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Welcome to The Visible Embryo, a comprehensive educational resource on human development from conception to birth.

The Visible Embryo provides visual references for changes in fetal development throughout pregnancy and can be navigated via fetal development or maternal changes.

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Today, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than 1 million visitors each month. The field of early embryology has grown to include the identification of the stem cell as not only critical to organogenesis in the embryo, but equally critical to organ function and repair in the adult human. The identification and understanding of genetic malfunction, inflammatory responses, and the progression in chronic disease, begins with a grounding in primary cellular and systemic functions manifested in the study of the early embryo.

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Pregnancy Timeline by SemestersFetal liver is producing blood cellsHead may position into pelvisBrain convolutions beginFull TermWhite fat begins to be madeWhite fat begins to be madeHead may position into pelvisImmune system beginningImmune system beginningPeriod of rapid brain growthBrain convolutions beginLungs begin to produce surfactantSensory brain waves begin to activateSensory brain waves begin to activateInner Ear Bones HardenBone marrow starts making blood cellsBone marrow starts making blood cellsBrown fat surrounds lymphatic systemFetal sexual organs visibleFinger and toe prints appearFinger and toe prints appearHeartbeat can be detectedHeartbeat can be detectedBasic Brain Structure in PlaceThe Appearance of SomitesFirst Detectable Brain WavesA Four Chambered HeartBeginning Cerebral HemispheresFemale Reproductive SystemEnd of Embryonic PeriodEnd of Embryonic PeriodFirst Thin Layer of Skin AppearsThird TrimesterSecond TrimesterFirst TrimesterFertilizationDevelopmental Timeline
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development
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Home | Pregnancy Timeline | News Alerts |News Archive Oct 21, 2014

 

 







CDC Growth Standards 0 to 2 Years of Age

 

 

 

PCBs can change thyroid hormone during pregnancy

Flame retardant chemicals called polychlorinated biphenyls (PCBs) can infiltrate the placenta during pregnancy and negatively affect development of the fetal brain.

A study of human placentas provides the strongest evidence to date that Endocrine Disrupting Chemicals (EDCs) can interfere with the thyroid in pregnant women.

The implication is that flame retardant chemicals called polychlorinated biphenyls (PCBs) can infiltrate the placenta during pregnancy and affect thyroid hormone activity at the cellular level, according to a new study published in the Endocrine Society's Journal of Clinical Endocrinology & Metabolism (JCEM).


PCBs were used in transformers and other electrical equipment, paints, adhesives and tape.

Although the endocrine-disrupting chemicals were banned in the United States in 1979, PCBs still are released into the environment from disposal sites or products manufactured prior to the ban.

Most people have been exposed to low levels of PCBs.


"As endocrine-disrupting chemicals, PCBs interfere with the way the thyroid hormone functions, but they don't actually change the amount of the hormone found in the body," said one of the study's authors, R. Thomas Zoeller, PhD, of the University of Massachusetts in Amherst, MA. "Although these effects are largely invisible in scientific studies that only judge thyroid activity by measuring hormone levels, they may be having a real impact on infants' brain development."

The birth cohort study examined the effects of low-dose chemical exposure in 164 pregnant women. Tissue from the placenta – the structure that develops in the uterus to provide oxygen and nutrients to the fetus during pregnancy – was analyzed for the enzyme CYP1A1.

This enzyme changes endocrine-disrupting chemicals into a form that can interfere directly with the body's thyroid hormone receptors.

Researchers found that in pregnancies where the placenta contained higher amounts of the enzyme, the tissue also exhibited signs of thyroid disruption. Levels of two thyroid-regulated genes tended to be higher in these pregnancies, although the mother's overall thyroid hormone levels did not change.

Zoeller: "Whatever is happening in the placenta likely reflects what is happening in the fetus. To truly understand how endocrine-disrupting chemicals may be affecting pregnancies, the findings show we need to study not only hormone levels, but hormone activity at the cell level."


The effects of endocrine-disrupting chemicals may be particularly insidious in people who smoke, adds Zoeller.

The enzyme CYP1A1 is supposed to clean the blood, and the body produces more of this enzyme when it is exposed to cigarette smoke.

Researchers found pregnant women who smoke tend to have higher levels of the enzyme in their placental tissue.


Other authors of the study include: Thomas Luke Wadzinski, Katherine Geromini, Judy McKinley Brewer and Ruby Bansal of the University of Massachusetts; and Nadia Abdelouahab, Marie-France Langlois and Larissa Takser of the University of Sherbrooke in Sherbrooke, Quebec.

The study, "Endocrine Disruption in Human Placenta: Expression of the Dioxin-Inducible Enzyme, Cyp1a1, is Correlated with that of Thyroid Hormone Regulated Genes," was published online, ahead of print.

Summary
Thyroid hormone (TH) is essential for normal development; therefore, disruption of TH action by a number of industrial chemicals is critical to identify. Several chemicals including polychlorinated biphenyls are metabolized by the dioxin-inducible enzyme CYP1A1; some of their metabolites can interact with the TH receptor. In animals, this mechanism is reflected by a strong correlation between the expression of CYP1A1 mRNA and TH-regulated mRNAs. If this mechanism occurs in humans, we expect that CYP1A1 expression will be positively correlated with the expression of genes regulated by TH. Objective: The objective of the study was to test the hypothesis that CYP1A1 mRNA expression is correlated with TH-regulated mRNAs in human placenta. Methods: One hundred sixty-four placental samples from pregnancies with no thyroid disease were obtained from the GESTE study (Sherbrooke, Qéubec, Canada). Maternal and cord blood TH levels were measured at birth. The mRNA levels of CYP1A1 and placental TH receptor targets [placental lactogen (PL) and GH-V] were quantitated by quantitative PCR. Results: CYP1A1 mRNA abundance varied 5-fold across 132 placental samples that had detectable CYP1A1 mRNA. CYP1A1 mRNA was positively correlated with PL (r = 0.64; P < .0001) and GH-V (P < .0001, r = 0.62) mRNA. PL and GH-V mRNA were correlated with each other (r = 0.95; P < .0001), suggesting a common activator. The mRNAs not regulated by TH were not correlated with CYP1A1 expression. Conclusions: CYP1A1 mRNA expression is strongly associated with the expression of TH-regulated target gene mRNAs in human placenta, consistent with the endocrine-disrupting action of metabolites produced by CYP1A1.

Founded in 1916, the Endocrine Society is the world's oldest, largest and most active organization devoted to research on hormones and the clinical practice of endocrinology. Today, the Endocrine Society's membership consists of over 17,000 scientists, physicians, educators, nurses and students in more than 100 countries. Society members represent all basic, applied and clinical interests in endocrinology. The Endocrine Society is based in Washington, DC. To learn more about the Society and the field of endocrinology, visit our site at http://www.endocrine.org. Follow us on Twitter at https://twitter.com/#!/EndoMedia.


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