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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.

The National Institutes of Child Health and Human Development awarded Phase I and Phase II Small Business Innovative Research Grants to develop The Visible Embryo. Initally designed to evaluate the internet as a teaching tool for first year medical students, The Visible Embryo is linked to over 600 educational institutions and is viewed by more than one million visitors each month.

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 14, 2013


Thomas Sudhof won a Nobel prize for his work in understanding how nerve cells communicate. They use junctions known as synapses to transmit chemical messengers to each other.

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The science behind Thomas Südhof's Nobel prize

Thomas Sudhof won his Nobel prize for his work in nerve cell communication — the firing patterns of our synapses that underwrite our consciousness, emotions and behavior.

Sudhof studied for almost 30 years how our brain transmits the simple act of taking a step forward or tasting a doughnut into millions of simultaneous and precise synaptic firing events throughout the brain and peripheral nervous system.

There are an estimated 200 billion neurons in a healthy adult brain. Any single neuron may share synaptic contacts with as few as one, or as many as 1 million, other neurons (the median is somewhere in the vicinity of 10,000), meaning that that the human brain likely holds 2 quadrillion synapses.

Much of a neuron can be visualized as a long, hollow cord whose outer surface conducts electrical impulses in one direction.

At various points along this cordlike extension are bulbous nozzles known as presynaptic terminals. Each houses a myriad of tiny, balloon-like vesicles of neurotransmitters. Each of these abutt a downstream (postsynaptic) neuron.

When an electrical impulse traveling along a neuron reaches a presynaptic terminal, calcium from outside the neuron floods in through channels that open temporarily in the terminal. A portion of the neurotransmitter-containing vesicles fuse with the terminal's outer membrane and spill their contents into the narrow gap separating it from the postsynaptic neuron's receiving end.

Südhof purified key protein constituents he found sticking to the surfaces of neurotransmitter-containing vesicles, protruding from nearby presynaptic-terminal membranes, or bridging them. Then, he explained how orchestrated interactions among these proteins contribute to membrane fusion. As a result, synaptic transmission is now one of the best-understood phenomena in neuroscience.

The proteins Südhof has focused on for the past three decades are disciplined in their recruitment of vesicles, bringing them into "docked" positions near the presynaptic terminal, herding calcium channels to the terminal membrane and—cued by calcium—interweave like two sides of a zipper, forcing the vesicles into such close contact with the terminal membrane that they fuse, releasing neurotransmitters into the synaptic gap.

Although these specialist cells perform defined roles at the synapses, similar proteins, discovered later by Südhof and others, play similar roles ranging from hormone secretion; fertilization of an egg during conception; to immune cells' defense against foreign invaders.

Südhof started focusing on the workings of the synapse in the early 1980s. It was then known that presynaptic terminals are filled with tens to hundreds of small synaptic vesicles, and that the release of these vesicles' contents were triggered by calcium. But nobody had any idea how any of it actually worked. Südhof helped solve the riddle.

More recently, Thomas Südhof’s work has focused on aspects of synaptogenesis and maintenance of the synaptic connection. Südhof discovered neurexins, present on presynaptic neurons, and neuroligins, present on postsynaptic neurons. Neurexins and neuroligins come together to form a physical protein bridge across the synapse.

The diversity in types of neurexins and neuroligins allows for a variety of unique binding opportunities between neurons and impart a specificity to synaptic connections. In additional studies, Südhof identified mutations in these proteins as a factor in inherited autism.

Original press releas: http://med.stanford.edu/ism/2013/october/nobel-explainer-1007.html