Gastrulation

Question 1

What do the cellular movements of gastrulation establish?

Answer 1

the three germ layers of the embryo

Question 2

From which cell mass is the embryo developed from?

Answer 2

the Inner Cell Mass

Question 2

As the morula divides, some cells find themselves on the inside. These cells begin to express different genes, including Oct4. What is Oct4 important for?

Answer 2

it is an important marker of pluripotent stem cells

Question 3

The Inner Cell Mass Cells split into what two layers?

Answer 3

• epiblast (above; high columnar; develop into embryo)
• hypoblast (below; low cuboidal; support tissue for epiblast)
• together these layers comprise the embryonic disk

Question 5

What two layers comprise the embryonic disk?

Answer 5

• epiblast (above; high columnar in appearance; all cells of the embryo itself will arise from the epiblast layer)
• hypoblast (below)

I think only epiblast cells have Oct4 at this point
6-7 days after fertilization, around the time of implantation

Question 6

The hypoblast appears to be a source of what for the epiblast?

Answer 6

source of nutrients and signaling molecules

the hypoblast seems to be a support tissue for the epiblast

Question 7

The epiblast separates into two layers to form what?

Answer 7

the amniotic cavity

• only those epiblast tissues in contact with the hypoblast will contribute to the embryo
• the roof of the amniotic cavity, the amniotic membrane, is an example of an extraembryonic tissue; the amniotic membrane forms a fluid-filled cavity that surrounds the embryo

Question 7

Embryonic cells divide how often?

Answer 7

about every 16-24 hours

Question 8

What cells will form embryonic tissues?

Answer 8

only the epiblast cells in contact with hypoblast

Question 9

During 7-14 days, the embryo undergoes rapid cell division. How does the disk change?

Answer 9

• rapid expansion in the size of the embryonic disk
• the disk changes from roughly circular to ovoid, broader at the cranial end
• most of the expansion is in the cranial direction

*despite the undifferentiated appearance of the embryonic disk, a number of signaling events have already occurred, these will later distinguish specific regions of the embryo

Question 10

What is the Germ Layer Concept?

Answer 10

All higher organisms go through a developmental period when they consist of three different tissue layers or germ layers. These are called ECTODERM, MESODERM, AND ENDODERM. During subsequent development, organs and tissues derived from the same germ layer will share certain traits, due to their common origin.

Question 11

The three germ layers, ectoderm, mesoderm, and endoderm are developed by what process?

Answer 11

gastrulation - physical process of cell movement

Question 12

During the third week, the primitive groove becomes visible at the caudal end of the embryo. The streak consists of a narrow groove (the primitive groove) that is surrounded by a thickening at each side and also at the cephalic end. The thickening at the cephalic end is called the primitive node (or just the node). What happens at the streak?

Answer 12

• the streak marks the position where epiblast cells that have migrated towards the midline, move inwards (ingress) to form a new layer of tissue in the embryo
• these migratory cells are often called mesenchyme (a not very specific term meaning undifferentiated migratory cells)

Question 13

What is mesenchyme?

Answer 13

undifferentiated migratory cells

Question 15

What do the first cells that ingress through the groove form?

Answer 15

these first cells displace the hypoblast and form the embryonic endoderm

Question 16

After cells ingress to form the endoderm, what do the next cells to involute (day 16 onwards) form?

Answer 16

they take up a position between the epiblast and the endoderm to form the mesoderm
*the mesodermal cells continue to migrate both laterally and cranially, between the ectodermal and endodermal layers, until they occupy almost the entire middle layer of the embryo

Question 16

The mesenchyma eventually do what to the hypoblast and epiblast?

Answer 16

push hypoblast away from epiblast

Question 17

The cells that continue to exist as the epiblast eventually become what?

Answer 17

your skin (ectoderm)

Question 18

Mesenchyme cells were originally epiblast cells that, once they migrated towards the center, were signaled to become migratory and become mesenchyme.

Answer 18

Check this

Question 19

EMT

Answer 19

f

Question 21

During gastrulation, mesodermal cells originating at the primitive node migrate forward (cranially) and differentiate into what?

Answer 21

the notochord

Question 22

What is the notochord?

Answer 22

• a rigid structure that serves as a simple embryonic spine
• it also plays an essential role as a source of signaling molecules
• notochord cells continue to migrate cranially until they reach the oral membrane plate
• during late embryonic development, the notochord acts as a scaffolding for assembly of the backbone
• the remnants of the notochord contribute (at least transiently) to the nucleus pulposis of the intervertebral disk, but appear to degenerate during later development (after birth)

Question 23

The mesoderm proliferates and migrates to separate the ectoderm and the endoderm in all but 2 places. What are these two locations?

Answer 23

• oral membrane (buccopharyngeal membrane), that will later form part of the oral cavity
• cloacal plate or membrane, which will eventually be located in the anal area
• at both of these positions, the ectoderm directly contacts the endoderm

Question 24

As development proceeds, the node and the associated primitive streak regress caudally. The streak finally disappears by about day 30. As the node regresses it lays down more notochord in the caudal region of the embryo. If the streak does not disappear completely, what is it thought to give rise to?

Answer 24

It is thought to give rise to highly invasive embryonic tumors called sacral teratomas (sacrococcygeal teratoma)

Question 25

What does the ectoderm form?

Answer 25

• skin epithelium (epidermis)

- neural tissues

Question 26

What does the endoderm form?

Answer 26

epithelial layer of most visceral organs, including the gut tube, bladder, liver, pancreas, and lungs

Question 27

What does the mesoderm form?

Answer 27

• skeletal and smooth muscle
• cardiovascular system
• bones
• dermis of the skin
• also, the supporting tissues between the skin and gut tube

Question 28

What is a totipotent cell?

Answer 28

cell that can differentiate into any tissue type

ex: inner cell mass cells (these give rise to all tissues in the embryo)

Question 29

What is a pluripotent cell?

Answer 29

cell with multiple, but limited, developmental potential

Question 30

What are human embryonic stem cells (hESC)?

Answer 30

• human inner cell mass cells that were maintained in culture
• these retain their totipotency as they divide and it is possible to obtain very large numbers of cells by continuous culturing

Question 31

What are induced pluripotent stem cells (iPSCs)?

Answer 31

differentiated cells converted into pluripotent stem cells by forced expression of several transcription factor genes
*generation of these cells does not require destruction of a human embryo