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The Human Embryo Advanced
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: August 27, 2016
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. The Human Embryo - Advanced
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The Human Embryo Advanced
• Understand what occurs during gastrulation.
• Learn about the development of the embryo from implantation to the eighth week after fertilization.
At one time, did we all really look alike?
We all start as a single cell and soon grow into an embryo. Notice the remarkable details beginning to form. The
eyes, backbone, and limb buds are obvious. Think about the amazing complexity that must be going on inside the
embryo and the tremendous amount of growth and development still to come. So, yes, at one time we all looked
similar.
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Growth and Development of the Embryo
An embryo is a developing human being between the time of implantation and the first eight weeks after fertilization. During this time, the embryo grows in size and undergoes three processes: gastrulation, differentiation, and
organogenesis.
Gastrulation
Gastrulation is the development of different layers of cells in the embryo. It generally occurs during the second
week after fertilization. During gastrulation, cells of the embryo migrate to form three distinct cell layers: the
ectoderm, mesoderm, and endoderm. These layers are shown in Figure 1.1. Each layer will eventually develop into
certain types of tissues and cells in the body.
• Ectoderm—Forms tissues that cover the outer body; it develops into cells such as nerves, skin, hair, and nails.
• Mesoderm—Forms tissues that provide movement and support; it develops into cells such as muscles, bones,
teeth, and blood.
• Endoderm—Forms tissues involved in digestion and breathing; it develops into cells such as lungs, liver,
pancreas, and gall bladder.
FIGURE 1.1
The three cell layers of the embryo develop into different types of cells.
For
example, the ectoderm develops into skin
cells, the mesoderm into muscle cells,
and the endoderm into lung cells.
Differentiation and Organogenesis
During the third week after fertilization, the embryo begins to undergo cellular differentiation. Differentiation is
the process by which unspecialized cells become specialized into one of the many different types of cells that make
up the body. During differentiation, certain genes are turned on, or activated, while other genes are switched off,
or inactivated. As a result of this process, cells develop specific structures and abilities that suit them for their
specialized roles in the body. Several examples of specialized cells are shown in Figure 1.1, along with the cell
layers from which they develop.
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Chapter 1. The Human Embryo - Advanced
Differentiation of cells leads to the development of specific organs within the three cell layers. This is called
organogenesis. All the major organs begin to form during the remaining weeks of embryonic development. A
few of the developments that occur in weeks 4 through 8 are listed in Figure 1.3.
FIGURE 1.2
Cellular differentiation occurs in the 3week-old embryo.
Embryonic Development During Weeks 4-8
FIGURE 1.3
Embryonic Development (Weeks 4-8). Most organs develop in the
embryo during weeks 4 through 8. If the embryo is exposed to toxins
during this period, the effects are likely to be very damaging. Can you
explain why? (Note: the drawings of the embryos are not to scale.)
As the embryo develops, it also grows in size. By the eighth week of development, the embryo is about 30 millimeters
long. It may also have begun to move.
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MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/139401
Summary
• An embryo is a developing human being between the time of implantation and the first eight weeks after
fertilization.
• During gastrulation, cells of the embryo migrate to form three distinct cell layers: the ectoderm, mesoderm,
and endoderm. Each layer will eventually develop into certain types of tissues and cells in the body.
• During differentiation, certain genes are turned on, or activated, while other genes are switched off, or
inactivated. As a result of this process, cells develop specific structures and abilities that suit them for their
specialized roles in the body.
Review
1.
2.
3.
4.
How many layers are formed during gastrulation?
What tissues and cells does the mesoderm develop into?
When do the major organs of the body form?
When do the lungs start to develop?
References
1. . . CK-12 Foundation
2. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
3. Two fetus images courtesy of Melchior Meijer/3DPregnancy.com; composite created by CK-12 Foundation. http://commons.wikimedia.org/wiki/File:6_weeks_pregnant.png; http://commons.wikimedia.org/wiki/F
ile:10_weeks_pregnant.png . CC-BY-SA 2.5
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