Fish and Bird Development

What are the stages of zebrafish development?

1. Synchronous cleavage

2. Asynchronous cleavage after MBT

3. Gastrulation

4. Head and tail formation

When considering fish zygotes, what is a key difference compared to amphibians?

They do not undergo cortical rotation and sperm entry point has no influence on where the organizer forms

At the blastula stage in zebrafish, what is accumulated at the Nieuwkoop center?

Like frogs, there is nuclear beta catenin accumulated here

What is the effect of beta catenin in zebrafish?

Beta-catenin activates organizer genes:

• squint (nodal-like) and bozozok (Siamois ortholog)

  • These activate Chordino, Noggin, and Dickkopf

Where does gastrulation in amniote embryos occur?

Through a linear structure: primitive streak

• contains a groove through which cells ingress to form the endoderm and mesoderm

• cells that do not ingress form the ectoderm

  • Much like the blastopore dorsal lip in amphibians

The cells of Hensen’s Node (just node in mammals) do what?

At the anterior end of the primitive streak serve as the amniote organizer, and therefore Hensen’s node is a source of cells expressing BMP inhibitors

What shape is early amniote embryos?

Flat compared to others we have seen so far

As soon as you see the node, what can you tell?

The anterior/posterior axis of the embryo

What characterizes reptiles, birds and mammals?

The membranes of the amniote egg

What do all amniotes have?

Amnion, chorion, yolk sac, and allantois

What is the role of the chorion?

Gas exchange

What is the role of the allantois?

Waste excrement

What is the importance of the amnion?

Allows development to take place out of water, by providing an aqueous environment within a sac to support development

What is the yolk composition of chick embryos during cleavage?

LOTS of yolk compared to us

• Blastodisc on top with dividing cells

What is the difference between the hypoblast and epiblast?

Hypoblast forms the extraembryonic membranes, and the epiblast forms the embryonic body

What is the area pellucida?

Makes up the epiblast from which the body of the embryo develops

What is the Koller’s sickle?

Thick area at the posterior edge of the area pellucida, where the streak starts to stretch out from

As the primitive streak regresses, what happens?

Cells ingress through it

• notochord is left behind

Epiblasts cells that remain close to the streak will form? Epiblast further from the streak will form?

Close = neural tissue, further = epidermis

How can we predict what structures particular cells will form?

Based on when and where they enter the streak (ingress)

How does space determine cell identity when ingressing?

Cells entering the streak more anteriorly will form more dorsal structures

How does time determine cell identity when ingressing?

Cells entering the streak first tend to form more anterior structures

Unlike gastrulation in amphibians, cells in chicks undergo ____ as they ingress through the streak

Epithelial to Mesenchymal transformation

• Amphibians undergo involution and epiboly

Cells that ingress through at the node will form what?

Pharyngeal mesoderm and notochord

What is the organizer of the avian embryo?

Cells of Hansen’s node

• these cells form the dorsal mesoderm and induce the formation of neural tissue after gastrulating

Cells of the posterior marginal zone serve as the avian ____

Nieuwkoop center

Vg1 in chicks are expressed___, unlike in xenopus where it is a ___

Zygotically, maternal mRNA

At the time Hensen’s node has developed, the portion of the epiblast that will form the nervous system is devoid of….

BMP expression AND activity