Final Exam - Chick Development

Question 1

Epiblast

Answer 1

• disc-like structure
• sheet of cells (epithelium) that will undergo gastrulation
• in mouse and chick embryos, a group of cells within the blastocyst or blastoderm, respectively, that gives rise to the embryo proper
• in the mouse, it develops from cells of the inner cell mass
• not spherical or hollow like the blastula in frogs/amphibian embryos
• there is initially no distinct regions of ectoderm, mesoderm or endoderm
• Cells of the epiblast converge and ingress towards and into the primitive streak
• part of the blastodisc in the chick
• lies on top of a fluid filled space called the subgerminal space

Question 2

Primitive streak

Answer 2

• region where gastrulation occurs
• extends to the centre of the embryo as gastrulation proceeds
• the streak starts in the future posterior and reaches the furthest anterior part of the embryo
• the earlier primitive streak is when the germ layers are specified and internalized
• lays down the anterior-posterior axis
• this is similar in mouse and chick and called streak elongation
• equivalent of the blastopore seen in amphibians
• the site of gastrulation in avian and mammalian embryos and the forerunner of the antero-posterior axis
• it is a strip of ingressing cells that extends into the epiblast from the posterior margin
• epiblast cells move through the streak into the interior of the embryo to form mesoderm and endoderm
• is highly regulative

Question 3

Ventral closure

Answer 3

• the coming together and closing of the lateral edges/sides of the chick or mammalian embryo on the ventral side of the body to form the gut
• lateral edges fold to create a gut tube (endoderm) surrounded by mesoderm and ectoderm
• brings together ventral tissues to the ventral aspect of the embryo

Question 4

Blastodisc / Blastoderm (also called epiblast)

Answer 4

• a sheet of cells that give rise to the three germ layers of the embryo
• in the chick, the blastodisc consists of the epiblast and hypoblast
• alternative name for the blastoderm in the chick
• blastoderm: a post-cleavage embryo composed of a solid layer of cells rather than a spherical blastula, as found in early chick, zebrafish, and Drosophila embryos
• arises through cleavage of the early embryo
• analogous to the frog blastula
• surrounded by albumin and protective membranes
• If blastodisc is sectioned into many pieces, each will form a primitive streak, this is how twins are formed

Question 5

Area pellucida

Answer 5

-the central clear area of the chick blastoderm

Question 6

Subgerminal space

Answer 6

• the cavity that develops under the area pellucida in the early chick blastoderm
• the epiblast lies on top of this fluid filled space

Question 7

Area opaca

Answer 7

-the outer dark ring on the outer edge of the chick blastoderm

Question 8

Hypoblast

Answer 8

• a sheet of cells in the early chick embryo that covers the yolk under the blastoderm and gives rise to extra-embryonic structures such as the stalk of the yolk sac
• part of the blastodisc in the chick
• lies on top of the yolk and is replaced by the endoblast (extra-embryonic tissues that aid in nutrition and development of epiblast)
• The hypoblast provides an inhibitory signal to streak elongation, as this is replaced by endoblast this inhibition is lost and streak elongation proceeds

Question 9

Embryo proper

Answer 9

-are the cells that are going to contribute to tissues in the embryo at birth

Question 10

Koller’s sickle

Answer 10

• important structure in chick
• a crescent shaped ridge of small cells lying at the front of the posterior marginal zone in the chick blastoderm
• defines the position of the primitive streak

Question 11

Posterior marginal zone (PMZ)

Answer 11

• major signalling center
• breaks the radial symmetry of the epiblast (also called blastodisc or blastoderm)
• where primitive streak will form
• is sufficient for streak formation
• The position of the PMZ is determined by gravity and also defines the dorsal-ventral axis
• is similar to the Nieuwkoop Centre
• forms at the junction of the area pellucida and area opaca
• this area specifies the dorsal and posterior side of the embryo
• region where gastrulation will initiate
• a dense region of cells at the edge of the blastoderm of the chick embryo that will give rise to the primitive streak
• transplantation of posterior marginal zone cells from one chick blastoderm to another can result in an additional primitive streak

Question 12

Ingression

Answer 12

• during gastrulation, cells are dividing and migrating, and pass through the streak as individual cells
• the movement of individual cells from the outside of the embryo into the interior during gastrulation
• cells are specified toward mesoderm and endoderm as they enter and migrate through the streak
• The first cells to move through the primitive streak are endoderm, these replace the endoblast that covers the yolk
• Later cells that migrate through the streak are mesodermal, the first to enter are most posterior, as the streak elongates more anterior identity is apparent
• cells of the epiblast move and converge upon the streak, pass through the streak and move into the inside of the embryo
• in mammalian and avian embryosm cells detach from the epiblast surface and ingress through the primitive streak

Question 13

Mesenchyme

Answer 13

• migratory cells
• loosely packed cells of mesodermal origin (in contrast to cells part of the epithelium)
• loose connective tissue, usually of mesodermal origin, whose cells are capable of migration
• some epithelia of ectodermal origin such as the neural crest undergo an epithelial to mesenchymal transition

Question 14

Endoblast

Answer 14

• in chick embryo, the layer of cells that grows out from the posterior marginal zone prior to the primitive streak formation and replaces the hypoblast underlying the epiblast
• extra-embryonic tissues that aid in nutrition and development of the epiblast
• replaces the hypoblast

Question 15

Hensen’s node

Answer 15

• major signalling center
• a condensation of cells at the anterior end of the primitive streak in chick embryos
• it corresponds to the Spemann-Mangold organizer in amphibians
• the equivalent region in mammals is just called the node
• cells of the node give rise to the prechordal plate and the notochord in chick embryos and to the notochord in mammals
• forms once the primitive streak has fully elongated (streak starts in the future posterior and reaches the furthest anterior part of the embryo)
• at the most anterior point the node forms and the node and streak regress back to the posterior = node regression
• thought to be a major organizing centre similar to the Spemann-Mangold organizer
• patterns the germ layers as it regresses
• anterior structures are patterned first

Question 16

Prechordal plate (mesoderm)

Answer 16

• cells that migrate anterior to the node during regression of the Henson’s node become the prechordal plate
• The mesoderm that migrates anterior to the Node to contribute to the head process (anterior to the notochord)
• the anterior-most mesoderm in the vertebrate embryo
• located anterior to the notochord
• gives rise to various ventral tissues of the head

Question 17

Head process

Answer 17

• forms anterior to the notochord during regression of Henson’s node
• the anterior end of the notochord that projects into the head of mammalian and avian embryos

Question 18

Lateral plate mesoderm

Answer 18

• mesoderm on the flanks of the embryo
• as the node regresses, the mesoderm lateral to the somites become lateral plate mesoderm
• mesoderm in vertebrate embryos that lies lateral and ventral to the somites and gives rise to the tissues of the heart, kidney, gonads, blood and the limb connective tissues

Question 19

Tailbud

Answer 19

• forms when the node reaches the posterior after regression
• this is a stem cell population that generates tail structures
• the structure at the posterior end of vertebrate embryos containing stem-like cells that give rise to the post-anal tail

Question 20

Neural plate

Answer 20

• ectoderm forms neural plates which fold to form the neural tube
• an area of thickened dorsal ectodermal epithelium at the anterior of a vertebrate embryo that gives rise to the nervous system through the process of neuruation

Question 21

Neural tube

Answer 21

ectoderm forms neural plates which fold to form the neural tube
-the ectodermally derived tubular structure that forms along the dorsal midline of a vertebrate embryo and gives rise to the nervous system

Question 22

Head fold

Answer 22

-an infolding of the three germ layers in the head region of the gastrula in chick and mammalian embryos that indicates the start of the development of the pharynx and the foregut

Question 23

Embryonic membranes

Answer 23

• membranes external to the embryo proper that are involved in its protection, nutrition, and oxygen exchange in avians (birds)
• in mammals, they include the amnion, chorion, allantois (placental tissues) and yolk sac
• Allantois analogous to structure that makes placenta, extends to edge of sac for gas exchange

Question 24

Primitive streak elongation failure and how to rescue it?

Answer 24

• Failure of streak elongation results in a ventralized embryo (blood tissue and some muscle)
• addition of activin (Vg-1 and Wnt-8c are the chick versions) activate Nodal in the posterior marginal zone and primitive streak to rescue dorsal structures (such as the notochord)
• Nodal (a signal) initiates mesoderm induction and formation in the primitive streak
• The hypoblast provides an inhibitory signal to streak elongation
• As this is replaced by endoblast this inhibition is lost and streak elongation proceeds
• Initiation of the streak is associated with Fgf expression
• Nodal and Fgf together induce streak formation and elongation
• BMP is expressed outside the streak and inhibits dorsal structures (e.g. axial notochord mesoderm)
• BMP in turn is inhibited by chordin expressed at the tip of the streak

Question 25

Vg-1

Answer 25

• is the activin version of the chick to rescue the dorsal structures during failure of streak elongation (results in ventralized embryo)
• with Wnt-8c, activate Nodal in the posterior marginal zone and primitive streak

Question 26

Wnt-8c

Answer 26

• is the activin version of the chick to rescue the dorsal structures during failure of streak elongation (results in ventralized embryo)
• with Vg-1, activate Nodal in the posterior marginal zone and primitive streak
• induce organizer and dorsal fate

Question 27

Nodal

Answer 27

• activated by Vg-1 and Wnt-8c
• low Nodal=ventral structure
• in the posterior marginal zone and primitive streak
• Nodal initiates mesoderm induction in the primitive streak (with BMPR1, Wnt-3, and Brachyury in the mouse)
• Nodal is a signal that induces mesodermal formation
• Nodal and Fgf together to induce streak formation and elongation
• induces organizer and dorsal fate
• in the mouse, Nodal and Wnt at their highest activate goosecoid (Gsc), this induces node formation and Nodal patterns mesoderm and endoderm during gastrulation

Question 28

Fibroblast Growth Factor (FGF)

Answer 28

• Initiation of the streak is associated with Fgf expression
• Nodal and Fgf together to induce streak formation and elongation

Question 29

BMP

Answer 29

• ventralizes the embryo
• BMP is expressed outside the streak and inhibits dorsal structures (e.g. axial notochord mesoderm)
• BMP in turn is inhibited by chordin expressed at the tip of the streak

Question 30

Chordin

Answer 30

• expressed at the tip of the streak

- inhibits BMP

Question 31

Distal visceral endoderm (DVE)

Answer 31

• in mouse gastrulation, the epiblast is adjacent to Distal Visceral endoderm
• formation of the DVE (distal visceral endoderm ) breaks the radial symmetry of the epiblast
• Distal since it is furthest from the extra embryonic ectoderm
• The DVE transitions to the AVE (anterior visceral endoderm) to mark the location of the head formation
• the endoderm located at the distal end of the cup-shaped mouse epiblast, which moves and extends away from the posterior side of the embryo, upon which it becomes the anterior visceral endoderm

Question 32

Anterior visceral endoderm (AVE)

Answer 32

• During gastrulation in the mouse, The DVE transitions to the AVE (anterior visceral endoderm) to mark the location of the head formation
• streak forms opposite of the AVE
• an extra-embryonic tissue in the early mouse embryo that is involved in inducing anterior regions of the embryo

Question 33

Extra-embryonic ectoderm

Answer 33

• in mammals, embryonic tissue that contributes to the formation of the placenta
• involved in implantation

Question 34

BMP-4

Answer 34

• seen in Xenopus embryo
• BMP in chick
• ventralizes the embryo and is inhibited by Noggin (Chordin in the chick)
• extra embryonic ectoderm
• induces streak formation in the mouse

Question 35

Wnt-3

Answer 35

-inducing mesoderm in the streak with BMPR1, Nodal and Brachyury during axes formation in the mouse

Question 36

BMP receptor type 1 (BMPR1)

Answer 36

-inducing mesoderm in the streak with Wnt-3, Nodal and Brachyury during axes formation in the mouse

Question 37

Brachyury

Answer 37

-inducing mesoderm in the streak with BMPR1, Nodal and Wnt-3 during axes formation in the mouse

Question 38

Node

Answer 38

• in avian and mammalian embryos, the embryonic organizing center analogous to the Spemann-Mangold organizer of amphibians
• it is also known as Henson’s node in birds
• in plants, that part of the stem at which leaves and lateral buds form

Question 39

Goosecoid

Answer 39

-in the mouse, Nodal and Wnt at their highest activate goosecoid (Gsc), this induces node formation

Question 40

Cerberus-like 1

Answer 40

-During mouse gastrulation, Cerberus and Lefly inhibits Nodal signalling in the primitive streak

Question 41

Dickkopf (Wnt inhibitor)

Answer 41

-in the mouse, is a Wnt signalling inhibitor

Question 42

Chick lifecylce

Answer 42

1) egg(development within oviduct)
2) blastoderm (embryo, development within oviduct)
3) cleavage (development within oviduct)
4) area opaca, area pellucida and posterior marginal zone forms (development within oviduct)
5) gastrulation
6) Stage 4- Henson’s node and primitive streak form
7) Stage 14- 22 somites
8) organogenesis
9) Stage 30
10) hatching
11) chick
12) adult

Question 43

How is the primitive streak formed by mesoderm induction and patterning?

Answer 43

1) signalling by epiblast cells at the posterior marginal zone
2) Nodal function antagonized by Cerberus produced by hypoblast
3) Hypoblast replaced by endoblast, Nodal and FGF induce formation of the primitive streak

Question 44

How is axes formation in the mouse accomplished?

Answer 44

1) specification of the distal visceral endoderm (DVE, 5.5 days)
2) Proliferation and movement of anterior visceral endoderm (AVE, 6 days)
3) Specification of anterior ectoderm and beginning of primitive streak (6.5 days)

Question 45

Why was the chick used as a model?

Answer 45

• Easy to manipulate by cutting a small incision in the egg and observe the development of the embryo inside
• Easy to culture
• Flat embryo
• Problem: genome is not good to manipulate

Question 46

What are some major deviations seen in chick (bird) development compared with frog (amphibian) development?

Answer 46

-Epiblast: The embryo is initially a layer of cells called an epithelium, Not spherical hollow blastula, In the epiblast there are initially no distinct regions of ectoderm, mesoderm or endoderm
-The timing of germ-layer specification is occurring before and during gastrulation
-There is considerable cell proliferation and migration = cell mixing
-Fate map does not exist since germ layers are all mixed up
-During gastrulation: The primitive streak becomes the equivalent of the blastopore, Cells of the epiblast converge and ingress towards and into the primitive streak
-Ventral closure is another defining feature: The gut cavity is created as the lateral edges of the embryo,
Fold to create a gut tube (endoderm) surrounded by mesoderm and ectoderm

Question 47

Epithelium

Answer 47

chick embryo starts off initially as this layer of cells

Question 48

What creates the first differences of cells in the embryo?

Answer 48

• position in the embryo (outside vs inside) creates the first differences
• will create the trophectoderm and inner cell mass

Question 49

In the chick, there are three important structures/signalling centres. What are they?

Answer 49

1) Koller sickle: crescent-shaped ridge, define the position of the primitive streak
2) Posterior marginal zone (PMZ): forms at the junction of the area pellucida and area opaca, this area specifies the dorsal and posterior side of the embryo, the region where gastrulation will initiate
3) Henson’s Node: the node in mammals, forms once the primitive streak has fully elongated (streak starts in the future posterior and reaches the furthest anterior part of the embryo), at the most anterior point the node forms and the node and streak regress back to the posterior = node regression, thought to be a major organizing centre similar to the Spemann-Mangold organizer, patterns the germ layers as it regresses, anterior structures are patterned first

Question 50

Node regression

Answer 50

• the node and streak regress back to the posterior, this is known as node regression
• the Henson’s node patterns the germ layers as it regresses (anterior structures are patterned first)
• the primitive streak lays down the anterior-posterior axis
• cells that migrate anterior to the node become the prechordal plate
• the head process forms anterior to the notochord
• the node as it regresses patterns and lays down the notochord and somites on either side
• mesoderm lateral to somites becomes lateral plate mesoderm
• when the node reaches the posterior, it stops and forms the tail bud (stem cell population that generates the tail structures)
• node regression patterns the tissues in an anterior to posterior direction

Question 51

Ingression in the chick vs involution in the frog

Answer 51

Ingression: during gastrulation of chick embryo, cells are dividing and migrating, then pass through the streak as individual cells
Involution: in the frog embryo, cells migrate as a coherent sheet through the streak

Question 52

During ingression, cells of the epiblast move and converge upon the streak, pass through the streak and move into the inside of the embryo. What happens to the cells based on their new position?

Answer 52

• the first cells to move through the primitive streak and are the furthest cells become specified towards endoderm (these replace the endoblast that covers the yolk)
• the later cells that migrate through the streak and are found in the middle become specified towards mesoderm (the first to enter are most posterior, as the streak elongates more anterior identity is apparent)
• remaining epithelial cells become ectoderm
• the endoblast is eventually replaced by endoderm

Question 53

Mesenchyme vs epithelium

Answer 53

mesenchyme=migratory mesodermal cells that are loosely packed
epithelium=a sheet of tightly packed cells bound to each other by adhesive cell junctions

Question 54

Gastrulation

Answer 54

• head to tail axis is patterned
• cells move as sheets
• put germ layers in the cell in the right position

Question 55

Axial mesoderm

Answer 55

mesoderm that forms along the main axis of the body, such as the notochord, vertebral column, somites and neural tube in vertebrates

Question 56

Paraxial mesoderm

Answer 56

mesoderm lying on either side of the dorsal midline and which gives rise to the somites

Question 57

Somite staged embryo

Answer 57

• Somite number is used to stage the development of tissues

- Organ development and tissue development during this time

Question 58

Maternal determinants

Answer 58

not as relevant in chick embryo

Question 59

Where does mesoderm induction and patterning happen in the chick embryo?

Answer 59

-occurs in the primitive streak

Question 60

What are the similarities between the chick embryo and the Xenopus embryo?

Answer 60

• The Posterior Marginal Zone is probably similar to the Xenopus Nieuwkoop Centre
• Hensen’s Node is analogous to Spemann-Mangold organizer
• Mesoderm patterning is similar
• XWnt-8/Wnt-8c and Nodal induce organizer and dorsal fate
• Low Nodal=ventral structure
• BMP/BMP-4 ventralizes the embryo and this is inhibited by Noggin/Chordin

Question 61

What are the differences between the chick embryo and the Xenopus embryo?

Answer 61

• Organizer is moving in the chick

- Cells are specialized as they migrate through the streak to become mesoderm and endoderm

Question 62

What happens during gastrulation in the mouse?

Answer 62

• The epiblast is adjacent to Distal Visceral endoderm
• Formation of the DVE (distal visceral endoderm ) breaks the radial symmetry of the epiblast
• Distal as it is furthest from the extra embryonic ectoderm (Involved in implantation)
• The DVE transitions to the AVE (anterior visceral endoderm) to mark the location of the head formation
• Cerberus and Lefly inhibits Nodal signalling in the primitive streak
• Streak forms opposite of the AVE

Question 63

How is mouse gastrulation different from chick?

Answer 63

• Translation of the primitive streak induces a second axis but without head formation
• In mouse, head development is separated from patterning of the rest of the body

Question 64

How is axes formation done in the mouse?

Answer 64

• Extra embryonic ectoderm BMP-4 to induce streak formation
• Primitive streak: BMP receptor 1 (BMPR1), Wnt-3, Nodal and Brachyury, This is inducing mesoderm in the streak
• Nodal and Wnt at their highest activate goosecoid (Gsc)
• This induces node formation
• Nodal patterns mesoderm and endoderm during gastrulation
• ThuVE prohibits mesoderm formation
• Cerberus-like 1 = Nodal inhibitor
• Dickkopf=Wnt inhibitor
• Prospective mesoderm with migration displaces ectoderm to initiate streak elongation

Question 65

ThuVE (in mouse axes formation)

Answer 65

prohibits mesoderm formation

Question 66

Late embryogenesis of the mouse

Answer 66

• The mouse embryo undergoes embryo turning, an exaggerated ventral closure
• Extra embryonic tissue assist in embryo nutrition and protection
• Allantois migrates out to the make contact with the placenta (the umbilical cord)