Third class

Question 1

1- ________ on epiblast gives rise to ectoderm, endoderm and mesoderm

2- Notochord arising from ____________ forms a midline induction center

Answer 1

1- Primitive streak

2- Primitive node

Question 2

During week three to four, the process of ___________ begins.

Answer 2

Neurulation

• Process of central nervous system formation
• Begins ~day 18 when notochord appears
• Notochord induces overlying ectoderm to thicken
• Thickened ectoderm called the neural plate rises on either side to form neural folds, central depression is neural groove

Question 3

Neural folds rise up & _________ sink in further until neural folds meet

Answer 3

1-Neural groove

–Fusion occurs first in mid-embryo

–Once fused, the tube made of neural plate is called neural tube and the space inside is neural canal

Question 4

•Neural folds fusion continues in either direction
–Cranial opening of the neural tube closes ~ day 24
–Caudal opening of the neural tube closes ~ day 26
–The bulbous cranial neural tube becomes the brain & neural canal here will become the brain ventricles
–Caudal neural tube becomes the spinal cord & neural canal becomes the central canal

Answer 4

Question 5

Neurulation, also gives rise to ___________.

–Where the neural folds meet, some ectodermal cells become amoeboid and migrate to either side of the forming neural tube

–What does neural crest give rise to?

Answer 5

Neural crest cells

develop into including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.

Question 6

What is the fate of the primitive streak?

Answer 6

Question 7

___________ occurs due to caudal agenesis or caudal dysgenesis
–Occurs when primitive streak degenerates prematurely
–Due to insufficient mesoderm
–Fusion of lower limbs
–Anomaly may also involve other organs

Answer 7

Sirenomelia

Question 8

__________ occurs due to tumor of the persistent primitive streak remnants.
–Most common tumor in newborns (1:35000), 80% female
–Most are benign

Answer 8

Sarcococccygeal teratoma

Question 9

Embryo Folding

• At the end of week 3, embryo is a flat, trilaminar germ disc
• In week 4, the trilaminar disc starts to fold due to differential growth of tissues

Answer 9

Question 10

Embryo HEAD fonding

Answer 10

After folding

Question 11

Comparison between head folding before and after

Question 12

Embryo TAIL folding

Answer 12

Embryo LATERAL folding

Question 13

Embryo folding summary

•The entire embryo is covered on the outside by the ectoderm & amniotic cavity surrounds the ectoderm covered embryo
•Endoderm lined yolk sac is incorporated inside the body as a gut tube (foregut, midgut, hindgut), suspended from dorsal body wall by the dorsal mesentery
•The rest of the yolk sac is pushed to outside of the body, but remains continuous with the midgut via vitelline duct
•The connecting stalk is positioned ventrally at umbilicus
•Intraembryonic cavity is separated from the chorionic cavity