The Embryonic Period
Stage 2 of Human Development
{WEEKS 2 - 8}
Read:
The following weeks of development after implantation of the blastocyst are marked by continued growth and gradual changes in the shape and orientation of the developing human. During the six week period which encompasses the Embryonic Period, the developing human is referred to as an embryo. During this period the embryo undergoes induction, proliferation, differentiation, morphogenesis, and maturation. (See definitions of these terms in the diagram below).
{WEEKS 2 - 8}
Read:
The following weeks of development after implantation of the blastocyst are marked by continued growth and gradual changes in the shape and orientation of the developing human. During the six week period which encompasses the Embryonic Period, the developing human is referred to as an embryo. During this period the embryo undergoes induction, proliferation, differentiation, morphogenesis, and maturation. (See definitions of these terms in the diagram below).
Image: Bath-Balough, M. and Fehrenbach, M. J. Illustrated Dental Embryology, Histology, and Anatomy. 2011. Diagram. Table 3-3. 23.
Watch:
We are going to look in detail at the second, third, and fourth week of development in the Embryonic Period. Please watch the following video that provides a review of the events of the Preimplantation Period as well as outlines the major events in the Embryonic Period that we will soon focus on.
Watch:
We are going to look in detail at the second, third, and fourth week of development in the Embryonic Period. Please watch the following video that provides a review of the events of the Preimplantation Period as well as outlines the major events in the Embryonic Period that we will soon focus on.
Video: Incredibleleo. "Human Development." Online video clip. YouTube. YouTube, 17 Apr. 2007. Web 24 Sept. 2013.
Read:
So as you just saw, there are some pretty dramatic changes that occur in the Embryonic Period! Let's break down these changes week by week to get a more clear understanding.
Week 2
Due to the continued growth of what was the implanted blastocyst, a bilaminar embryonic disc is now evident in what was the inner cell mass or embryoblast layer of the blastocyst in the previous developmental period. As the name implies, the bilaminar disc consists of 2 layers. The superior or upper layer is termed the epiblast layer and the inferior or lower layer is termed the hypoblast layer. There are also two fluid filled cavities apparent during the second week of gestation. One of which is the amniotic cavity which faces the epiblast layer, and the other is yolk sac which faces the hypoblast layer.
Look:
Look at the following diagram and find the 2 layers of the bilaminar embryonic disc as well as the 2 fluid filled cavities that you just read about.
Read:
So as you just saw, there are some pretty dramatic changes that occur in the Embryonic Period! Let's break down these changes week by week to get a more clear understanding.
Week 2
Due to the continued growth of what was the implanted blastocyst, a bilaminar embryonic disc is now evident in what was the inner cell mass or embryoblast layer of the blastocyst in the previous developmental period. As the name implies, the bilaminar disc consists of 2 layers. The superior or upper layer is termed the epiblast layer and the inferior or lower layer is termed the hypoblast layer. There are also two fluid filled cavities apparent during the second week of gestation. One of which is the amniotic cavity which faces the epiblast layer, and the other is yolk sac which faces the hypoblast layer.
Look:
Look at the following diagram and find the 2 layers of the bilaminar embryonic disc as well as the 2 fluid filled cavities that you just read about.
Image: Bath-Balough, M. and Fehrenbach, M. J. Illustrated Dental Embryology, Histology, and Anatomy. 2011. Diagram. Figure 3-6. 24.
Read:
Week 3
At the beginning of the third week of pregnancy what is referred to as the primitive streak forms within the bilaminar disc. The primitive streak causes the disc to have bilateral symmetry or to have a right and left sides that are mirror images of each other. Due to the continued growth and differentiation of the embryo's cells during this week a new embryonic layer is formed. Now instead of presenting as a bilaminar disc, the embryo resembles a trilaminar disc with, you guessed it, 3 distinct layers! These layers are termed ectoderm (which was the old epiblast layer), endoderm (which was the old hypoblast layer), and a new layer in between called mesoderm.
Look:
Look at the diagram below and find the 3 layers of the trilaminar embryonic disc. Also, read about (and memorize!) what each primitive tissue layer will become.
Read:
Week 3
At the beginning of the third week of pregnancy what is referred to as the primitive streak forms within the bilaminar disc. The primitive streak causes the disc to have bilateral symmetry or to have a right and left sides that are mirror images of each other. Due to the continued growth and differentiation of the embryo's cells during this week a new embryonic layer is formed. Now instead of presenting as a bilaminar disc, the embryo resembles a trilaminar disc with, you guessed it, 3 distinct layers! These layers are termed ectoderm (which was the old epiblast layer), endoderm (which was the old hypoblast layer), and a new layer in between called mesoderm.
Look:
Look at the diagram below and find the 3 layers of the trilaminar embryonic disc. Also, read about (and memorize!) what each primitive tissue layer will become.
Image: Bath-Balough, M. and Fehrenbach, M. J. Illustrated Dental Embryology, Histology, and Anatomy. 2011. Diagram. Figure 3-7. 25.
Read:
Week 4
During the fourth week of pregnancy the trilaminar disc undergoes embryonic folding which helps to put the embryo's primitive structures into correct anatomical position. The folding of the trilaminar embryonic disc results in endoderm lying inside of the ectoderm, with mesoderm filling in areas in between.
We will not spend time discussing the remaining 4 weeks of the Embryonic Period as the major events have already occurred. Please understand that continued induction, proliferation, differentiation, morphogenesis, and maturation are going in the embryo at a cellular level.
Watch:
Please scroll back to the top of this page and re-watch the YouTube video that covers the major events in both the Preimplantation and Embryonic Period before continuing on.
Once you have watched the video for the second time, you are ready to complete the Embryonic Period Activities. Click on the "Next Page" button to continue on to the activities page. To go back to the previous page, click on the "Back" button.
Read:
Week 4
During the fourth week of pregnancy the trilaminar disc undergoes embryonic folding which helps to put the embryo's primitive structures into correct anatomical position. The folding of the trilaminar embryonic disc results in endoderm lying inside of the ectoderm, with mesoderm filling in areas in between.
We will not spend time discussing the remaining 4 weeks of the Embryonic Period as the major events have already occurred. Please understand that continued induction, proliferation, differentiation, morphogenesis, and maturation are going in the embryo at a cellular level.
Watch:
Please scroll back to the top of this page and re-watch the YouTube video that covers the major events in both the Preimplantation and Embryonic Period before continuing on.
Once you have watched the video for the second time, you are ready to complete the Embryonic Period Activities. Click on the "Next Page" button to continue on to the activities page. To go back to the previous page, click on the "Back" button.