Chapter 5 Flashcards
The most characteristic event occurring during the third week of gestation is ____, the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo.
gastrulation
Gastrulation begins with formation of the ____ on the surface of the epiblast
primitive streak
The cephalic end of the streak, the ___,
consists of a slightly elevated area surrounding the small primitive pit
primitive node
The most characteristic event occurring during the third week of gestation is gastrulation, the process that establishes all three _____ (ectoderm, mesoderm, and endoderm) in the embryo
germ layers
These prenotochordal cells become intercalated in the hypoblast so that for a short time, the midline of the embryo consists of two cell layers that form the ____
notochordal plate
The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small ___
primitive pit
Cell migration and specification are controlled by_____, which is synthesized by streak cells themselves
fibroblast growth factor 8 (FGF8)
_____ invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plate
Prenotochordal cells
Once the cells have invaginated, some displace the hypoblast, creating the embryonic ____, and others come to lie between the epiblast and newly
created endoderm to form ___. Cells
remaining in the epiblast then form ___
endoderm - mesoderm - ectoderm
Gradually, they migrate beyond the margin of the disc and establish contact with the extraembryonic mesoderm covering the yolk sac and amnion. In the cephalic direction, they pass on each side of the____
prechordal plate
The prechordal plate itself forms between the tip of the notochord and the ____ and is derived from some of the first cells that migrate through the node in the midline and move in a cephalic direction
oropharyngeal membrane
Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the _____
prechordal plate
As the hypoblast is replaced by endoderm cells moving in at the streak, cells of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the _____, which underlies the neural tube and is a signaling center for inducing the axial skeleton
definitive notochord
Establishment of the ____, _____ (A—P; craniocaudal), _____ (D—V), and _____ (L—R), occurs early in embryogenesis and is probably initiated during the late morula to blastocyst stages of development with the A—P and D—V axes specified prior to the L—R
body axes - anterior—
posterior ; dorsoventral; left-right
At the point where the pit forms an indentation in the epiblast, the _____ temporarily connects the amniotic and yolk sac cavities
neurenteric canal
When the cloacal membrane appears,
the posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk. This diverticulum, the ___, appears around the 16th day of development
allantoenteric diverticulum or allantois
The ____ is formed at the caudal end of the embryonic disc. This membrane, which is similar in structure to the oropharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm.
cloacal membrane
By the blastocyst stage, the A—P axis is determined and cells that are destined to form the ____ at the cranial end of the endoderm layer of the bilaminar disc migrate toward what will become the head region
anterior visceral endoderm (AVE)
At this bilaminar disc stage, cells in the AVE express genes essential for head formation, including the transcription factors OTX2, LIMI , and HESXI, and the secreted factors ___ and ___ (members of the TGF-B family), which inhibit nodal (a member of the TGF-B family) activity, thereby establishing the cranial end of the embryo
cerberus and leftyl
In the presence of this protein and ___, mesoderm will be ventralized to contribute to kidneys (intermediate mesoderm), blood, and body wall mesoderm (lateral plate mesoderm)
FGF
Another member of the TGF-B family, _____, is secreted throughout the embryonic disc
bone morphogenetic protein 4 (BMP4)
Thus, ____ (activated by the transcription factor GOOSECOID), noggin, and follistatin antagonize the activity of BMP4
CHORDIN
Thus, CHORDIN (activated by the transcription factor __), noggin, and follistatin antagonize the activity of BMP4
GOOSECOID
Thus, CHORDIN (activated by the transcription factor GOOSECOID), ___, and ___ antagonize the activity of BMP4
noggin, and follistatin
As mentioned, _____ is involved in initiating and maintaining the primitive streak. Similarly, HNF—3B maintains the node and later induces regional specificity in the forebrain and midbrain areas.
Nodal
As mentioned, Nodal is involved in initiating and maintaining the primitive streak. Similarly,____ maintains the node and later induces regional specificity in the forebrain and midbrain areas.
HNF—3B
Regulation of dorsal mesoderm formation in middle and caudal regions of the embryo is controlled by the _____ expressed in the node, notochord precursor cells, and notochord.
BRACHYURY (T) gene
___ encodes a sequence-specific DNA-binding protein that functions as a transcription factor
BRACHYURY
____ also is established early in development. Normally, many organs exhibit asymmetries, including the heart,
lungs, gut, spleen, stomach, liver, and others
Laterality (L—R sidedness)
When the primitive streak appears, ___ is secreted by cells in the node and primitive streak, and this growth factor induces expression of ____
FGF8 - NODAL
NODAL expression is then restricted to the left side of the embryo by the accumulation of _____ on the left side
serotonin (5-HT)
Ultimately, Nodal protein in the left lateral plate mesoderm initiates a signaling cascade that includes LEFTY2 to upregulate ____, is a homeobox-containing transcription factor that is a “master gene” responsible for establishing left sidedness, and its expression is repeated
on the left side of the heart, stomach, and gut primordia as these organs are assuming their normal asymmetrical body positions
PITX2
If the gene is expressed ectopically (e.g., on the right side), this abnormal expression results in _____, including situs inversus and dextrocardia (placement of the heart to the right side
laterality defects
Note that the neurotransmitter ___ also plays a critical role in this signaling cascade that establishes laterality
5-HT
Genes regulating right-sided development are not as well-defined, although expression of the transcription factor ___ is restricted to the right lateral plate mesoderm and probably regulates effector genes responsible for establishing the right side
SNAIL
Why the cascade is initiated on the left remains a mystery, but the mechanism
may involve ____ on cells in the node that beat to create a gradient of Nodal toward the left or by a signaling gradient established by ___ and small ion transport
cilia - gap junctions
For example, animal studies indicate that high doses of alcohol at this stage kill cells in the anterior midline of the germ disc, producing a deficiency of the midline in
craniofacial structures and resulting in ___
holoprosencephaly
Gastrulation itself may be disrupted by genetic abnormalities and toxic insults. ln ____, insufficient mesoderm is formed in the caudalmost region of the embryo.
caudal dysgenesis [sirenomelia]
Sometimes, remnants of the primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors, known as ___, which commonly contain tissues derived from all three germ layers
sacrococcygeal teratomas
Teratomas may also arise from ____ that fail to migrate to the gonadal ridge
primordial germ cells
_____ refers to the normal positioning of
the internal organs
Situs solitus
____ refers to a condition where the positioning of all organs is reversed in a mirror image arrangement
Situs inversus
Discordant organ positioning with respect to symmetry, where one or more organs are abnormally reversed in position or if isomerisms [i.e., both atria in the heart look the same] or inversions [i.e., the two ventricles in the heart are reversed] are present is called _____ or ____
situs ambiguus or heterotaxy
Also, approximately 20% of patients with complete situs inversus have bronchiectasis and chronic sinusitis because of abnormal cilia _____
[Kartagener syndrome]
In contrast, patients with ______ have a high risk of having a wide variety of other birth defects, including midline malformations, such as neural tube defects, cleft palate, anal atresia, etc.
situs ambiguous
[heterotaxy]
Mutations in the zinc finger transcription factor ___, a gene located on the X chromosome, cause _____
ZIC3 - X-linked heterotaxy
The ____ is an important signal molecule for establishing laterality, and animal studies show that disrupting 5-HT signaling results in cases of situs inversus, heterotaxy, dextrocardia, and a wide variety of heart defects
neurotransmitter serotonin [5-HT]
By the beginning of the third week, the trophoblast is characterized by ____ that consist of a cytotrophoblastic core covered by a syncytial layer
primary villi
During further development, mesodermal cells penetrate the core of the primary villi and grow toward the decidua. The newly formed structure is known as a ____
secondary villus
By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system. The villus is now known as a ___ or a ____
tertiary villus or definitive placental villus
Meanwhile, cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. Here, they establish contact with similar extensions of neighboring villous stems, forming a thin _____
outer cytotrophoblast shell
Villi that extend from the _____ to the decidua basalis (decidual plate: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur.
chorionic plate
Villi that extend from the chorionic plate to the ____ (decidual plate: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur.
decidua basalis
Villi that extend from the chorionic plate to the decidua basalis (______: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur.
decidual plate
Villi that extend from the chorionic plate to the decidua basalis (decidual plate: the part of the endometrium where the placenta will form) are called ___ or ____. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur.
stem or anchoring villi
Villi that extend from the chorionic plate to the decidua basalis (decidual plate: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are ____, through which exchange of nutrients and other factors will occur.
free (terminal) villi
The chorionic cavity, meanwhile, becomes larger, and by the 19th or the 20th day, the embryo is attached to its trophoblastic shell by a narrow _____
connecting stalk
The connecting stalk later develops into the ____, which forms the connection between the placenta and embryo
umbilical cord
