Download episode 3.1 - Amy Himsel, Ph.D.

P16 Podcast Transcript, Amy Himsel, Ph.D.
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whether they shared the same placenta or not. Not all identical twins share the same placenta.
Also, they may have had different experiences outside of the womb that could interact with those
genes in different ways.
Think about how multifactorial interaction could affect the development of biological siblings. If
theyʼre not monozygotic twins, then each child is genetically unique, right? And letʼs say that
each child was raised by the same two parents. The type of parenting each child receives is not
just a matter of the parents though; itʼs also a matter of the child. Different children require
different approaches, in large part due to each childʼs genetic predispositions. A difficult child
with colic elicits a difference response than a calm baby who sleeps through the night at 2 months
of age. An outgoing, boisterous, and daring toddler triggers a very different response from
parents than a quiet and reserved toddler.
Itʼs my hope the next time you are having a conversation with someone about whether genes or
environment makes a person have a certain characteristic, you will say, “Itʼs always both!”
3.1-– PRENATAL DEVELOPMENT
EPISODE 2.2
We never see more rapid and fascinating development than what occurs during the 38 weeks of
pregnancy after sperm meets egg.
From Zygote to Newborn
It takes 38 weeks to make a baby, from zygote to newborn. Sometimes youʼll hear people talk
about a 40-week pregnancy. Whatʼs the difference? It depends on when you starting counting.
If you start with the date of the womanʼs last period, itʼs a 40-week pregnancy; if you start with the
date of conception, itʼs a 38-week pregnancy. Sometimes doctors prefer to use the 40-week
schedule because they feel that day of the womanʼs last period is more easily determined than
the exact day conception occurred. It could take several days for sperm to reach the egg, so just
because a couple had sex, it no way indicates that thatʼs the day conception occurred. In fact we
can be pretty confident that is not the day that conception occurred.
We follow the scientistsʼ lead in this class, referring to a 38-week pregnancy that can be broken
down into three distinct periods of development: zygotic, embryonic, and fetal. This isnʼt the
same thing as trimesters, by the way, although itʼs easy to learn how they overlap: All three
prenatal periods – the zygote, the embryo, and the fetus – occur during the first three months, the
first trimester. The last period, the period of the fetus, begins during the first trimester, yes, but
extends from the end of the first trimester all the way through to birth. The nice thing about
prenatal periods, in contrast to trimesters, is that they are divided by the activities going on. The
period of the zygote extends from conception to implantation, the period of the embryo involves
the beginnings of all major bodily systems (organs, circulatory system, etc.), and the period of the
fetus is when all major body systems become fully functional. Letʼs talk more in depth about
each.
Most pregnant women donʼt even know theyʼre pregnant until after the germinal period has
passed. Nausea and other symptoms of pregnancy typically donʼt begin until after the zygote has
become embedded in the uterus lining around 2 weeks after conception (and this is the event that
ends the germinal period). But, rewinding a little bit, after conception and prior to implantation,
the main action of the germinal period is cell division. Lots and lots of cell division. Very early on,
before the zygote reaches the 8-cell stage, the cells could become anything at all. They are
called stem cells. Scientists hope someday to be able to use stem cells to generate cells to cure
many of our most vexing disorders and diseases. After the 8-cell stage, DNA instructions for the
cells kick in, and cells start to differentiate (specialize) and then move where they are needed.
P16 Podcast Transcript, Amy Himsel, Ph.D.
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The embryonic period follows the germinal period. It begins once implantation is completed,
which is around two weeks, and lasts through week 8. Remember, weʼre still in the first trimester
here. Some women will experience “morning sickness” during this time, although itʼs really more
accurate just to call it nausea because many women feel it at different times of the day. Some
women will not even experience much nausea at all, or any. Others may notice their sense of
smell or taste becomes unbearably acute, which, in it itself, can also bring on feelings of nausea!
The embryo is really busy developing the start of all major body systems. The cardiovascular
system begins to function. All organs get their start. Regardless of gender, the embryo at this
point in time has a genital tubercle right now, which is a little flap of tissue between the legs that
will develop into the sexual organs during the fetal period. Throughout pregnancy, development
proceeds in a proximodistal fashion, meaning that the inner parts (organs) are maturing earlier
than the outer parts (fingers).
Moving along onto the fetal period. The fetal period is long. It lasts from 9 weeks to 38 weeks.
Notice again the start of the fetal period is within the first trimester and then it extends throughout
the entire remainder of the pregnancy. The fetal period is a time for all major body systems and
organs to gradually, gradually reach maturity. Fetuses with chromosomes XY develop testes,
which begin to produce testosterone, which turns the genital tubercle into male sexual organs and
influences brain development as well. You know that sense of ʻI am a boyʼ or ʻI am a girl,ʼ that the
vast majority of us just feel when we donʼt have confusion about our gender? That has to do with
the way that hormones influence our brains during prenatal development. A very small portion of
individuals have different experiences where their hormonal production during this time is not
gender typical. That can help you understand why a person can honestly feel like they are a
different gender then their body dictates. But like I said, for the majority of us this all lines up
together. The testosterone influences the development of organs, and also influences brain
development. Without testosterone, female sex organs develop and a female-ish brain develops
as well. By the way, let me just stop here though and clarify that although there are some
differences between the male and female brain, there are fewer differences than you probably
think, Weʼll talk about what things differ and what things donʼt across the semester. The fetus
experiences dramatic growth, from .25 oz (on average) at week 8 to 7.5 lbs (on average) at week
38. Fat layers develop near the very end of the pregnancy as well.
The organ that takes the longest to develop is the brain, as neurons are created, sent to their
designated positions, and begin making connections with other neurons. Brain development
progresses gradually from back to front, so gradually, in fact, that the very front portion of the
cortex (the prefrontal cortex) does not become fully mature until late adolescence or early
adulthood! Prenatal development, in general, is cephalocaudal, meaning that the brain is a
priority. The age of viability, a time when a preterm baby might be able to survive outside of the
womb, is determined largely by brain development. This occurs somewhere between 22-26
weeks after conception, so thatʼs during the fetal period. Thinking about the cephalocaudal
development, the priority of the brain, notice how the proportion of the fetusʼs body is so different
from ours, with a huge head compared to the rest of the body. That huge head is containing that
prioritize brain. Eventually, as the infant develops into childhood, the rest of the body will catch
up to the brain and we will see more adult-like proportions.
The priority of brain development, and our enormous heads that go with it, can make birth much
more difficult for us compared to other species. And, the fact that we walk upright (instead of on
all fours) led us to evolve with more narrow hips than our primate ancestors. A large fetus head
and narrow maternal hips = a challenging birth. Luckily, in the vast majority of cases today, birth
proceeds with a hitch. Although this certainly was not the case in the past when it was much
more common for the mother or the baby or both to die during childbirth. Thankfully we have csections and other medical interventions to help if there are problems during birth. Some, though,
would suggest that the modern hospital birth, with hormones used to hasten contractions, etc.,
P16 Podcast Transcript, Amy Himsel, Ph.D.
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may actually prolong and complicate the birth process and lead to more c-sections than would be
necessary. Itʼs true that c-section births have increased, and thatʼs something to be aware of and
to keep investigating, but itʼs also true that without procedures like c-sections, many lives
wouldʼve been lost.
Complications
One troubling aspect of the birthing experience in the United States is the increasing rates of
babies born weighing less than 5.5 lbs, a marker for low birthweight. A low birthweight baby may
be preterm (born early). Some are not born early, but are born small for their gestational age.
They may be 38 weeks of age, but still less than 5.5 lbs. This is often due to substance abuse
(including smoking) or malnutrition thatʼs occurred during the pregnancy. In general, nurture plays
a stronger role in the low birthweight experience than nature does, especially considering that
rates of low birthweight are decreasing in other developed nations while they increase here in the
U.S. Stress may also impact prenatal development. When a pregnant mother is extremely
stressed out, her bodyʼs resources are diverted to her brain and to her muscles for survival. Itʼs
like the body prioritizes things and says, “We canʼt focus that much on this developing baby in the
uterus; weʼve got to make sure to keep mom alive.” So what does this mean for the motherʼs
body? Well, blood flow to the uterus is decreased when a mother is extremely stressed out and
the fetus, then, is deprived of oxygen and nutrients.
One final reason for an increased rate of low birthweight is infertility treatments that result in
multiple births. A fetus who has had to share the uterus with others is almost always smaller than
one who had the whole place to him- or herself. Sometimes itʼs just a matter of space.
I mentioned how substance abuse can impact prenatal development. Letʼs look more closely at
things like that. Today, the idea that the developing human is affected by whatever the mother
ingests or is exposed to seems obvious. But it wasnʼt always that way. For a long time, scientists
incorrectly believed that the placenta shielded the developing human against harmful substances.
It wasnʼt until 1941 that the first teratogen was identified: rubella (German measles). In the early
1960s, the thalidomide disaster occurred. Thalidomide was a drug prescribed to relieve morning
sickness during pregnancy. Thousands of children across the world were born with limb
deformities due to thalidomide. After this, scientists started looking more closely at other potential
teratogens, such as alcohol. Fetal Alcohol Syndrome was officially recognized in 1973. Even a
small amount of alcohol can take a toll on prenatal development (especially on the brain), as
evidenced by the fact that alcohol is the leading teratogenic cause of mild mental retardation.
Teratogens that influence the brain, by the way, are called behavioral teratogens.
The impact a teratogen has on the developing human depends upon three key factors: genetic
vulnerability, the timing of the exposure, and the amount of the exposure. The embryo or fetusʼs
genes may make them more vulnerable to the effects of a teratogen. Take alcohol, for example.
Some of us metabolize alcohol more quickly than others, and this is influenced by our genes. An
embryo without such genes metabolizes alcohol more slowly, meaning that it stays in his/her
system longer and has a more devastating impact on development. So, genetic vulnerability
certainly matters, although the best advice for a pregnant woman is to avoid this risk altogether
by not drinking during pregnancy. How about the timing of the exposure? Again, letʼs focus on
alcohol. Maternal drinking during the embryonic period will impact the development of facial
features and the brain. In fact, children with FAS (Fetal Alcohol Syndrome) have unique facial
features because of this. But, if a mother drinks during the fetal period only, the child may be
born with typical facial features but have significant brain damage nevertheless. The amount of
the teratogen the developing human was exposed to plays an important role as well. With
alcohol, sustained heavy drinking results in FAS, whereas light or moderate drinking may have a
lesser impact (although that impact – usually mental retardation – is still quite significant). Some
substances do not become teratogenic at all until they reach a certain dosage, something referred
P16 Podcast Transcript, Amy Himsel, Ph.D.
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to as a threshold effect. Take vitamin A, which is healthy in small doses but can result in extreme
birth defects if taken in large doses. Accutane is a medication for acne and itʼs basically a
megadose of vitamin A, which is why many doctors require women on Accutane to be on birth
control at the same time.
Even if the beginning of life involves complications, thatʼs not the end of the story. With low
birthweight babies and those exposed to teratogens, we still have all of the years ahead for a
positive environment and responsive parenting to make a difference.