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Pregnancy Timeline by SemestersDevelopmental TimelineFertilizationFirst TrimesterSecond TrimesterThird TrimesterFirst Thin Layer of Skin AppearsEnd of Embryonic PeriodEnd of Embryonic PeriodFemale Reproductive SystemBeginning Cerebral HemispheresA Four Chambered HeartFirst Detectable Brain WavesThe Appearance of SomitesBasic Brain Structure in PlaceHeartbeat can be detectedHeartbeat can be detectedFinger and toe prints appearFinger and toe prints appearFetal sexual organs visibleBrown fat surrounds lymphatic systemBone marrow starts making blood cellsBone marrow starts making blood cellsInner Ear Bones HardenSensory brain waves begin to activateSensory brain waves begin to activateFetal liver is producing blood cellsBrain convolutions beginBrain convolutions beginImmune system beginningWhite fat begins to be madeHead may position into pelvisWhite fat begins to be madePeriod of rapid brain growthFull TermHead may position into pelvisImmune system beginningLungs begin to produce surfactant
CLICK ON weeks 0 - 40 and follow along every 2 weeks of fetal development


The force is strong — with embryo cells

The strength of inner contractions determines whether a cell becomes part of the embryo or part of the placenta. At the cell level, this is determined by the staggered amount of proteins produced which drives a cell to contract or stop contracting.

For a cell in an embryo, the secret to becoming part of the baby's body instead of the placenta is to contract more and keep on 'dancing,' according to scientists at the European Molecular Biology Laboratory (EMBL). The study, published in Nature, could one day have implications for assisted reproduction.

After a sperm cell fertilises an egg cell, that fertilised egg divides repeatedly, forming into a ball of cells — the morula. Shortly before the embryo implants in the womb, some of those cells moves into the center of the ball. These are the cells that will develop into the baby's body. Cells remaining on the surface become the placenta, connecting the embryo to the mother's uterus.

EMBL group leader Takashi Hiiragi and Jean-Leon Maître, then a postdoc in Hiiragi's lab, found that, in a mouse embryo, whether a cell moves to the middle or stays on the surface depends on how strongly it can contract. By combining computer modelling and experiments in the lab, the scientists determined that cells that contract at least one-and-a-half times more strongly than their neighbours — move inwards.

But why do some cells contract more than others? The answer, EMBL scientists found, is they receive unequal amounts of apical proteins — proteins which form at the top of the cell membrane. These molecules can stop the cell from contracting. Cells that inherit small amounts of apical proteins contract more, and move inwards and give rise to the embryo. By contrast, cells with large amounts of apical proteins contract less and stay on the surface, becoming the placenta. But when the EMBL scientists prevented all cells from contracting, they were surprised at the result.

"We found that if you interfere with contractility, the cells stay at the surface. But instead of becoming placenta cells, as you'd expect, they become embryonic," explains Jean-Leon Maître PhD, formerly a student assistant to Hiiragi and now beginning his own laboratory at the Institut Curie in Paris, France. "This tells us that sensing the forces around them may be important for cells to know where they are, and what to become."

Last year, Hiiragi and Maître discovered that in the first few days of an embryo's life, all of its cells perform a little 'dance', as a wave of contractions sweep around their surfaces. Now scientists saw that in an embryo 3 to 4 days old, the cells becoming placenta no longer dance.

Although the study looked at mouse embryos, the first stages of development of both species are so similar, researchers expect their findings to be true for both. If so, one day researchers and clinicians doing pre-implantation diagnostic tests on in vitro fertilised (IVF) embryos would know exactly which cells to test based on their "dance."

Screening IVF embryos for genetic disorders before implanting them in the mother involves taking a [single] cell from an embryo for analysis of its DNA. Knowing cells which will become the placenta don't dance - may help clinicians choose which embryonic cell to test.

For their part, Hiiragi and Maître will continue to investigate the fundamentals of how embryos take shape, and how differences between cells arise.

"We now understand a lot more about the mechanical aspect of how cells move inwards and become the embryo, but it's also raised a lot of questions about how that is translated into gene expression," says Hiiragi. "We're also looking at other factors that could be involved, like how cells communicate, and sense contacts with each other."

During pre-implantation development, the mammalian embryo self-organizes into the blastocyst, which consists of an epithelial layer encapsulating the inner-cell mass (ICM) giving rise to all embryonic tissues1. In mice, oriented cell division, apicobasal polarity and actomyosin contractility are thought to contribute to the formation of the ICM2, 3, 4, 5. However, how these processes work together remains unclear. Here we show that asymmetric segregation of the apical domain generates blastomeres with different contractilities, which triggers their sorting into inner and outer positions. Three-dimensional physical modelling of embryo morphogenesis reveals that cells internalize only when differences in surface contractility exceed a predictable threshold. We validate this prediction using biophysical measurements, and successfully redirect cell sorting within the developing blastocyst using maternal myosin (Myh9)-knockout chimaeric embryos. Finally, we find that loss of contractility causes blastomeres to show ICM-like markers, regardless of their position. In particular, contractility controls Yap subcellular localization6, raising the possibility that mechanosensing occurs during blastocyst lineage specification. We conclude that contractility couples the positioning and fate specification of blastomeres. We propose that this ensures the robust self-organization of blastomeres into the blastocyst, which confers remarkable regulative capacities to mammalian embryos.
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Aug 5, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

Embryonic cell movement;EMBL

Watch as the PINK cell wiggles into the center of this blastula to become part of the fetus.
Image Credit:



Phospholid by Wikipedia Embryonic cell movement