Chapter 12 Part 2: Bird Development- Gastrulation/Primitive Streak Formation

Question 1

T/F: gastrulation occurs through the blastopore

Answer 1

false. gasrulation takes place through primitive streak which is basically an elongated blastopore lip

Question 2

the primitive streak arises from the ___ ___ and epiblast located aboce it.

Answer 2

arises from the kollers sickle

Question 3

a ___ ___ forms within the primitce streak, and becomes the area where most migrating cells pass though

Answer 3

primitve groove

Question 4

a blastopore is equivalent to the :

Answer 4

primitive groove

Question 5

the anterior end of the primitive streak is a regional thickening of cells called ___ ___, also known as ___ ___

Answer 5

hensen’s node, also known as primitive knot.

Question 6

Hensens node on anterior end contains a depression known as ___ ___: where cells can enter to form the notochord and prechordal plate.

Answer 6

Hensens node on anterior end contains a depression known as PRIMITIVE PIT: where cells can enter to form the notochord and prechordal plate.

Question 7

hensen’s node is the functinal equivalent of:

Answer 7

the dorsal lip in amphibians.

Question 8

how does primitive streak define the major body axis of avian embryo?

Answer 8

• the primitive streak extends from posterior to anterior. the migrating cells enter through its dorsal side and move to its ventral side.

the axis of the streak also separates left portion of embryo from right portion.

Question 10

Cell fates:

Cells that migrate through hensen’s node of primitive streak:

Cells that migrate through middle of streak:

Cells that migrate through posterior end of streak:

Epiblast cells outside of the streak that do not ingress will form:

Answer 10

Cells that migrate through hensen’s node of primitive streak: gives rise to prechordal mesoderm, notochord, and medial aspect of the somites

Cells that migrate through middle of streak: forms lateral party of somites, heart, kidneys

Cells that migrate through posterior end of streak: forms lateral plate, and extreaembryonic structures

*think about the similarities in amphibian. Posterior regions of organizer specify lateral mesoderm too. anterior regions of organizer specify pharyngeal endoderm for anterior head formation, and prechordal mesoderm + notochord)

Epiblast cells outside of the streak that do not ingress will form: medial and dorsal structures like neural plate (ecotderm also forms nervous system, remember). Epiblast cells further from the streak will become epidermis.

Question 11

How does the primitive streak ELONGATE?

Answer 11

• as cells enter the primitive streak region, they undergo an epithelial to mesenchymal transformation, and the basal lamina beneath them breaks down, allowing them to slide by.
• The streak elongates through convergent extension, towards the future head region (anterior). Anterior cells migrate towards the center of the embryo, causing it to elongate and narrow.

Recall that the groove is not formed yet, and that the streak is still forming without gastrulation/cell migration occuring.

• some cells from the anterior portion of the primitive streak form Hensen’s node.
• at the same time, posterior secondary hypoblast cells continue to migrate anteriorly.

Question 12

How do hypoblast cells direct the movement of cells through the primitive streak (which formed from koller’s sickle of epiblast cells)?

Answer 12

Hypoblast cells produce FGF, activating the Wnt planar cell polarity pathway in the epiblast, causing cells to come down the sides of the epiblast to undergo an intercalation of cells in the posterior margin where the P.S is formting.

Question 13

Germ layer identity is established before gastrulation starts, but specification of cell type is controlled by inductive influences during and after what event?

Answer 13

migration through the primitive streak. AS soon as the PS forms, cells start migrating through it. The streak has a continuously changing cell population

Question 14

cells migrating through the anterior end of the primitive streak pass down into the blastocoele cavity and migrate more anteriorly to form;

Answer 14

the endoderm, head mesoderm, notochord.

Question 15

cells migrating through the posterior end of the PS become:

Answer 15

majority of the mesodermal tissues.

Question 16

the first cells to migrate through the hensen’s node (anterior portion) are desteined to become:

Answer 16

the pharyngeal endoderm of the foregut.

Question 17

as mesodermal ingression (cell migration in the posterior end of the PS) continues, the PS starts to regress, moving the Hensen’s node to a more posterior position as opposed to being in the center of the area pellucida. This regression creates the ___ axis of the embryo, including the notochord.

By the end of regression stage, all endodermal and mesodermal cells have entered the embryo by this stage.

Answer 17

This regression creates the dorsal axis of the embryo, including the notochord (recall that dorsal mesoderm creates notochord [chorsal mesoderm]). the notochord is created head to tail.

Question 18

pharyngeal endoderm + head mesoderm form the ____ portions of the brain. The notochord, formed during regression when hensen’s node starts to move back to posterior region, will form the ___ __ and ___ ___.

After all these components have migrated into the embryo through the primitive streak, the epiblast is now only made up of presumptive ____ cells, which proliferate and surround the yolk via ____.

Answer 18

pharyngeal endoderm + head mesoderm form the ANTERIOR portions of the brain. The notochord, formed during regression when hensen’s node starts to move back to posterior region, will form the HIND BRAIN and SPINAL CORD.

After all these components have migrated into the embryo through the primitive streak, the epiblast is now only made up of presumptive ECTODERM cells, which proliferate and surround the yolk via EPIBOLY.

Question 19

After all the endoderma nd mesoderm cell shave migrated through the primitve streak, the ectoderm cells are what’s left of the epiblast, and they proliferate. What allows these cells to migrate around the yolk?

Answer 19

ectodermal cells develop FILOPODIA to attach to FIBRONECTIN in the vitelline envelope and assist in migration of cells around the yolk.

Question 20

General NOTES:

the anterior end of the embryo is always developing ahead of the posterior region. As cells migrate into the posterior region of the primitive streak, the anterior end is already beginning organogenesis (because they migrated into the streak earlier).

BY the end of gastrulation: the ecotderm surrounds the embryo. The endoderm REPLACES the hypoblast region below the ectoderm. The mesoderm is between the ecoderm and the mesoderm.