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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

Developmental biology - Cell Differentiation

Embryos Remember Chemicals They Encounter

How cells become individually identified in development...

We all start out as a clump of identical cells. As these cells divide and multiply, then gradually take on distinct identities as they acquire traits needed to form into muscle, bone, or nerves for example. A recent study from Rockefeller University scientists offers new insight into how these cellular identities are cultivated during embryonic development.

According to the study published in eLife, cells retain memory of the chemical signals to which they are exposed. Researchers reveal how embryos that fail to form these contact "memories" remain a clump of clones, never realizing any unique biologic potential.

Activating embryos

Over 25 years ago, Ali H. Brivanlou demonstrated in Developmental Biology how the protein Activin promotes embryonic frog cells to initiate traits specific to certain tissue types, in a process called differentiation. Since that observation, for decades Activin has been thought to instigate the transition from homogenous clump to specialized cells.
"Activin was the textbook definition of a molecule that is necessary and sufficient for differentiation. Researchers had shown that the dose of the protein determines cellular fate. At a very high dose, for example, you get gut and muscle; and at a very low dose, you get nerve tissue."

Ali H. Brivanlou PhD, Laboratory of Stem Cell Biology and Molecular Embryology; the Robert and Harriet Heilbrunn Professor, Rockefeller University, New York, USA.

Despite ample research from animal studies, questions remained about how Activin guides development in human cells. Working with Brivanlou and Eric D. Siggia PhD, graduate fellow Anna Yoney investigated whether "Activin" protein triggers differentiation in laboratory-generated embryos developed from human stem cells.

These embryos mimic the behavior of human cells during very early stages (13 days) of development. Researchers expected these synthetic embryos to respond just like Brivanlou's frogs. Yet, after applying Activin to these cells, nothing happened. "Anna put Activin on the embryos and we waited - and waited and waited. And absolutely nothing happened! That was shocking," says Brivanlou.

Memorable molecules

Undeterred, Yoney considered possible explanations for her results. "I thought, Ok, we don't get a response from Activin alone," she recalls. "What additional signals might we need to see differentiation?"

She ultimately homed in on WNT, a molecule known to regulate the movement of cells during development. In her next experiment, she exposed the cells to WNT before adding Activin; and this time, they differentiated in the normal manner initiating gastrulation. Now bilateral symmetry of the embryo becomes distinguished by the Primitive Streak which appears to separate the embryonic disk "in half." Cells now migrate into the streak and continue to differentiate into the endoderm, mesoderm and ectoderm layers and those bodily systems within each layer.
"The cells that "saw" WNT reacted to Activin with the full range of response - just as we find in the frog and other animals. But cells that hadn't "seen" WNT were totally unresponsive, as if Activin wasn't even there."

Ali H. Brivanlou PhD.

Researchers saw that differentiation requires both WNT and Activin signaling. Crucially, they also found cells needn't be exposed to both chemicals simultaneously.
"We blocked WNT signaling during the Activin treatment phase and found that the cells still differentiated. So we concluded that the cells actually remembered that they had previously been exposed to WNT."

Anna Yoney, Laboratory of Stem Cell Biology and Molecular Embryology, and Center for Studies in Physics and Biology, The Rockefeller University, New York, USA.

Researchers call this phenomenon "signaling memory" with WNT appearing to permanently affect cell behavior. Earlier research has failed to uncover evidence of embryonic cellular memory, says Brivanlou.

Yoney adds: "In model animal systems, cells encounter a series of signals before scientists manipulate them. But Anna's artificial embryos came from stem cells that hadn't been exposed to any signals - and this makes them perfect tools for discovering the individual roles of signals. As beautiful as the model systems are, sometimes they can lead you to miss things."

The researchers hope to further explore how and where cellular memories are stored. Yoney suspects signals are recorded in each cell nuclei as this is where modifications to the epigenome - a multitude of chemical compounds - tells DNA in genes when to function and for how long. Additional research in this area could have major implications for understanding human and other species development.

Self-organization of discrete fates in human gastruloids is mediated by a hierarchy of signaling pathways. How these pathways are integrated in time, and whether cells maintain a memory of their signaling history remains obscure. Here, we dissect the temporal integration of two key pathways, WNT and ACTIVIN, which along with BMP control gastrulation. CRISPR/Cas9-engineered live reporters of SMAD1, 2 and 4 demonstrate that in contrast to the stable signaling by SMAD1, signaling and transcriptional response by SMAD2 is transient, and while necessary for pluripotency, it is insufficient for differentiation. Pre-exposure to WNT, however, endows cells with the competence to respond to graded levels of ACTIVIN, which induces differentiation without changing SMAD2 dynamics. This cellular memory of WNT signaling is necessary for ACTIVIN morphogen activity. A re-evaluation of the evidence gathered over decades in model systems, re-enforces our conclusions and points to an evolutionarily conserved mechanism.

Anna Yoney, Fred Etoc, Albert Ruzo, Thomas Carroll, Jakob J Metzger, Iain Martyn, Shu Li, Christoph Kirst, Eric D Siggia and Ali H Brivanlou.

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Nov 14, 2018   Fetal Timeline   Maternal Timeline   News   News Archive

Special imaging techniques allow researchers to visualize the location of every nucleus in a lab-generated embryo (RED - left). When exposed to Activin alone, cells along the embryo's border (GREEN - right) react briefly, pulling away from the center - but did not fully differentiate. Credit: Laboratory of Stem Cell Biology and Molecular Embryology at The Rockefeller University

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