Development Weeks 1-8

Development Weeks 1-8


This lesson will identify the stages of development of an embryo during the first 8 weeks of pregnancy.

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Source: Video and Images Created by Amanda Soderlind

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Welcome to this lesson today on development, weeks one through eight.

Today we will be talking about what happens as an embryo develops from one week through eight weeks of pregnancy.

Basically, during these weeks one through eight, the basic body plan of an embryo will began to develop. The first thing that happens is an embryonic disk will form. This embryonic disk is a thin disk of cells. It's basically the first stage of setting up a body plan of this embryo. And then from there, the neural tube will form. This neural tube forms from within the embryonic disk, and it will actually become the brain and spinal cord.

This neural tube, as I mentioned, is formed within the embryonic disk, will become the brain and spinal cord, and it's actually formed from the ectoderm, which is one of the three layers of germ cells.

Following this, other germ cell layers-- so you have three layers of germ cells-- those other germ cell layers will also begin to specialize, and will give rise to certain body features, and organs and organ systems and such.

By the end of week eight, the embryo will actually begin to resemble a human being. Prior to that, it doesn't quite resemble a human being yet, but by the end of week eight, if you are able to look at an embryo at the end of week eight, it actually does start to resemble a human being.

During these weeks, one through eight, the reproductive structures will also begin to develop, depending on the inheritance of the sex chromosomes. The sex chromosomes are the chromosomes that determine the sex of the individual. If the individual inherited an X and a Y, it will become male, and male reproductive structures will form. If the individual inherited an X and an X, it will become a female, and female reproductive structures will then form.

At the end of week eight, the embryo is then called a fetus. So it's an embryo prior to week eight, and then, by the end of week eight, we call it a fetus.

During these weeks one through eight, there is always a chance that a miscarriage can occur. Miscarriages can also occur leader in development, although it's much less common. A miscarriage is when an embryo or fetus will be spontaneously expelled from the uterus. So basically, the cause for this-- there can be many factors that caused this-- but often times the factor that causes it is a genetic abnormality. So as this body plan is being laid out, and as this embryo is starting to develop into a fetus and taking on a human form, if there's some sort of genetic abnormality, that embryo will not be able to properly develop. And therefore, the body will expel it from the uterus because of these genetic abnormalities. So that's what a miscarriage is. It's when the embryo or fetus is spontaneously expelled from the uterus.

This lesson has been an overview on development of an embryo and a fetus from weeks one through eight.

  • Embryonic Disk

    Part of the inner cell mass of the blastocyst, the embryonic disk is the cellular framework that begins embryonic development; the nervous system develops from ectoderm of this.

  • Neural Tube

    A tube formed from the ectoderm of the blastocyst that serves as the framework for the development of the central nervous system (brain and spinal cord).

  • Sex Chromosomes

    The X & Y chromosomes that when paired determine the gender of the child and carry other genetic traits as well; XX = female while XY = male.

  • Fetus

    The developing baby after the first 8 weeks of development up to birth.

  • Miscarriage

    Also called spontaneous abortion, a miscarriage is when the embryo is expelled from the uterus.