lec 17- early development in chick embryos

Question 1

where does fertilization occur in the hen?

Answer 1

inside, in the oviduct before the shell forms around the egg

Question 2

when does cleavage occur in the hen and what is the blastoderm?

Answer 2

after fertilization when the egg is still in the oviduct, gives rise to a single layer of cells called the blastoderm

Question 3

are fertilized eggs laid before gastrulation occurs?

Answer 3

yes

Question 4

what occurs during cleavage in chick embryos?

Answer 4

-the first cleavage extends downwards from the surface, they do not extend into the yolky area and the blastoderm remains continuous with the yolk
-the next cleavages are oriented in the other directions, the blastoderm is divided into tissue that is a few layers thick
-subgerminal space is formed
-area pellucida is formed
-area opaca is formed
-eggs are laid

Question 5

is the subgerminal space?

Answer 5

A space between the blastoderm and the yolk

Question 6

what is the area pellucida?

Answer 6

The region of the blastoderm that overlies the central cavity and is translucent in appearance

Question 7

what is the are opaca?

Answer 7

The surrounding area of the area pellucida and is darker in appearance

Question 8

what are the steps to the formation of the AP axis?

Answer 8

-the first observable sign of the AP axis is the formation ofthe Kollers sickle
-epiblast forms
-hypoblast forms
-it gives rise to extra-embryonic structures that connect the embryo to the source of nutrients in the yolk
-A little later in development, the hypoblast cells are displaced by the cells that grow out from the posterior, the new layers of cells is called the endoblast

Question 9

what is Kollers sickle?

Answer 9

the first observable sign of the AP axis is the formation of a crescent shaped ridge of small cells at the posterior end

Question 10

what is the epiblast?

Answer 10

the layers of cells lying over the subgerminal space beside Kollers sickle

Question 11

what is the hypoblast?

Answer 11

A layer of cells that develops over the yolk to form a floor on the cavity

Question 12

where does gastrulation begin?

Answer 12

at the posterior end, where Kollers sickle is located, and moves anteriorly

Question 13

where does ingression take place during gastrulation?

Answer 13

the primitive streak, which is equivalent to the blastopore in xenopus embryos

Question 14

when does primitive streak start to form?

Answer 14

it starts to form when the endoblast is formed

Question 15

what is ingression?

Answer 15

when cells pass through primitive streak as individual cells, the cells spread out below to form endoderm and mesoderm

Question 16

in the Xenopus, the cavity gives rise to the gut formation during gastrulation, does that happen in chicks?

Answer 16

no, there is no cavity formation during gastrulation

Question 17

what do cells do once through the primitive streak and moved outwards?

Answer 17

internalized cells form the head, forgut, mesoderm, notochord, and other endodermal and mesodermal tissues

Question 18

what marks the end of gastrulation?

Answer 18

Hensens node, which marks the anterior end of the primitive streak

Question 19

what is Hensens node?

Answer 19

a funnel shaped major organizing center of the embryo, equivalent to the Spemann-mangold organizer

Question 20

what happens to the Hensens node at the end of gastrulation?

Answer 20

it regresses, moves towards the posterior end of the embryo leaving behind cells that have been specified for different developmental fates

Question 21

what happens during neurulation?

Answer 21

-hensens node reaches anterior, causing head process to form
-cells migrate from the node forwards under the epiblast
-the primitive streak now begins to regress, Hensens node moves towards the posterior end of the embryo
-as the node regresses, the head notochord, and somites are laid down

Question 22

what are somites?

Answer 22

blocks of mesoderm tissue from which muscles and skeletons will form

Question 23

what forms first during neurulation and what occurs?

Answer 23

-the head process
-neural folds have risen up on either side and will form a tube when they meet in the midline
-the mesenchyme in this region will form the head
-endoderm forms a pouch that gives rise to the anterior portions of the gut, also gives rise to mid and hind gut

Question 24

what happens in the more posterior regions during Neurulation?

Answer 24

-the mesoderm and endoderm will form structures appropriate for the position along the AP and DV axes
-forms somites, heart, kidney and more

Question 25

what does Kollers sickle mark?

Answer 25

the site at which the primitive streak will develop at gastrulation and the posterior end of the embryo

Question 26

how is the posterior end of the chick embryos specified?

Answer 26

-by gravity, as the egg passes through the oviduct, it moves pointed end first and slowly rotates -the spinning motion causes the yolk and embryo to shift so that the lighter components become positioned on one side of the blastoderm, this end of the blastoderm tips up and become the posterior portion of the embryo where primitive streak formation begins

Question 27

what evidence proves that the entire marginal zone is capable of initiating primitive streaks?

Answer 27

-if the blastoderm is cut up into fragments before gastrulation, each piece undergoes gastrulation and a body axis forms in each fragment -but when embryos are left intact, only the cell of the posterior marginal zone form a single primitive streak -once a primitive streak has formed, it inhibits other areas of the marginal zone from forming additional primitive streaks

Question 28

why is the posterior marginal zone considered an organizing center?

Answer 28

-when the posterior marginal zone of one embryo is grafted onto another position in a second embryo it can induce the formation of another primitive streak in the second embryo -but only one of the two axes develop further, the one that forms first tends to inhibit the development of the second one because the posterior marginal zone induces the formation of a new body axis when it is transplanted into a new location on another embryo, it can be thought of as an organizing center

Question 29

what type of proteins initiate the primitive streak?

Answer 29

by multiple signalling proteins, one of the earliest is Vegetal-1 (Vg-1)

Question 30

where is the signalling protein vg-1 expressed?

Answer 30

in the posterior marginal zone

Question 31

in what family does vg-1 belong to?

Answer 31

belongs to Transforming growth factor-B (TGF-B) family of signalling proteins

Question 32

what are the steps to primitive streak initiation?

Answer 32

-epiblast cells in the posterior marginal zone overlying kollers sickle secrete Vg-1 and Wnt signals, these signals induce expression of Nodal (a signal in the TGF pathway) in the adjacent epiblast cells -The Nodal signal is initially blocked by the inhibitory signal released from the hypoblast, the inhibitory signal is Cerberus a protein that binds to Nodal and prevents it from binding to its receptor -When the hypoblast is displaced by endoblast, Nodal signalling is released from its repression, Nodal together with FGF signals from Kollers sickle induce the internalization of epiblast cells and formation of the primitive streak

Question 33

explain the TGF-B signalling pathway:

Answer 33

-Vg-1 and Nodal belong in this family -Nodal-related signaling molecules are dimeric ligands that bind receptors on the cell surface -the receptors are serine/threonine kinases that form heterodimers of two different subunits, type I and type II -binding of ligand causes subunit II to phosphorylated subunit I which in turn phosphorylates intracellular signalling proteins called Smads -The phosphorylated Smads bind to another type of Smad -the complex enters the nucleus where it either activates or represses target genes -different receptors phosphorylate different Smads and different Smads activate and/or repress different genes, it is the type I subunit of the receptor that determines which SMads are phosphorylated

Question 34

explain how Nodal signals are used to direct many different developmental processes and the overall steps:

Answer 34

-there are different places where pathways can be modified -one point involved the production of nodal protein -nodal is synthesized and secreted as an inactive precursor protein -it is converted to an active form by proteolytic cleavages that are carried out by proprotein convertases -if the convertases are not present, there will be no nodal signalling even if nodal protein is expressed -the presence/absence of inhibitors of nodal signalling represent another control point -there are several extracellular inhibitors that bind to nodal or its receptors and either prevent the signal from reaching the target cell or prevent signalling from the receptor -Cerberus and Lefty are examples

Question 35

explain the third control point and 4th level of regulation of the Nodal signals:

Answer 35

-A third control point in the nucleus, the expression of both nodal and its antagonists (Cerberus) can be increased in response to nodal signalling -this leads to either positive or negative feedback loops -A 4th level of regulation involves inhibiting translation of mRNAs corresponding to either nodal or antagonists of nodal by microRNAs -these microRNAs bind to specific mRNA sequences and target them for degradation before they can be translated into proteins -the ability of cells to modify the Nodal pathway in multiple ways ensures that the correct levels of signalling occurs in the right place at the correct times during development