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Timeline of embryonic development: Weeks 1 and 2

by Peter Ward, PhD
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    00:01 Welcome to this talk on the major events that occur during the process of embryonic development.

    00:05 Now, what are the things that always bothered me when I was studying embryology? Was that we could study the formation of the heart, formation of the lungs, formation of the limbs, but I never got a sense of how it all fit together and how the various events are organized altogether chronologically as we move through the embryologic period.

    00:25 And that?s exactly what this lecture is going to do is discuss how every organ system is developing as we proceed from a single cell zygote all the way to the end of the embryonic period and the beginning of the fetal period.

    00:38 Because of that, I heavily recommend you view several of the other topics first and make sure you have a fairly robust understanding of embryology before coming to this lecture which will hopefully bring it all together.

    00:51 Now as we follow the process of pregnancy we typically split it into three trimesters and of roughly thirteen weeks or three months each.

    01:00 The embryonic period only takes up the first eight weeks of that entire process.

    01:06 The first eight weeks of the first trimester is pretty much what we spend our entire time viewing because that?s the time all of the organs system are coming from nothing and creating their more or less final appearance, and then, develop further, elongate and grow during the rest of the fetal period.

    01:25 Embryonic growth at this time is usually calculated via the ultrasound technique of measuring the crown to rump length which is illustrated on the screen.

    01:34 Looking at the longest distance we can cover between the crown of the head and the rump of the body.

    01:39 Another common measurement that you used as the fetus gets older is going to be measuring the crown to heel length while we measure the line from crown to rump, rump to knee, and knee to heel.

    01:51 So starting off on day 1, Fertilization.

    01:56 We have fertilization.

    01:58 The sperm meets the egg and the male and female pronuclei come together as the sperms penetrates the zona pellucida.

    02:06 On day 2, we have our first split and we enter the two-cells stage.

    02:11 By the time we get to day 3, we have reached the morula.

    02:15 At least eight cells comprising an inner and an outer cell mass.

    02:20 By the time we get to day 4, the zona pellucida has started to breakdown just a bit and fluid has penetrated it.

    02:27 This is gonna form the early blastocyst as the blastocyst cavity is coming into existence.

    02:33 As we move to day 5, we enter the late blastocyst stage with the distinct blastocyst cavity but also a distinct embryoblast, a cluster of inner cells on one side and trophoblast cells surrounding it.

    02:47 As we move in to day 6 and 7, we?re gonna begin the process of implantation.

    02:53 For that to occur, the trophoblast is going to need to distinguish itself into this cytotrophoblast, which maintains the distinctions between nearby cells and syncytiotrophoblast, which looses it?s individual cell walls and becomes an invasive, multinucleated structure that helps the embryo burrow into the uterine lining.

    03:14 So implantation into the endometrium begins once the syncytiotrophoblast begins migrating and pulling the embryo with it.

    03:23 At this time, the blastocyst has the bilaminar embryo start to form, so that embryoblast is gonna separate into a hypoblast and epiblast.

    03:34 As we move into day 8, not only do we have hypoblast, series of cells in contact with the early yolk sac which is the remnant of the blastocyst cavity, but we have taller cells up above them, the epiblast, developing their own cavity, and we can see that early amniotic cavity present in this picture end on day 8.

    03:55 As we move into day 9, we?ve implanted far more completely into the uterine lining and the primitive yolk sac has formed as the hypoblast cells have migrated around and completely covered the blastocyst cavity.

    04:11 Lacunae are beginning to form inside the syncytiotrophoblast as it fills with maternal blood, allowing the maternal blood to reach the developing embryo for the first time.

    04:21 As we move into day 10, the embryo is fully surrounded by the endometrium, it is fully moved in with maybe a small closing plug behind it.

    04:30 The extra embryonic mesoderm is beginning to separate the epiblast, the hypoblast, and the yolk sac from the surrounding cytotrophoblast.

    04:40 Moving in to day 11, the lacunae, full of maternal blood are getting larger and more distinct as the cytotrophoblast is allowing the embryo to stay distinct from the uterine lining, but the syncytiotrophoblast keeps moving further and further in opening up that maternal vessels and glands.

    05:01 Inside the extraembryonic mesoderm we have little cavities beginning to form that will enlarge and form the chorionic cavity, which will eventually separate the developing embryo and yolk sac from the developing placenta.

    05:16 By day 12, the secondary yolk sac is formed and we?ve moved the little remnant of the primary yolk sac to the opposite side of the developing embryo.

    05:26 Moving into day 13, we can see that the primary yolk sac is gone, the secondary or definitive yolk sac is present and little remnants of it are on the other side of the cell.

    05:37 In terms of the placenta, we start to see primary villi of the cytotrophoblast growing outward into the syncytiotrophoblast.

    05:47 Day 14, we?re gonna focus now on the embryo itself.

    05:51 This view is showing us the roof of the amniotic cavity having been exposed, and we?re looking at the epiblast cells.

    05:59 The connecting stalk is holding this developing embryo close to the cytotrophoblast and will eventually form the umbilical cord.

    06:07 The prechordal plate is not visible, but it?s beginning to be noted by the cells that are in the epiblast and the hypoblast, and the developing embryo will orient itself relative to that prechordal plate, and this marks where the eventual head is going to form.


    About the Lecture

    The lecture Timeline of embryonic development: Weeks 1 and 2 by Peter Ward, PhD is from the course Conception, Implantation and Fetal Development. It contains the following chapters:

    • Timeline of Embryonic Development
    • Embryonic Development: Week 2

    Author of lecture Timeline of embryonic development: Weeks 1 and 2

     Peter Ward, PhD

    Peter Ward, PhD


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