Neurulation & Folding

Question 1

What is the neural groove and what surrounds it?

Answer 1

By day 21, the neural plate starts to proliferate over the paraxial mesoderm, forming neural folds. These surround and form the neural groove at the midline

Question 2

How does neural folding proceed?

Answer 2

Once the neural folds have formed, the ectodermal cells begin to pinch off the neural tube. This proceeds from the initial point of fusion in the cervical region both cranially and caudally

Question 3

What are the rostral and caudal neuropores?

Answer 3

The end openings of the neural tube before it seals, the way it communicates with the amniotic cavity

Question 4

When does fusion of the neural tube complete and what does that mark?

Answer 4

Completes by end of 4th week, neural tube is closed and broader at the cranial end (brain vesicles). CNS is developing

Question 5

What are brain vesicles?

Answer 5

The three expanded regions at the cephalic end of the neural tube, including prosencephalon, mesencephalon, and rhombencephalon (primary vesicles) that give rise to different parts of brain

Question 6

What is the prosencephalon and its two secondary subdivisions?

Answer 6

Forebrain brain vesicle

1. Telencephalon
2. Diencephalon

Question 7

What is the mesencephalon?

Answer 7

Midbrain brain vesicle

Question 8

What is the rhombencephalon and its two secondary subdivisions?

Answer 8

Hindbrain brain vesicle

1. Metencephalon
2. Myelencephalon

Question 9

What type of epithelium makes up the neural tube and what is special about it?

Answer 9

Pseudostratified columnar epithelium (neuroepithelium) which lies between both external and internal limiting membranes which are basal lamina

These give rise to all CNS neurons and glia

Question 10

What are the zones of the neural tube epithelium?

Answer 10

Ventricular zone (inner) - proliferating neuronal progenitor cells
Mantle (intermediate) zone - migrating neuroblasts
Marginal zone - The axons of the developing neurons

Question 11

Where does the neural crest arise from and up to what level?

Answer 11

Neural crest cells are a distinct layer in the edges of the neural folds in the cranial region. They are derived from neuroectoderm, and are produced up to the diencephalon level, but not telencephalon.

Question 12

Why does telencephalon not produce neural crest cells?

Answer 12

They only produce neuroepithelial cells that form ectodermal placodes, which form small aggregates of specialized ectoderm within the surface ectoderm

Question 13

What is the general fate of neural crest cells?

Answer 13

They undergo EMT and migrate to different locations in the embryo after losing contact with neuroectoderm

Question 14

What do neural crest cells in the trunk region become?

Answer 14

Dorsal route: leads to skin, give rise to the melanocytes of epidermis and hair follicles.
Ventral route: They travel between anterior parts of somites and will form dorsal root ganglia, sympathetic ganglia, parasympathetic ganglia, para-aortic ganglia, Schwann cells, and C-cells of adrenal medulla

Question 15

What do neural crest cells in the head become?

Answer 15

They leave before the tube is totally closed, and migrate tot their final destination within the developing pharyngeal arches, and most importantly the sensory neurons and glia of cranial ganglia

Question 16

Where do ectodermal placodes appear? What is their role?

Answer 16

They are ectodermal thickenings that appear lateral to neural tube and crest. They have an important role in sensory organs, pituitary gland, teeth, and cranial ganglia (including sensory, along with neural crest cells)

Question 17

What is the primary driving force of the sagittal head folding?

Answer 17

Rapid growth of cranial neural folds rising above surface ectoderm

Question 18

How does relative positioning change in sagittal head folding?

Answer 18

The most cranial structures up to the buccopharyngeal membrane move ventrally and caudally. This makes the septum transversum, primordial heart, and intraembryonic coelom more caudal, in that order. The pivot point is the prosencephalon.

Question 19

What forms the foregut?

Answer 19

During sagittal head folding the cranial end of the endoderm folds ventrally and caudally to form it. It is ended at the ectodermal derived stomodeum (the depression into the buccopharyngeal membrane) which will form the mouth

Question 20

What does the septum transversum do after folding inward?

Answer 20

extends dorsally on each side of developing gut, separating pericardial cavity from the rest of the intraembryonic coelom (which is open to the extraembryonic coelom)

Question 21

What are the three sections of the intraembryonic coelom after sagittal head folding?

Answer 21

1. Pericardial cavity - most cranial portion, closed off
2. Pericardio-peritoneal canals - two which run on each side of foregut dorsally.
3. Lateral edges of intraembryonic coelom which will form the peritoneal cavity

Question 22

What is the pivot point for the sagittal folding of the tail region and what moves?

Answer 22

The end of the neural tube. The allantois and connecting stalk will now lie cranial to the cloacal membrane. The hindgut is formed

Question 23

What forms the midgut?

Answer 23

The portion of the gut lying between the foregut and hindgut, but still connected to the yolk sac. Forming the midgut pinches off the yolk sac somewhat and forms the vitelline duct

Question 24

What is the shape of the intraembryonic coelom after folding in sagittally?

Answer 24

It is like a horseshoe, closed at the end, which folds in on itself. So the top of the horseshoe (pericardial) is connected at both sides dorsally (pericardioperitoneal canals) and then runs backwards into the peritoneal cavity

Question 25

After folding, where does the intraembryonic coelom lie in relation to the developing primordial heart?

Answer 25

Slightly ventral

Question 26

What must happen before transverse folding can proceed?

Answer 26

The somatopleure and splanchnopleure grow ventrally towards the midline (early week 4)

Question 27

What forms the definitive foregut/hindgut?

Answer 27

When the splanchnopleure fuses at the midline. The endodermal layer will be on the inside, lined by the splanchnic mesoderm.

Question 28

What forms the peritoneal cavity?

Answer 28

The transverse folding brings ends of the somatopleure together, forming the ventral wall of the peritoneal cavity. It is lined with splanchnic mesoderm (around the gut), inside of outer wall is somatic mesoderm, and outside of outer wall is surface ectoderm

Question 29

Why is the definitive foregut / hindgut different than the definitive midgut?

Answer 29

There is a space where the splanchnopleure and somatopleure cannot fuse because the yolk sac is preserved. The small connecting tube is called the vitelline duct

Question 30

What is the primitive umbilical ring?

Answer 30

The oval line between the amnion and the surface ectoderm of the embryo

Question 31

What structures pass through the umbilical ring?

Answer 31

Connecting stalk, allantois, and umbilical vessels (two arteries and one vein). Vitelline duct with vitelline vessels (from primitive hemangiogenesis), and canal connecting peritoneal cavity to extraembryonic cavity

Question 32

How is the primitive umbilical cord formed?

Answer 32

Whenever the amnion begins to fill the chorionic cavity after formation of primitive umbilical ring. It is formed from vitelline duct and umbilical vessels, some intestinal loops (too big to fit inside abdominal cavity) and allantois remnants. Yolk sac remains connected to intestinal loops via vitelline duct

Question 33

What happens by the third month of umbilical development?

Answer 33

The amnion has completely crowded out the chorionic cavity, and only the chorionic sac remains. The yolk sac shrinks, along with the vitelline duct and its vessels. It is degraded, and intestinal loops return to the abdominal cavity.

Question 34

What is the fate of the allantois?

Answer 34

Form a fibrous cord, the median umbilical ligament