lec 17- early development in chick embryos Flashcards
where does fertilization occur in the hen?
inside, in the oviduct before the shell forms around the egg
when does cleavage occur in the hen and what is the blastoderm?
after fertilization when the egg is still in the oviduct, gives rise to a single layer of cells called the blastoderm
are fertilized eggs laid before gastrulation occurs?
yes
what occurs during cleavage in chick embryos?
-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
is the subgerminal space?
A space between the blastoderm and the yolk
what is the area pellucida?
The region of the blastoderm that overlies the central cavity and is translucent in appearance
what is the are opaca?
The surrounding area of the area pellucida and is darker in appearance
what are the steps to the formation of the AP axis?
-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
what is Kollers sickle?
the first observable sign of the AP axis is the formation of a crescent shaped ridge of small cells at the posterior end
what is the epiblast?
the layers of cells lying over the subgerminal space beside Kollers sickle
what is the hypoblast?
A layer of cells that develops over the yolk to form a floor on the cavity
where does gastrulation begin?
at the posterior end, where Kollers sickle is located, and moves anteriorly
where does ingression take place during gastrulation?
the primitive streak, which is equivalent to the blastopore in xenopus embryos
when does primitive streak start to form?
it starts to form when the endoblast is formed
what is ingression?
when cells pass through primitive streak as individual cells, the cells spread out below to form endoderm and mesoderm
in the Xenopus, the cavity gives rise to the gut formation during gastrulation, does that happen in chicks?
no, there is no cavity formation during gastrulation
what do cells do once through the primitive streak and moved outwards?
internalized cells form the head, forgut, mesoderm, notochord, and other endodermal and mesodermal tissues
what marks the end of gastrulation?
Hensens node, which marks the anterior end of the primitive streak
what is Hensens node?
a funnel shaped major organizing center of the embryo, equivalent to the Spemann-mangold organizer
what happens to the Hensens node at the end of gastrulation?
it regresses, moves towards the posterior end of the embryo leaving behind cells that have been specified for different developmental fates
what happens during neurulation?
-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
what are somites?
blocks of mesoderm tissue from which muscles and skeletons will form
what forms first during neurulation and what occurs?
-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
what happens in the more posterior regions during Neurulation?
-the mesoderm and endoderm will form structures appropriate for the position along the AP and DV axes
-forms somites, heart, kidney and more
what does Kollers sickle mark?
the site at which the primitive streak will develop at gastrulation and the posterior end of the embryo
how is the posterior end of the chick embryos specified?
-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
what evidence proves that the entire marginal zone is capable of initiating primitive streaks?
-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
why is the posterior marginal zone considered an organizing center?
-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
what type of proteins initiate the primitive streak?
by multiple signalling proteins, one of the earliest is Vegetal-1 (Vg-1)
where is the signalling protein vg-1 expressed?
in the posterior marginal zone
in what family does vg-1 belong to?
belongs to Transforming growth factor-B (TGF-B) family of signalling proteins
what are the steps to primitive streak initiation?
-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
explain the TGF-B signalling pathway:
-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
explain how Nodal signals are used to direct many different developmental processes and the overall steps:
-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
explain the third control point and 4th level of regulation of the Nodal signals:
-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
