeine in the brain results from caffeine accumulating over time, as shown by an increase in brain mass when exposed to caffeine but a decrease in the metabolites that are known to metabolize caffeine. Caffeine metabolites are naturally decreased in the pregnant mother by natural biological mechanisms, though it is unsure why this phenomenon occurs. The inability to metabolize caffeine results in caffeine buildup over time and this also relates to disruption of neurotransmitter signals which may affect development through organogenesis and growth in the third trimester of humans (Li et al., 2012). The overall ideas found in these experiments are shown in the figure below:



The chicks were administered various levels of caffeine //in vitro// and the effects were measured and analyzed. Of the caffeine-administered chicks, failure to close the caudal region of the neural tube occured whereas the control did show complete caudal neural tube closure (Li et al., 2012). The highest caffeine amounts that were shown in the experimental groups received 1.5 mg/mL and 1.0 mg/mL of caffeine respectively, and the third group receiving "low" caffeine amounts was administered 0.5 mg/mL. The highest concentration group exhibited a flattened neural plate at the point when neural tube bending and shaping around the medial hinge point and dorsolateral hinge points should have occurred. The group receiving low caffeine showed some signs of neural tube formation in the stage of bending the neural plate around the medial hinge point. In all the chicks that were experimentally effected, not only did neural tube defects occur, but the prosencephalon’s mesenchymal cell layer was also noticeably thicker, though the mechanism of the prosencephalon wall thickening is unknown. Overall, slower development in the anterior neural tube and failure of closure in the caudal regions of neural tube occurred when exposed to caffeine (up to 1.5 mg/mL). High amounts (>2.0 mg/mL) of caffeine administered to chicks resulted in death of the embryo. Neural crest cell migration was also affected, as HNK1 expressing cells showed erratic migratory trajectories, rendering this transcription factor a key player in the altered neural crest cell migration. Furthermore, when neuronal cells were removed from the neural tube and treated individually with caffeine //in vitro// for 24 hours there was a decrease in the number of neurofilament positive cells as well as decreases in neuronal length, showing that caffeine inhibits neuronal lengthening and neurofilament production (Ma et al., 2012).

In addition to failure of neural tube closure and altered migration, another more recent study found that though increasing caffeine initially increased brain mass, after nine days of exposure to high dosage but not fatal doses of caffeine (10 micromolar/egg) the embryos showed decreased brain mass. The results also indicated a dose-dependent increase in embryo abnormality. Caffeine accumulates in the brain of the embryo and as development continues, the embryo accumulates more and more caffeine which results in more severe abnormalities in the brain and in neurotransmitter function. Accumulations of caffeine initially increase brain mass but then the brain slows maturation, specifically in the ability to signal for organogenesis which results in a lower brain mass after nine days in chicken embryos. The caffeine accumulation disrupts the maturational signals sent to the embryonic organs and tissues, preventing proper development of later stage tissue growth and organogenesis. Caffeine may interrupt these signals by preventing serotonergic neurotransmitters from fully maturing and being fully functional. Without these transmitters maturing, they improperly function and are related to behavioral problems and diseases that carry through adulthood, such as ADHD, as a direct result of faulty serotonin neurotransmission (Li et al., 2012).

__Teratogenic Effects of Mercury__
Mercury can exist as a metallic element, inorganic salt, and organic compound. The form that is most associated with teratogenicity is the organic form. More specifically, methylmercury is the organic form of mercury that results in teratogenic effects when exposed to developing embryos. Methylmercury synthesis occurs when bacteria interact with the metallic mercury present in the sediments found in bodies of water. The methylmercury then accumulates to very high concentrations within the tissues of the aquatic animals, like fish, which are then consumed by humans. Because methylmercury is capable of easily crossing the placenta, the major route of embryo exposure to methylmercury is through consumption of these contaminated animals by the mother (Trasande, Landrigan, & Schechter, 2005).

Methylmercury is classified as a neurotoxicant due to resulting damage of the central nervous system and is one of the six most dangerous chemicals in the world’s environment, according to the International Program of Chemical Safety. Several studies have shown evidence that the effects of fetal exposure to methylmercury are much more severe than childhood or adult exposure. Furthermore, when a fetus shows adverse effects to mercury exposure, the mother very rarely shows any sign of being affected. The brains of infants and animals exposed to methylmercury //in utero// exhibit many differences when compared to brains that have developed under normal conditions. Neuronal migration and distribution patterns appear to be significantly affected when exposed to methylmercury //in utero//. Additionally, cell loss, reduced brain size, and brain scarring can be observed. Several hypotheses attempt to explain the differences in the appearance of the brain and include intracellular cytoskeletal structure rearrangements, increased reactive oxygen species (ROS), alterations of membrane function and signal transduction, decreased production of proteins, and shifts in neurotransmission (Gilbert & Grant-Webster, 1995). The images below display a couple of these hypotheses. Figure 1 looks at how methylmercury exposure can disrupt cytoskeletal structures within neurons, which in turn affects signal transduction and neurotransmission. Figure 2 shows a study conducted by Ni et al. (2010) in which the relationship between methylmercury exposure and ROS production was examined in the living microglial cells of rats.





Although the exact pathway(s) being disrupted due to methylmercury exposure has not been pinpointed, research indicates that exposure to even small concentrations can result in severe neurological defects. High incidence of cerebral palsy, seizures, blindness, abnormal reflexes, deafness, failure to walk, talk, or stand, and even death results from exposure to methylmercury //in utero// (Gilbert and Grant-Webster, 1995). There is no known safe level of exposure to methylmercury and exposure can cause adverse effects anytime during development. (Bose-O’Reilly, McCarty, Steckling, & Lettmeier, 2010).

**__Treatment Options & Ways to Avoid Teratogenicity__**
The obvious way to avoid the various adverse effects caused by exposure to caffeine and mercury in developing fetuses is for the mother to avoid consuming foods and drinks containing these compounds. By limiting the amount of caffeine-containing products, such as coffee, tea, chocolate, or medications, and mercury-containing products, such as water-dwelling animals that may be contaminated with mercury, during the time of potential pregnancy or when trying to become pregnant, women can significantly decrease the possibility of neuronal defects in their baby. However, this can be challenging due to the fact that many women are not aware of this information prior to becoming pregnant. Also, women don't always know that they will become pregnant, because many women do not plan their pregnancy. This results in increased risk of fetuses being harmed by the teratogenic effects of caffeine and mercury.

The medical community has no defined time span that indicates caffeine intake is not acceptable during pregnancy. However, various studies do show that large amounts of caffeine can be detrimental to the fetus and potentially have adverse effects after birth. Infante-Rivard et al. (1993) found that pregnant women that increased their caffeine intake had a much greater risk of losing the fetus. For every 100mg consumed beyond the average, the risk increased by a factor of 1.22. Other studies have shown that prolonged effects can be seen after the birth of the child. Some effects that have been associated with the caffeine consumption by the mother during pregnancy are spina bifida, attention deficit disorders, low birth weight, smaller head circumference, and others. These conditions can be treated with various medications depending on the negative effects they have on the fetus.

Mercury has been shown to cause multiple adverse effects in the fetuses of pregnant women. Effects that have been linked to mercury poisoning include autism and Alzheimer's disease, among other brain malfunctions. There are many substances that should be avoided to lower the chances of ingesting mercury during pregnancy. Keeping the intake amount of tuna and other fish to a minimum, avoiding getting amalgam tooth fillings, and not receiving a flu vaccination while pregnant are advised to help ensure safety of the fetus. The time frame to avoid mercury is throughout the entire duration of pregnancy. Studies have also indicated that the pregnant woman could potentially be harmed from the mercury poisoning as well as a newborn infant (Koos & Longo, 1976).

Overall, caffeine is safe to consume at low to moderate amounts during pregnancy where as mercury is not safe to comsume at all in pregnany. Major issues with both of these teratogens are that the effects cannot be reversed nor can they always be detected before the damage has been incurred. Therefore, treatments are difficult to acquire and the best way to avoid teratogenicity is to avoid substances containing caffeine or mercury and monitor the intake accordingly.

Resources:

Bose-O'Reilly, S., McCarty, K. M., Steckling, N., & Lettmeier, B. (2010). Mercury exposure and children's health. //Current Problems in Pediatric and Adolescent// //Health Care,// 40(8), 186-215.

Gilbert, Scott F. (2010). //Developmental Biology// (9th ed.). Sunderland, MA: Sinauer Associates, Inc.

Gilbert, S. G. & Grant-Webster, K .S. (1995). Methylmercury exposure. //Environmental Health Perspectives//, 103(6), 135-142.

Infante-Rivard, C., Fernandez, A., Gauthier, R., David, M., & Rivard, G. (1993). Fetal loss associated with caffeine intake before and during pregnancy. //The Journal of// //American Medical Association, 270(24),// 2940-2943.

Koos, B. & Longo, L., (1976). Mercury toxicity in the pregnant woman, fetus, and newborn infant. //American Journal of Obstetrics and Gynecology, 126(3),// 390-409.

Li, X. D., He, R. R., Qin, Y., Tsoi, B., Li, Y. F., Ma, Z. L.,. . . Kurihara, H. (2012). Caffeine interferes embryonic development through over-stimulating serotonergic system in chicken embryo. // Food //// and Chemical Toxicology, // 6517, 1-6.

Ma, Z. L., Qin, Y., Wang, G., Li, X. D., He, R. R., Chuai, M.,. . . Yang, X. (2012). Exploring the caffeine-induced teratogenicity on neurodevelopment using early chick embryo. // PLoS ONE, // 7(3):e34278, 1-8.

Moms Clean Air Force. How mercury poisoning works. Retrieved from []

Ni, M., Li, X., Yin, Z., Jiang, H., Sidoryk-Wegrzynowicz, M., Milatovic, D.,. . . Aschner, M. (2010). Methylmercury induces acute oxidative stress, altering nrf2 protein level in primary microglial cells. //Toxicological Sciences//, 116(2), 590-603.

Trasande, L., Landrigan, P. J., & Schechter, C. (2005). Public health and economic consequences of methyl mercury toxicity to the developing brain. //Environmental// //Health// // Perspectives, // 113(5), 590-596.