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Human Prenatal Development

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by

Jennifer Mayr

on 16 February 2016

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Transcript of Human Prenatal Development

February 2016
Human Prenatal Development
Fertilization to Implantation
1st Trimester
3 weeks 2-chambered heart begins beating
4 weeks arm and leg buds develop, early eyes form
7 weeks heart has 4 chambers, fingers/toes free
9 weeks sensitive to touch
fetus weighs about 1 ounce at end of first trimester
2nd Trimester
10 weeks finger and toenails begin to grow
11 weeks sex organs differentiate
12 weeks palate fuses, bowel movements occur
15-20 weeks mother feels fetus move
20 weeks response to sounds
21-22 weeks if born, survival is possible
25 weeks brain function detectable
weighs 1-1.5 pounds at end of second trimester
3rd Trimester
32 weeks alveoli form in lungs
36 weeks surfactant production in lungs accelerates
subcutaneous fat stored
brain weight doubles in the last 9 weeks of pregnancy
fetus typically doubles in overall weight during the last 11 weeks of pregnancy
Birth and the "4th Trimester"
baby born with large brain, can't stay in mother
thermoregulation, feeding, breathing take energy
nervous system still developing
first 3 months of baby's life not interactive, colicky, needs swaddling, suckling, shushing, rocking
Spontaneous Abortion
estimated that up to half of all fertilized eggs die and are lost (aborted) spontaneously (usually before the woman knows she is pregnant)
among those women who know they are pregnant, miscarriage rate is about 15-20%
most miscarriages occur during the first 7 weeks of pregnancy
causes include chromosomal abnormalities in the fetus, drug/alcohol use, infection, environmental toxins, obesity, smoking, and problems with mother's immune or reproductive system
father's DNA enters oocyte
start
oocyte finishes meiotic division; pronucleus forms around mother's DNA; separate pronucleus forms around father's DNA
2 hr.
membranes around pronuclei dissolve = zygote (0.1 mm)
12-18 hr.
first cell division
22-26 hr.
0.1 mm
40 d 50 d
Félix, born at 24 weeks
photo by Red Méthot
April 23, 2015
The experiment with human embryos was
dreaded
, yet widely anticipated. Scientists somewhere, researchers said, were trying to edit genes with a technique that would permanently alter the DNA of every cell so any changes would be passed on from generation to generation.
Those
concerns
drove leading researchers to issue
urgent
calls in major scientific journals last month to
halt
such work on human embryos, at least until it could be proved safe and until society decided if it was
ethical
.

Now, scientists in China report that they tried it.

The experiment
failed
, in precisely the ways that had been
feared
.

The Chinese researchers did not plan to produce a baby — they used defective human embryos — but did hope to end up with an embryo with a precisely altered gene in every cell but no other
inadvertent DNA damage
. None of the 85 human embryos they injected fulfilled those criteria. In almost every case, either the embryo died or the gene was not altered. Even the four embryos in which the targeted gene was edited had
problems
. Some of the embryo cells overrode the editing, resulting in embryos that were genetic mosaics. And speckled over their DNA was a sort of
collateral damage
— DNA mutations caused by the editing attempt.

February 1, 2016
In April 2015, in the first known use of Crispr on human embryos, researchers led by Junjiu Huang of Sun Yat-sen University in China tried to
correct
a defective gene that causes a blood disorder known as beta thalassemia. Though the experiment was
ethically defensible
— all the embryos were unviable because of a fatal defect — it also demonstrated the possible dangers of the technique because of the many things that went wrong. It was this experiment that gave urgency to the steps leading to the three academies’ call for a worldwide moratorium on modifying the human germ line.
Dr. Niakan, a developmental biologist, has no intention of implanting the altered embryos in a womb. According to a report in Nature, she will let the embryos expire when they are seven days old and have reached the blastocyst, or implantation, stage. The usual source of such embryos is fertility clinics that have generated more than their clients need.

Dr. Niakan’s goal is to
understand
the cascade of genetic switches that are thrown as the fertilized egg progresses through its first few divisions. Her experiment will lead to no specific medical treatment, only to better knowledge of the basic biology of development. This may prove
useful
in treating certain cases of infertility, given that many fertilized eggs fail before they reach the blastocyst stage.
Preimplantation Genetic Diagnosis (8 cells,
in vitro
)
15-20 weeks
10-13 weeks
Full transcript