Third week of embryonic development Flashcards
what is the beginning of morphogenesis
Gastrulation
what is gastrulation
process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc
what gives rise to specific tissues and organs
ectoderm, endoderm, and mesoderm
what does gastrulation start with
formation of the primitive streak on the surface of the epiblast
how does the streak look like on a 16 day embryo
clearly visible as a narrow groove with slightly bulging regions on either side
what is the cephalic end of the streak and what does it consist of
primitive node, consisting of a slightly elevated area surrounding the small primitive pit
what do the epiblast cells migrate towards
primitive streak
Cell migration and specification are controlled by
fibroblast growth factor 8 (FGF8)
how does the GF control cell movement
downregulating Ecadherin, a protein that normally binds epiblast cells together
FGF8 then controls cell specification into the _____________ by regulating ___________
mesoderm
Brachyury (T) expression
what happens once Cells of the epiblast have invaginated
some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm
what happens to the cells remaining in the epiblast
form the ectoderm
how is the notochordal process formed
Some mesenchymal cells (prenotochordal cells) migrate cranially from the primitive node and pit, forming a median cellular cord, the notochordal process
what does the notochordal process acquire
a lumen, the notochordal canal
The notochordal process grows cranially between the ectoderm and endoderm until it reaches _______
prechordal plate
what is the function of the prechordal plate
organizer of the head region.
what forms the definitive notochord
hypoblast replaced by endoderm cells notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord
what is the function of the definitive notochord
it underlies the neural tube and serves as the basis for the axial skeleton
where do the notochord and prenotochordal cells extend to
cranially to the prechordal plate (an area just caudal to the oropharyngeal membrane) and caudally to the primitive pit
what is the neurenteric canal and what is its function
where the pit forms an indentation in the epiblast and it temporarily connects the amniotic and yolk sac cavities
where is the cloacal membrane formed
at the caudal end of the embryonic disc
what is the structure of the cloacal membrane
consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm
what happens to the posterior wall of the yolk sac when the cloacal membrane appears
forms a small diverticulum that extends into the connecting stalk
what is the diverticulum
allantoenteric diverticulum, or allantois
what is the function of the allantois in
a) in lower vertebrates
b) in humans
a) reservoir for excretion products of the renal system
b) remains rudimentary but may be involved in abnormalities of bladder development
what do these epiblast cells become (fate map)
a)cells that ingress through the cranial region of the node
b) migrating at the lateral edges of the node from the
cranial end of the streak
c) migrating through the midstreak region
d) migrating through the more caudal part of the streak
e) migrating through the caudal most part of the streak
a) prechordal plate and notochord
b) paraxial mesoderm
c) intermediate mesoderm
d) lateral plate mesoderm ( mesenteries: periton, pleura pericardium)
e) contribute to extraembryonic mesoderm
what happens to the embryonic disc
becomes elongated, with a broad cephalic and a narrow caudal end (mainly in cephalic reagion)
what happens to the primitive streak at the end of the fourth week
shrinks and disappears
what begins to differentiate first, cephalic or caudal region
cephalic- mid 3rd week
caudal- end of 4th week
i.e embryo to develop cephalocaudally
Appearance of the notochord and prechordal mesoderm induces the overlying ectoderm to thicken and form the _____________
neural plate
cells of the neural plate make up the ___________, and their induction represents the initial event in the process of ______________
neuroectoderm
neurulation (process whereby the neural plate forms the neural tube)
what happens to the neural plate by the end of the third week
the lateral edges of the neural plate become elevated to form neural folds, and the depressed midregion forms the neural groove
where does fusion of the neural fold begin
in the cervical region (fifth somite) and proceeds cranially and caudally
how doe the cephalic and caudal ends of the neural tube communicate with the amniotic cavity until fusion is complete
by way of the anterior (cranial) and posterior (caudal) neuropores respectively
intraembryonic mesoderm on each side proliferates to form _________________
a thick, longitudinal column of paraxial mesoderm
what happens to the paraxial mesoderm by the end of the third week
differentiates and begins to divide into paired cuboidal bodies, or somites, on each side of the developing neural tube
what are used to determine the embryos age
somites because they are so prominent during the fourth and fifth weeks
where do the first somites appear
near the cranial end of the notochord
what do somites give rise to
most of the axial skeleton and the associated musculature, as well as to the adjacent dermis of the skin
what sequence do somites form
craniocaudal sequence
where does vasculogenesis begin
in the extraembryonic mesoderm of the umbilical vesicle, connecting stalk, and chorion
what is the trophoblast characterized by the end of the third week
by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer
how is the secondary villus formed
mesodermal cells penetrate the core of primary villi and grow toward the decidua
what is another name for tertiary villus and how is it formed
definitive placental villus formed when mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system
Remnants of the primitive streak may persist and give rise to a large tumor known as __________________
sacrococcygeal teratoma (most common tumors in newborn infants)
why does sacrococcygeal teratoma contain tissues derived from all three germ layers in incomplete stages of differentiation
Because it is derived from pluripotent primitive streak cells
