3rd Week of Development Flashcards
What is the 3rd week of development characterized by?
Gastrulation
What is gastrulation?
The process that establishes all 3 germ layers in the embryo (ectoderm, mesoderm, endoderm)
Gastrulation begins with the formation of what?
The primitive streak
Where does the primitive streak appear?
Caudally in the median plane of the dorsal aspect of the embryonic disc
The primitive streak results from what?
The proliferation and movement of cells of the epiblast to the median plane of the embryonic disc
How does the primitive streak elongate?
by addition of cells to its caudal end
The cranial end of the primitive streak proliferates to form what?
The primitive node
What is the primitive pit?
A depression in the center of the primitive node, connecting to the notochord
What is the primitive groove?
a shallow groove that appears in the primitive streak
What can we identify as soon as the primitive streak appears?
The embryo’s cranicaudal axis,cranial and caudal ends, dorsal and ventral surfaces, and right and left sides
How and the primitive groove and primitive pit formed?
From the invagination of epiblastic cells
Describe how endoderm, mesoderm, and ectoderm form
Once the epiblast cells have invaginated, some displace the hypoblast, thereby creating the embryonic ENDODERM. Other epiblast cells come to lie between the epiblast cells and newly created endoderm to form the embryonic MESODERM. Cells remaining in the epiblast the form the ECTODERM.
What gives rise to the 3 germ layers of the embryo?
The epiblast
The _____ gives rise to all tissues and organs in the embryo.
epiblast
What is a sacrococcygeal teratoma?
Type of germ cell tumor that contains tissues derived from pluripotent primitive streak cells
The embryonic disc is bilaminar at what 3 sites by the end of the 3rd week?
1) Oropharyngeal Membrane
2) In the median plane cranial to the primitive node, where the notochord process is located
3) Cloacal membrane
If the embryonic disc is bilaminar what can’t happen and why?
Mesenchymal cells cannot migrate between the endoderm and ectoderm because they are fused
What will the paraxial mesoderm become?
The structures right along the midline (vertebral clumn and associated muscles)
What will intermediate mesoderm become?
Kidneys and urogenital system
What will the lateral plate mesoderm become?
the body wall
What will the extrembryonic mesoderm become?
The chorion
Differentiation occurs in a _______ direction as gastrulation continues.
Cephalocaudal
What is the allantois?
A small diverticulum from the caudal wall of the yolk sac (umbilical vesicle) that extends into the connecting stalk
When does the allantois appear?
Approximately day 16
What is the importance of the allantois in humans?
It remains very small, but allantoic mesoderm expands beneath the chorion and forms umbilical arteries that will serve the placenta
What is the urachus?
The proximal part of the original allantoic diverticulum that extends from the bladder to the umbilical region
What represents the urachus in adults?
Median Umbilical Ligament
How do secondary villi form?
Mesodermal cells penetrate the core of th eprimary villi and grow toward the decidua
What are tertiary villi?
Mesodermal cells in the core of the secondary villus begin to differentiate into blood cells and smell blood vessels and the villus is now called tertiary
Why does the heart begin to beat around the same time tertiary capillaries are formed?
Capillaries in the tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk. These vessels then establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo.
