Embryology Flashcards
1
Q
What are the two sets of tissue in embryology?
A
embryonic (which turns into the baby)
extramembryonic
2
Q
Where is the oocyte fertilised?
A
oviduct
3
Q
What is the oocyte surrounded by?
A
zona pellucida
4
Q
Define cleavage
A
the series of mitotic divisions of the zygote, where the cells become smaller every division, thus the mass of the cell remains constant
5
Q
What are cells that are formed through cleavage called?
A
Blastomeres
6
Q
What is compaction?
A
When the cells are loosely packed and then become tightly packed
7
Q
What is the morula?
A
16 Blastomeres
8
Q
What are the cells in the inside of the morula called?
A
inner cell mass or embryoblast
9
Q
What is the trophoblast?
A
Cells forming on the outside surface of the morula
10
Q
What gene is expressed in trophoblast cells ut not inner cell mass cells?
A
Yap gene
this yap gene activates a transcription factor called Tead4
11
Q
Explain the mosaic model
A
Cells randomly become inner cell mass cells or trophoblast cells randomly and then migrate to specific area
12
Q
Explain the positional model
A
Cells differentiate into ICM or trophoblast cells depending on whether they are in contact with the external environment or not
13
Q
Explain the polarity model
A
Cells can divide in two ways:
If cell division is PARALLEL to axis of polarisation, two cells will become trophoblast cells
If cell division is ORTHOGONAL to axis of polarisation, one daughter cell will form trophoblast while apolar cell will form ICM cells
14
Q
On day 4-5 of development what happens to the blastocyst?
A
fluid moves through the trophoblast into the centre of the morula through osmotic gradients, which results in a cavity. CAVITATION has occurred. Now the embryo is called the blastocyst with a blastocoel.
15
Q
What creates the osmotic gradient when the blastocoel expands?
A
Na+/K+ ATPase forms a TRANSEPITHELIAL gradient
16
Q
What happens when the embryo hatches too early from the zona pellucida?
A
ectopic pregnancy
17
Q
What two different layers does the Icm divide into?
A
hypoblast and epiblast
18
Q
What does the epiblast form in the future foetus?
A
all of the tissues of the human foetus and the extra-embryonic support structures
19
Q
What does the hypoblast form in the future foetus?
A
only the extra-embryonic support structures
20
Q
List the contents of the TROPHOBLAST
A
cellular trophoblast blastocoel epiblast hypoblast syncytial trophoblast
21
Q
List the contents of the ICM
A
cellular trophoblast blastocoel epiblast hypoblast syncytial trophoblast amnion amniotic cavity
22
Q
What type of cells lines the amniotic cavity?
A
squamous cells
23
Q
What is the purpose of the yolk sac?
A
to nourish the embryo
provides blood cells to embryo
gives rise to germ line that will migrate into the gonads
24
Q
What is a similarity between all of
ICM, Trophoblast, Epiblast and Hypoblast?
A
Both have cells completely surrounded by cells and cells are also in contact with space
25
What is the difference between the ICM+trophoblast compared with the Epiblast+Hypoblast?
ICM and trophoblast differentiate based on position (thus position is crucial)
Epiblast and hypoblast use the mosaic model where cells randomly become epiblast and hypoblast cells and then migrate to correct locations (the position is not crucial)
26
As the third week of development begins it enters the period of...
gastrulation
27
Define gastrulation
Gastrulation is the process by which the bilaminar embryo is transformed into one that has 3 germ layers ( ectoderm , mesoderm and endoderm ), all of which are derived from the epiblast.
28
The ectoderm will give rise to
the skin and nervous system.
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The mesoderm will give rise to
bones, muscles and mesenteries.
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The endoderm will give rise to
the lining of the gut and large glands such as liver and pancreas.
31
The first sign of gastrulation is the formation of a
primitive streak
32
The formation of the primitive streak defines both the
midline and the caudal end of the embryo.
33
How does the primitive streak form?
The primitive streak begins to form when cells in the epiblast start migrating first towards the midline on the caudal end of the epiblast and then 'downwards' towards the hypoblast. As cells from the epiblast reach the primitive streak they change shape and pass through the streak on their way to form the germ layers below the epiblast.
As more and more cells join the stream of movement towards the midline and downwards the primitive streak elongates rostrally (towards the head). Later on, as fewer cells are actively migrating, the primitive streak shortens (so that the tip of the primitive streak is seen as moving backwards towards the tail).
34
What is the primitive node?
At the anterior end of the primitive streak there is a small, well-defined accumulation of cells that is known as the primitive node . The primitive node is a very important structure that helps guide the development of vertebrates.
35
Explain how the endoderm is derived:
The first cells to migrate through the primitive streak move first downwards towards the hypoblast, and then sideways underneath the epiblast. These cells push the hypoblast towards the sides of the embryo, pushing the hypoblast cells towards the inner wall of the blastocyst cavity. In this way, the first wave of migrating epiblast cells end up replacing the hypoblast in the embryonic disc giving rise to the endoderm . The cells of the hypoblast have now moved to cover the inside walls of the primitive blastocyst cavity which is now called the yolk sac.
36
Explain how the mesoderm is formed:
Once the endoderm is formed, the second wave of migrating cells also moves downwards but begin to place themselves between the endoderm and the epiblast. These cells will form the second embryonic germ layer, the mesoderm
37
Are mesoderm cells tightly packed or loosely packed?
Unlike the ectoderm and endoderm that are formed by tightly packed cells, the cells in the mesoderm are loosely packed, and said to form a mesenchyme.
38
How is the ectoderm formed?
The remaining epiblast (after the formation of the endoderm and the mesoderm) now becomes known as the ectoderm.
39
explain the relative location of the ectoderm:
now sitting as the floor of the amniotic cavity
40
explain the relative location of the mesoderm:
nested between the ectoderm and endoderm
41
explain the relative location of the endoderm:
forming the roof of the blastocyst cavity which is now called yolk sac
42
At the rostral end of the trilaminar disc, a depression forms on the head end called the
oropharyngeal membrane
43
at the caudal end, a depression forms called the
cloacal membrane
44
Which two structures degenerate giving rise to the openings of the mouth and anus?
oropharyngeal membrane (mouth) and cloacal membrane (anus)
45
What characteristics do cells of the epiblast have?
While they are part of the epiblast cells have characteristics typical of epithelial cells.
46
What happens when the epiblast cells enter the primitive streak?
the cells lose their basal lamina, and their apical-basal polarity, change the cell surface cell adhesion proteins they express and become mesenchymal , which means they are able to migrate as individual cells.
47
What are the parts of the mesoderm in the three layered embryo? (from middle to edges)
```
axial mesoderm (notochord)
paraxial mesoderm (somites)
intermediate mesoderm
lateral plate mesoderm
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48
As the streak begins to regress, the embryonic disc also elongates so that by the end of gastrulation it has acquired a what shape?
oblong
49
What are morphogens?
molecules that act at a distance, influencing the differentiation of different body parts.
50
What are the regions that secrete morphogens called?
organising centres (or organisers)
51
In mammals what are the 2 main early organisers?
the primitive node of the primitive streak (the tail organiser)
the Anterior Visceral Endoderm ( AVE ) (The head organiser).
52
Which forms first - the mesoderm or AVE?
AVE
53
What is the role of the AVE?
involved in patterning head structures and also contributes to the specification of the mesodermal regions that will eventually form the heart.
54
Where is the AVE derived from?
The AVE is derived from the hypoblast.
55
Which two structures work together to make the embryonic head?
node and the AVE
56
Explain the signals produced by the AVE
Signals from the AVE promote the formation of head structures while inhibiting formation of caudal structures.
57
Explain the signals produced by the node
Signals from the node promote the formation of caudal structures and inhibit the formation of rostral structures.
58
What are Hox genes?
As cells migrate through the primitive streak Hox genes begin to be expressed around the streak. Hox genes are important in determining the Antero-Posterior (or Rostro-Caudal) axis.
Hox genes are first expressed in the epiblast and then in the germ layers as they are forming. They are expressed in specific patterns that provide a code that tells the embryonic tissue what part of the body to form.
59
When longitudinal bending occurs - what axis does this happen on?
The flat disc of the trilaminar embryo bends along the rostro-caudal axis, so that if the embryo is seen from its side it is seen to gradually acquire an inverted C-shape.
60
During longitudinal bending what happens to the endoderm?
As the head and tail move downwards, the endoderm moves towards the middle, forming tubes with blind ends that will eventually become the gut.
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Label diagram on pg 122
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What will the endoderm differentiate into?
digestive tube
associated glands of the digestive tube, such as the liver, gallbladder and pancreas.
The respiratory tube forms as an outgrowth of the digestive tube, and is therefore also of endodermal origin.
allantois
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Chordamesoderm
forms as a rod-like structure (notochord) along the midline of the embryo, almost all the way to the tip of what will become the head, by cells migrating through node
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Paraxial (somitic) mesoderm
will form at both sides of the midline.
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Intermediate mesoderm
will form as paired cylindrical structures lateral to the paraxial mesoderm
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Lateral plate mesoderm
will form as flattened sheets lateral to the intermediate mesoderm.
75
Where is the notochord formed?
along the midline up to the level of the hindbrain (to about the level of where the ear will form).
76
What is the only non-paired mesodermal structure?
the notochord
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The notochord plays an important role in the development of the
nervous system
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What are three actions of the notochord?
converting overlying ectoderm so it can produce neural tissue
specifying certain cell types in the developing neural tube
influencing cells from the somites to become spinal vertebrae
79
The paraxial mesoderm gives rise to the
axial skeleton, trunk and limb skeletal muscle, portions of the trunk dermis and vasculature.
80
When does the patterning of the paraxial mesoderm get established?
as the primitive streak regresses during gastrulation.
81
How are somites formed?
cells from the somitomeres will aggregate to give rise to paired segmented structures known as somites . Cells of caudal somitomeres become compacted, splitting into well-defined segmental and paired units called somites that look like little balls on both sides of the neural tube at the level of the neck and trunk, starting at about the level where the ear will form.
82
Which pairs of somitomeres do not form somites?
The first 7 pairs of somitomeres in the cranial region do not form somites.
83
Define Somitogenesis
Refers to the segmentation of the paraxial mesoderm that gives rise to somites
84
Some of the structures that somites will form include:
```
The cartilage of vertebrae and ribs;
The muscles of the trunk and limbs;
The tendons that connect muscles and bones;
The dermis of the dorsal skin;
Structures of the circulatory system
```
85
In which direction do somites form?
head to tail
86
In the clock and wavefront model, what are the cells that are dividing at the caudal end exposed to (these are the cells that are not old and are relatively new compared to the cells at the rostral end)
Cells are exposed to varying concentrations of Fibroblast Growth Factor 8 (FGF8) that is high where the cells are dividing at the caudal end.
87
Explain the wavefront part of the clock wavefront model:
Towards the head, cells are older, and are becoming ready to begin to form somites. At the same time, these cells become increasingly closer to newly formed somites.
At some point, a specific combination of precise signals tell the presomitic mesodermal cells to cross a "threshold" that makes them ready and able to turn into a somite. As the body elongates a wave of "able" cells progresses along the antero posterior axis.
Thus, a "wave" of signals that make the presomitic mesoderm ready and able to form somites (threshold) travels from the head to the tail. But they still need the "go ahead".
88
Explain the clock part of the clock wavefront model:
The clock consists of rhythmic production of mRNA for a specific set of genes. The segmentation clock is therefore a molecular oscillator that leads to oscillations in transcription activity in the presomitic mesoderm. The period of oscillation matches the period of somite formation. Somites form in pairs adding one by one in a sequence that starts at the neck and moves backwards towards the tail.
89
Explain the Notch/Delta signalling pathway:
Notch is a transmembrane protein that has a cytoplasmic domain and binds to a ligand called Delta. Whenever Notch binds delta, the cytoplasmic end of the Notch is cleaved and then travels to the nucleus where it activates genes of a family of genes known as the bHLH family. Once the cytoplasmic end of Notch reaches the nucleus, it activates the promoter of a gene that produces a protein called Lunatic Fringe. Lunatic Fringe, therefore is one of the genes that are expressed as part of the clock wave. Once Lunatic Fringe is expressed, it is able to delimit domains of Notch activity, thus allowing the formation of the inter-somitic boundaries.
90
What happens to the somites after somite formation?
epithelialisation of the presomitic mesoderm
91
What type of cells are in these somites after epithelialisation?
somites are typically a ball with a wall of columnar epithelial cells and mesenchymal cells in the cavity of the somite (called somitocoel)
92
What gene is initially expressed by early somites?
Pax3
93
In the somitocoel, what two genes are expressed in the dorsal and ventral side of the somite?
Pax1 is expressed on the ventral side
| Pax 3 is expressed on the dorsal side
94
How is Pax1 expression induced in the ventral side of the somite?
Sonic Hedgehog (Shh) secreted by the notochord and Noggin secreted by the primitive node appear to act synergistically to induce the expression of Pax1 in the ventral somite.
95
The dorsal epithelium of the somite that expresses Pax3 is maintained and will become the
dermamyotome
96
What will specify the dermomyte's state?
It will receive signals from the dorsal neural tube and the surface ectoderm
97
The dermamyotome becomes subdivided into the
dermatome and the myotome
98
What happens to the cells of the dermatome?
The cells from the dermatome will migrate underneath the non-neural ectoderm where it will form the dermis of the skin and trunk.
99
What happens to the cells of the myatome?
The cells of the myotome will migrate to eventually give rise to muscles of the vertebrae and back of the body, and the muscles of the body wall and limbs.
100
What happens to the cells in the ventral side of the somite closer to the midline?
They lose their epithelial characteristics and become mesenchymal again. This part of the somite is known as the sclerotome.
101
How is the syndetome formed?
The sclerotome receives signals from the myotome to form a thin layer of cells called the syndetome.
102
What is the difference between the syndetome and the sclerotome?
Cells of the sclerotome migrate towards the notochord where they will give rise to the body of the vertebrae and ribs, whereas cells of the syndetome will give rise to the tendons that connect the muscles to the bones.
103
Name the four mesodermal structures from inside to outside
sclerotome
syndetome
myotome
dermatome
104
What are the 2 types of signals that are involved in the regional specialisation of somites?
Signals which may help decide whether a cell in the somite will give rise to muscle or cartilage, for example
Signals that would tell these cells specifically what body part they should form (eg, a vertebra, a rib, etc)
105
The intermediate mesoderm will give rise to
the urogenital system (kidneys, gonads and associated ducts).
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What happens with the head somitomeres?
In the cranial (head) region the somitomeres do not become compacted to form the well-defined somites that are typical of the neck and trunk.
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What will the head mesoderm give rise to?
The head mesoderm will give rise to those connective tissues and muscles of the head and neck that are not derived from the neural crest.
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The lateral plate mesoderm will give rise to
the heart, blood vessels and blood cells, the lining of the body cavities and the bones of the limbs.
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What are the two sheets called that the lateral plate mesoderm forms?
visceral (splachnic) lateral plate mesoderm
| parietal (somatic) lateral plate mesoderm
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Where is the parietal (somatic) lateral plate mesoderm and what does it form?
next to the ectoderm
| it forms the parietal peritoneum, a smooth layer of tissue that lines the inside of the body wall.
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Where is the visceral (splachnic) lateral plate mesoderm and what does it form?
next to the endoderm,
| it gives rise to the heart and to the smooth muscle and connective tissue of the wall of the gut
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Cells of which germ layer are not present in the oropharyngeal membrane?
A. Ectoderm
B. Mesoderm
C. Endoderm
D. All are present
```
B. mesoderm
113
The part of the ectoderm closer to the midline will develop into the
nervous system and will also give rise to the neural crest cells.
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What will the rest of the ectoderm not close to the midline develop into?
epidermis of the skin
115
What becomes neural or non-neural ectoderm is determined by the proximity to the
notochord
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The ectoderm that is overlying the notochord transforms into a thickened patch called the
neural plate
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What forms the pigmented cells of the skin (melanocytes)?
neural crest cells
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What shape changed do the cells on the ectoderm that are placed along or near the midline go through?
they become tall and columnar.
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The neural plate will curl up to form the
neural tube
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The neural tube will give rise to the
CNS (brain and spinal cord)
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What are the 4 stages in the formation of the neural tube?
Formation of the thickened neural plate
Shaping of the neural plate which makes it narrower and longer
Neurulation, which is the folding of the plate as a consequence of the elevation of the edges of the plate
Fusion and formation of the neural tube, which happens when the edges meet at the dorsal end and fuse.
122
Where on the neural plate will the brain form?
The neural plate is wider in the rostral end where it will give rise to the brain
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Where on the neural plate will the spinal cord form?
narrower towards more caudal levels where it will give rise to the spinal cord
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What is primary neurulation?
When the central area of the neural plate starts to shift “downwards” while the outer edges of the neural plate begin to rise and curl upwards and inwards
The raised edges of the neural pate are called the neural folds .The neural folds eventually meet up in the midline where they fuse.
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The tube does not initially close completely, leaving openings at the rostral and caudal ends that are called the
rostral neuropore and caudal neuropore respectively.
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In humans, failure to close the rostral neuropore results in a condition known as
anencephaly
127
Failure to close the posterior neuropore causes
spina bifida
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Cells at the edge of the neural plate will give rise to the
neural crest
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What type of cells are neural crest cells?
mesenchymal cells - meaning they are loosely packed and can migrate.
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At the level of the spinal cord, the neural crest cells will give rise to the
peripheral nervous system, as they produce motor and sensory neurons
They also produce the hormone secreting cells of the adrenal gland and
the pigmented cells of the skin (melanocytes).
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In the cranial region, neural crest cells can also give rise to
cartilage, bone and muscles of the face and neck.
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By the fourth week of development, the rostral region of the neural tube exhibits 3 fluid filled brain vesicles or enlargements:
forebrain vesicle
midbrain vesicle and
hindbrain vesicles
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What gives rise to the forebrain?
The telencephalic (or prosencephalic) vesicle will give rise to the forebrain
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What gives rise to the midbrain?
The mesencephalic vesicle will give rise to the midbrain;
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What gives rise to the hindbrain?
The rhombencephalic vesicle will give rise to the hindbrain;
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What gives rise to the spinal cord?
The rest of the neural tube will give rise to the spinal cord.
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What is the cranial neural crest?
specialised neural crest cells that form the head
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Cranial neural crest cells give rise to
neurons, glia and melanocytes
| and muscle, cartilage and bone that form the lower part of the face.
140
Which muscles are not formed the mesoderm?
some of the muscles and bones of the head, which are formed from cranial neural crest cells
141
What germ layers constitute the pharyngeal arches?
The pharyngeal arches are formed by derivatives of the three embryonic germ layers. Each arch is covered on the outside by ectoderm, on the inside by endoderm, with mesoderm in the middle.
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The mesoderm of the pharyngeal arches will contribute to
head and neck muscles and blood vessels.
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The ectoderm of the pharyngeal arches will form the
epithelium of the mouth and face and also some sensory ganglia of the peripheral nervous system.
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The endoderm of the pharyngeal arches will form the
some glands and the epithelium of the pharynx.
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Cranial neural crest migrate from the dorsal edges of the neural tube into the
pharyngeal arches
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In pharyngeal arches, Hox genes are involved in the patterning of cranial structures derived from
2nd and 3rd arch but not 1st arch derivatives
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The first pharyngeal arch divides into 2 subunits:
the mandibular and the maxillary arches.
| These will form the jaw bones and part of the inner ear bones.
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Neural crest cells migrating into the first pharyngeal arch will form the
trigeminal nerve and ganglion which innervates the teeth and jaw
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the future head will form from the
frontonasal prominence
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What are nasal placodes?
They will form the nostrils of the baby
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What gives rise to the bottom of the face?
The maxillary prominence and the mandibular prominence
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The circulatory system derives from the
lateral plate mesoderm, but will also receive contributions from cells of the neural crest (cardiac neural crest).
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What is the first functional organ to develop in the embryo?
heart
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The cells that will give rise to the heart are found as
two patches near the rostral end (anterior third) of the primitive streak.
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Hw does blood initially flow through the heart with only two chambers?
Initially blood flows in at the caudal end of the heart where the sinus venosus (SV) lies and blood is pumped out through the cranial end (bulbus cordis, BC) into the aortic arches. Between the two are the primitive atrium (pA) and the primitive ventricle (pV).
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Explain the three parts to the embryonic circulatory system
It required that there be an intraembryonic circuit to serve the tissues of the embryo, a second vitelline circuit to gather nutrients from the yolk sac and a third allantoic or umbilical circuit to get rid of nitrogenous waste into the allantois.
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Why are most embryos lost during or before implantation?
These spontaneous abortions may be a mechanism that prevents the development of embryos with genetic abnormalities.
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What is the extra embryonic mesoderm?
Newly formed cells that migrate to become placed underneath the trophoblast (between the trophoblast and the membrane surrounding the yolk sac and between the trophoblast and the amnion).
165
cavities in the extra embryonic mesoderm grow in size until they eventually come together to form a single large cavity which is called the
extraembryonic coelom
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What will the mesodermal stalk become?
the umbilical cord
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by the end of gastrulation, the three layered embryo is surrounded by a series of 3 cavities or chambers:
the amniotic cavity
the yolk sac and
the extraembryonic coelom
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Once the blastocyst attaches to the wall of the uterus what does the blastocyst do?
enzymes secreted by the embryonic trophoblast begin to digest away first the epithelium of the uterus and then the extracellular matrix of the uterine wall allowing the blastocyst to bury itself deep inside it.
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Once the embryo is implanted, the endometrium is called the
decidua
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The trophoblast secretes
human chorionic gonadotrophin (hCG)
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What is the role of hCG
prevents further menstruations.
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What is different about the way that mitosis occurs in the cells of the syncytial trophoblast?
the cells that divide to form the syncytial trophoblast undergo nuclear division but in the absence of cytoplasmic division. As a result, the nuclei of these cells end up sitting in a continuous cytoplasmatic mass (syncytium).
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What are lacunae?
As the embryo buries itself deep into the uterine wall, it will form large openings in the uterine tissue where the mother’s blood leaks into.
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What three layers constitute the chorion?
the syncytial trophoblast
the cellular trophoblast and
the extraembryonic mesoderm.
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The chorion and the decidua together form the
placenta
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What functions does the placenta have in placental mammals?
In placental mammals the chorion evolved nutritional, immune and endocrine functions in addition to those related to gas exchange.
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Why does chronic villi form?
The chorionic villi increase the surface area that is exposed to maternal blood in the lacunae helping maximise exchange between the maternal and foetal blood.
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The decidua can be divided into:
decidua basalis, that surrounds the embryo separating it from the rest of the uterus
decidua capsularis that separates the embryo from the uterine cavity, and
decidua parietalis (the rest of the endometrium)
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Which decidua contributes to the placenta
decidua basalis only
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What is the role of the amniotic sac?
prevent the embryo from drying out,
shock absorption
storage for metabolic residues
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Amniotic fluid is initially secreted by the
amnion
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Where does the embryo release its urine into?
the amniotic sac
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What is the purpose of the yolk sac in birds and reptiles?
In birds and reptiles, the yolk sac is filled with nutrients that allow the embryo to develop inside the isolated egg.
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What is the purpose of the yolk sac in placental mammals once the placenta has formed?
produces the cells that will form the embryo’s primordial germ line that will migrate into the developing reproductive system to form sperm and egg mother cells.
The yolk sac also contains precursors for blood cells.
185
The yolk sac receives its blood supply through the
vitelline circuit
186
How does the allantois form?
When the cloacal membrane is formed, a small sac evaginates from the yolk sac called the allantois.
187
What is the role of the allantois in reptiles and birds?
In modern reptiles and birds, this sac is large and highly vascularised where it serves as a waste storage and contributes to gas exchange.
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What is the role of the allantois in mammals?
It becomes surrounded by the stalk of the extraembryonic mesoderm and contributes to the formation of the umbilical cord. The walls of the allantois are well vascularised and these blood vessels will contribute to the umbilical arteries and veins that transport blood between the foetus and the placenta: the umbilical circuit.
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What constitutes the umbilical cord
umbilical arteries x2
| umbilical vein
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What are parts of the embryonic heart tube?
sinus venoses
primitive atrium
primitive ventricle
bulbous cordis
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What structures and morphogens are involved in separating the dorsal and ventral sections of the paraxial mesoderm (somites)?
Pax 3 is expressed by both ventral and dorsal sides of the somites
The notochord secrets shh protein which is a morphogen expressed by sonic hedge hog gene which causes Pax 1 to be expressed by the ventral side of the mesoderm.
Noggin and chordin is also released by notochord and primitive node, which influences dorsal and ventral development of the somites.
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What morphogen is essential for neural plate developement?
shh secreted by notochord!
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noggin released from notochord induces the formation of
nervous tissue
