Twinning – Week 2 of Embryogenesis

by John McLachlan, PhD

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    00:00 become pregnant. Now, of course, we can also have twinning. About one pregnancy in 90 will give rise to twins. And about a third of the twins are identical twins. But there are quite a lot of variations in the numbers of twins, and that’s as a result of in vitro fertilization or assisted reproductive techniques. It has been common to re-implant more than one fertilized egg, typically, two or sometimes three. And if both are all three of these take, if they all implant and give rise to a pregnancy, then you’ll significantly increase the number of non-identical twins as a result. Twin pregnancies are always that little bit more challenging than a normal pregnancy. So the chance of something going wrong is increased in a twin or multiple pregnancies and therefore, that in itself can have negative consequences. And that’s why largely, as a result of that observation, in vitro fertilization clinics are increasingly moving towards the idea of just returning one fertilized egg as a result. Different kinds of twins can result in different arrangements of the extra-embryonic tissues. So, you can see in the left-hand side of the diagram that there has been a division very early on, under two completely separate blastocysts, which will go and implant completely independently. So, these twins will have a completely independent set of extra-embryonic membranes within the uterus. If, however, fertilization gives rise to twinning at a particular stage or the inner cell mass cell stage, then each of those inner cell masses can give rise to an embryo, and each of them will then share membranes with each other. And there are technical names for each of those which we’ll explore in another lecture. If, however, two embryos were really close together, this probably happens round about the primitive streak stage. If they are developing close together and come in contact with each other, then you may well get conjoined twins. Twins can be conjoined in a variety of different places, at the head, for instance, or in the midline in the abdomen as a result of coming too close together at some point during the early stages of development. So then, in summary, what have we looked at in this lecture? We’ve been looking at the second week of the pre-embryo. We saw the formation of the primitive streak and gastrulation, which gives rise to the main body axis, and also, the three body layers, ectoderm, mesoderm, and endoderm. Subsequently, the neural tube begins to form in the process of neurulation, and if that is incomplete, if that remains open at any point, then you’ll have a neural tube defect. That could be at the head end as an anencephaly, or it could be further down the spine as in spina bifida. Spina bifida defects may be open or closed, and the severity depends on your nature. The more open, the more serious the defect is. Finally, we looked briefly at the possibility of twinning and the different arrangements of extra- embryonic membranes that you can get as a result of the twinning process.

    03:33 Thank you very much.

    About the Lecture

    The lecture Twinning – Week 2 of Embryogenesis by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.

    Author of lecture Twinning – Week 2 of Embryogenesis

     John McLachlan, PhD

    John McLachlan, PhD

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