Developmental Abnormalities – Weeks 3-8 of Embryogenesis

by John McLachlan, PhD

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    00:01 Now, I’ve said several times that things might go wrong. So let’s explore that in a little more detail.

    00:07 I’m going to ask some questions and offer answers as best we can. There is still much that we do not know about this complex and challenging area of development, and indeed,medicine.

    00:21 So we’ll look at how common they are, and we’ll look at the forms that major abnormalities can take. We will make our best guesses as to what may cause them, and there are three categories; genes, substances or teratogens, and unknown factors. Then we’ll look at the timing for teratogens for agents which might cause abnormalities.

    00:43 And then we’ll look at the nature of them. We’ll comment on how people might feel about this.

    00:49 This is a very sensitive and very important area. It often worries the expectant parents greatly. They’re worried that something might be wrong with their baby. And sometimes that can lead them to feel guilty when, of course, there is no cause for them to feel guilty about something that happened to their baby. Then finally, we’ll look at how frequency can be reduced, and that would feedback into our initial question of how common they are.

    01:14 Fortunately, in Western Europe, the frequency of developmental abnormalities has been falling, and there’s every reason to expect that we could make it fall still further in the future.

    01:25 So, how common are developmental abnormalities? The answer is they’re actually much more common than people realize. In Western Europe where rates are relatively good, about 2.5% of all babies have a significant developmental abnormality detected at birth. That’s one baby in 40. That’s far more common than people, even sometimes doctors realize.

    01:50 And we’re actually speaking about abnormalities which might have an effect on function if you’re to look at minor variations. Well, to be honest, almost everybody has minor variations of one kind or another. But in addition to that, there’s another 2.5% that are detected in infancy and childhood. So, there are things like deafness, for instance, which may not be detected at birth, or problems with movement which become evident when the child when the infant starts to walk. At that point, heart defects can also become detectable because the extra work required from the heart might show up a problem that had previously been undetected. In addition to that, an unknown number of fertilizations do not lead to pregnancies.

    02:42 They terminate spontaneously after fertilization has taken place. Now obviously, if fertilization does not lead to a pregnancy, it’s very hard to know that it happened at all, and therefore, that’s why we’re not sure about the total number. The highest estimate I know is that perhaps 75% of all fertilizations terminate spontaneously. And if one ever manages to retrieve one of those fertilizations, it looks as if it’s the genetic cause that something significant has gone wrong with the genetic expression in the developing fertilized egg and that it has terminated itself spontaneously as a result of that. As I said, that’s something of a guess and we don’t actually know exactly what proportions do terminate. We merely know that it’s a significant proportion.

    03:35 Of course, we haven’t looked at complex problems, which are not detected until much later in life. So, there is already evidence that things that happened during the fetal period can have an influence on how likely you are to have a number of conditions as an adult, and that includes cancer and diabetes, heart disease, and even possibly mental conditions such as schizophrenia, something has happened in the course of development which has increased the chances that you will suffer from these as an adult and at the moment, we have very little idea of what these factors affecting the pregnancy might have been. But obviously, the implications of these are massive, and if we could tell what those factors were, we might have another way of addressing these adult illnesses during the early stages of development.

    04:30 Now this is a table from a survey of abnormalities in Western Europe and you can see that the rate here is expressed per 10,000 pregnancies. So 256 per 10,000 means that that’s our 2.5% that we spoke about previously. And looking at the frequency of each of these, you can see that congenital heart defects are the most common single kind moving down through limb defects, defects of the urinary system on and then the nervous system. And part of that includes neural tube defects. So they come under the total of our nervous system through the digestive system, things affecting the face, and then things affecting the respiratory system and then the eyes and the ears. So those are the most common ones. The table on the right-hand side, the column on the right-hand side, are the same figures but excluding non-chromosomal causes such as Trisomy 21 Down syndrome.

    05:30 You can see that the non-chromosomal defects contribute a small part, but only a small part to the overall frequency of the developmental defects.

    About the Lecture

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

    Author of lecture Developmental Abnormalities – Weeks 3-8 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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