Major Development Periods – Weeks 3-8 of Embryogenesis

by John McLachlan, PhD

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    00:01 Hello, and welcome to this lecture on the three major stages of development, but focusing on the embryonic period, from the third week to the eighth week after fertilization.

    00:11 And the reason we’re focusing on that particular time is because this is the sensitive period when major morphological abnormalities develop. That’s major abnormalities affecting the body systems, such as the limbs, the skull, the brain, and so on. Because of this, it’s also emotionally challenging. It’s a disturbing subject to talk about, and therefore, we have to be aware of that when we’re dealing with it, both for ourselves and also for others.

    00:40 In the lecture, we’ll explore this critical sensitive period of development. Then we’ll look at the causes of some major morphological abnormalities, and the implications that this may have for parents and for children as a consequence. The three major periods of development in human development are, first of all, the pre-embryo, then the embryonic period, and then subsequently, the fetal period. We have a little calendar to this side to illustrate that there’s time involved here and these are of very different lengths. So the pre-embryo is from fertilization to about two and a half weeks later. The embryonic period is from that two and a half weeks up to about eight weeks after fertilization, and then the fetal period is from nine weeks after fertilization up to the time of birth. These are technical terms. So people will often talk about embryos when they really mean the pre-embryo or a fetus when they really mean the embryonic stage. But there’s value in having technically defined terms in medical terminology so that we can speak with precision. Just be aware that it’s not the same as the common language usage of these particular terms. You can see there are also very different lengths. The fetal period is much longer than pre-embryo and embryo and occupies the major part of the developmental process. Another thing to point out at this point is the [inaudible 00:02:14]around the timing of pregnancy or the time of gestation.

    02:17 Since nobody knows exactly when they became pregnant, despite the belief that you might be able to tell that, it’s normal to time a pregnancy from the flow phase of the last menstrual period. On average, that’s about two weeks before ovulation when pregnancy can actually begin. Therefore, time of pregnancy, as described by doctors and nurses, is the time after fertilization plus two weeks. So, when it’s commonly said that pregnancy lasts for 40 weeks, that’s the time of pregnancy. The time which you’re actually pregnant is 38 weeks, and this can lead to confusion. So, in these lectures, I will generally specify the time after fertilization if that’s for timing. Also, it’s conventional to describe pregnancy into three trimesters, each of which is about three months long. So, doctors or nurses may speak about the first, second, or third trimester. These are chronological divisions and they do not correspond to significant periods of change within the embryo itself whereas, the term pre-embryo, embryo, and fetus refer to quite specific periods of time when very different things are happening. For instance, in the pre-embryonic period, that’s zero to two and a half weeks, the pre-embryo is highly regulative.

    03:51 So these are the stages at which twins can form to completely separate, or even more individuals can form during the pre-embryonic period. Similarly, if some cells were damaged during the pre-embryonic period, there’s a very reasonable chance that the remaining cells will just divide more frequently and make up for that damage. So, perhaps unexpectedly in that pre-embryonic period, the pre-embryo is quite resistant to various kinds of damage.

    04:22 However, that very quickly moves into the embryonic period from two and a half weeks to the end of the eighth week after fertilization. And that’s a highly sensitive period.

    04:33 It’s when the major body systems are laid down, and at that stage, they are very sensitive to disturbance, and therefore, this is when major morphological abnormalities can develop.

    04:46 The remaining fetal period should not be assumed to be entirely safe from outside influences either.

    04:54 During this period, tissues are maturing. So, for instance, the bones are forming, the teeth are forming. Perhaps most crucially, the brain itself is developing.

    05:06 Therefore, things which happen during the fetal period may have an influence on these tissues as they mature. So it would not bring about a major morphological change but it could bring about a significant functional change.

    About the Lecture

    The lecture Major Development Periods – Weeks 3-8 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Major Development Periods – Weeks 3-8 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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