Exam 1 9/24 Davenport Flashcards
2nd week of development is characterized by:
- continued implantation and expansion of the trophoblast (synciotrophoblast) until it surrounds the entire embryo
- initial establishment of uteroplacental circulation
- formation of bilaminar disc (epiblast and hypoblast)
- blastocyst cavity replaced by primary yolk sac and then secondary yolk sac
- amniotic and chorionic cavities appear
During the second week, what happens to the trophoblast cells?
They divide into 2 layers: cytotrophoblast and synciotrophoblast
Cytotrophoblast surrounds:
blastocyst cavity
What happens to the ICM during the second week of development?
the embryoblast is compressed due to the formation of the amniotic cavity and the primary yolk sac –> forms true bilaminar disc containing epiblast and hypoblast
What does the synciotrophoblast do?
Proliferation of cytotropblast cells that implant into the endometrium, cell membranes break down and cytoplasm/nuclei spill out
Why does synciotrophoblast form?
It is more invasive, invades into endometrial laminopropria submucosa to set up appropriate circulation with maternal blood supply
What extraembryonic structures begin their formation during the second week?
- amniotic cavity
- amnion
- yolk sac
- connecting stalk
- chorionic sac
How does amnion form?
Amnioblasts separate from epiblast layer and line amnion (above epiblast)
The formation of the amnion results in:
formation of bilaminar disc
Which cell layer consists of high columnar cells?
Epiblast
Which cell layer consists of small cuboidal cells?
Hypoblast
Primary yolk sac another name
Exocoelomic cavity
The _____ is related to the amniotic cavity
epiblast
The _____ is related to the exocoelomic cavity
hypoblast
How does the blastocyst cavity get replaced by primary yolk sac?
Cells from hypoblast proliferate and go down to reline the cavity, resulting in primary yolk sac
What gives rise to extraembryonic mesoderm?
yolk sac endoderm cells/trophoblast (day 11-12)
Where is extraembronic mesoderm found?
Surrounds amnion and yolk sac; found between exocoelomic membrane and cytotrophoblast
Chorion
outermost of the two fetal membranes
What does the chorion develop?
Villi - vascular finger like projections
Chorionic villi develops into the:
placenta
When do the primary chorionic villi appear?
second week
What are chorionic villi?
vascular finger like projections
What splits the extraembryonic mesoderm into 2 layers?
Extraembryonic coelom
What does the extraembryonic mesoderm split into?
- Extraembryonic somatic mesoderm
- Extraembryonic splanchnic mesoderm
What has been accomplished at the end of the second week?
- embryonic disc is flat and bilaminar
- hypoblast form a rostral thickened circular area (prechordal plate) which is the future side of the mouth, and caudal end (cloacal plate)
Extraembryonic somatic mesoderm covers:
cytotrophoblast and amnion
Extraembryonic splanchnic mesoderm covers:
yolk sac
What comprises the chorion?
trophoblast and extraembryonic somatic mesoderm
How does extraembryonic coelom form?
cavities within the extraembryonic mesoderm quickly fuse (they are loosely arranged around amnion and yolk sac)
How does the yolk sac reduce in size?
embryo folds so that we have the embryo encased in amnion
Umbilical vesicle function
storage of waste when organs become functional
Umbilical cord begins to form from structures in the:
connecting stalk
What is gastrulation
formative process by which the 3 germ layers and axial orientation are established; beginning of morphogenesis
Cephalocaudal axis of embryo is defined by the:
primitive streak
What week does gastrulation begin
week 3
Primitive streak definition
Linear thickening of epiblast cells on the caudal part of the dorsal epiblast of the bilaminar disc
How does definitive endoderm form?
Epiblast proliferates and replaces underlying hypoblast
How does definitive mesoderm form?
epiblast continues to proliferate and forms a layer of cells between upper layer and endoderm, forming mesoderm and the epiblast becomes definitive ectoderm
How is the prechordal plate maintained in week 3?
ectodermal and endodermal layers fuse at the cephalic and caudal ends of the embryonic disc
True or false: in the prechordal and cloacal plates, there is no intervening mesoderm
True
_____ migrate around and cephalic to the prechordal plate to establish the cardiogenic and septum transversum
Mesodermal cells
When does formation of notochord begin?
Around day 16
When does the allantois form?
Around day 16
Allantois function
diverticulum of yolk sac extending into the connecting stalk; responsible for early blood formation, which took place previously extraembryonically
First sign of gastrulation at the beginning of the 3rd week
Primitive streak
Where does the primitive streak form?
Caudally in median plane on dorsal aspect
The area that forms most rostrally of the primitive streak:
primitive node, primitive pit
_____ is responsible for the establishment of the craniocaudal axis
Primitive streak
Primitive streak is the origin of cells that will:
leave the surface and form mesenchyme
Primitive streak is formed by:
epiblast proliferation and migrates to the midline, forming a ridge like structure that develops a trough in it
_____ actively forms mesoderm until the early part of the 4th week
Primitive streak
How does the notochord form?
mesenchymal cells migrate cranially from the primitive node and pit, forming a median cord
As mesenchymal cells migrate to form the notochord, this process acquires:
lumen (notochordal canal)
____ induce the notochord formation
Inducers from the primitive streak
True or false: the notochord becomes the vertebral column
False - serves as a basis
Purpose of the notochord:
- defines the primordial axis of the embryo and gives it some ridigity
- serves as a basis for development of the axial skeleton (head and vertebral column)
- indicates future site of vertebral bodies
Prior to folding, notochordal plate is intercalated in the:
embryonic endoderm
What is neurulation?
Process involved in the formation of the neural plate and neural folds to form the neural tube
When is neurulation completed?
By end of 4th week when caudal neuropore closes
During neurulation, the embryo is called the:
neurula
What induces the neural tube to form?
Notochord
Which germ layer thickens during neurulation?
ectoderm
Neural plate/tube gives rise to:
CNS (brain and spinal cord)
____ broadens to extend cranially to the oropharyngeal membrane
Neural plate/tube
How do the neural groove/folds form?
Neural plate expands and invaginates along its central axis
First signs of brain development
Neural plate folds become prominent at cranial end
When do the neural folds move together and fuse into neural tube?
End of 3rd week
Neural tube separates from:
surface ectoderm, drops below it
Where do neural crest cells come from?
Cells that become orphaned from the crests of the neural folds as the neural tube forms
What do neural crest cells form?
- sensory ganglia of spinal cord and cranial nerves
- dorsal root ganglia, ANS ganglia, ganglia of CN V, VII, IX, X
- neurolemmal sheaths of peripheral nerves and meninges of brain and spinal cord
- formation of pigment cells, adrenal medulla, skeletal and muscular components of the head and neck
As the notochord and neural tube develop, the intraembryonic mesoderm proliferates and organizes to form:
paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm
Lateral plate mesoderm is continuous with _____ and covers _____
extraembryonic mesoderm; yolk sac and amnion
Fate of embryonic ectoderm
epidermis, CNS, PNS, retina of the eye, etc.
Embryonic endoderm fate
lining of respiratory passages and GI tract (including glands)
Fate of embryonic mesoderm
Smooth muscle, CT, most of CV system, blood and bone marrow, skeleton, striated muscle, reproductive and excretory organs
