Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
EPISODE 5 - CH. 3 - PRENATAL DEVELOPMENT 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 in 1no 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 1 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. 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. 2 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 c- sections 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., 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 3 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 4 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 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. 5