Development of endoderm and mesoderm derivatives

Question 1

What is the location of the endodermal germ layer?

Answer 1

Covers the ventral surface of the embryo and forms the roof of the yolk sac

Question 2

What is the main organ system derived from the endoderm?

Answer 2

Epithelial lining of the primitive gut (GIT tract)

Question 3

What are some other derivatives of the endoderm?

Answer 3

1) Epithelial lining of the respiratory tract
2) Parenchyma of the thyroid, parathyroids, liver and pancreas
3) Reticular stroma of the tonsils and the thymus
4) Epithelial lining of the urinary bladder and the urethra
5) Epithelial lining of the tympanic cavity and auditory tube

Question 4

What are the two types of folding that occur in the development of the primitive gut?

Answer 4

Craniocaudal/cephalocaudal and lateral folding

Question 5

What occurs during craniocaudal folding?

Answer 5

Ventral movement of head and tail regions to curve the fetal position due to lengthening of the neural tube

Question 6

What happens during lateral folding?

Answer 6

Ventral movement of two lateral body wall folds to close the ventral body wall

Question 7

What are the results of such folding?

Answer 7

1) The folds pull the amnion down such that the embryo lies within the amniotic cavity
2) The ventral body wall closes completely except for the umbilical region where the connecting stalk and yolk sac duct remain attached
3) A continuously larger portion of the endodermal germ layer is incorporated into the body of the embryo to form the gut tube
4) Partial incorporation of the allantois into the body of the embryo where it forms the cloaca

Question 8

What is the fate map during gastrulation?

Answer 8

The ultimate fates of the epiblast cells that are decided according to the region of the primitive streak that the epiblast cells migrate and ingress through.

Question 9

Cells that ingress through the cranial region of the primitive node become?

Answer 9

Prechordal plate and notochord

Question 10

Cells that ingress through the lateral edges of the node and from the cranial end of the streak become?

Answer 10

Paraxial mesoderm

Question 11

Cells migrating through the midstreak region become?

Answer 11

Intermediate mesoderm and cardiogenic mesoderm

Question 12

Cells migrating through the more caudal part of the streak form?

Answer 12

Lateral plate mesoderm

Question 13

Cells migrating through the caudalmost part of the streak become?

Answer 13

Extraembryonic mesoderm

Question 14

What is the notochord?

Answer 14

Invagination of the prenotochordal cells in the primitive node

Question 15

How is the notochordal plate formed?

Answer 15

When prenotochordal cells become intercalated in the hypoblast

Question 16

What connects the amniotic and yolk sac cavities?

Answer 16

Temporary neurenteric canal

Question 17

How is the definitive notochord formed?

Answer 17

Proliferation of the notochordal plate and detachment from the endoderm

Question 18

What are the two components of the paraxial mesoderm?

Answer 18

Somitomeres/head mesoderm and somites

Question 19

Where do somites originate from?

Answer 19

The occipital region caudally

Question 20

Where do the first pair of somites originate from?

Answer 20

Occpital region of the embyro at approx 20th day of development

Question 21

What happens after the first pair of somites appear?

Answer 21

New somites appear in the craniocaudal sequence at a rate of three pairs per day until the end of the fifth week where there are 42-44 pair present

Question 22

How can somites tell the age of an embryo?

Answer 22

Because somites appear with a specified periodicity so we can count somites to tell the age of the embryo

Question 23

What are the walls that each somite has?

Answer 23

1) Ventral and medial walls (ventro-medial portion)
2) Dorsal and lateral walls (dorso-lateral portion)

Question 24

What does the ventro-medial portion give rise to?

Answer 24

Sclerotome

Question 25

What are the three regions of the dorso-lateral portion?

Answer 25

A) Dorsomedial edge: muscle cells
B) Ventrolateral edge: muscle cells
C) Cells between these two edges: dermatome

Question 26

What is the intermediate mesoderm?

Answer 26

A structure that temporarily connects paraxial mesoderm with the lateral plate

Question 27

What urogenital structures does the intermediate mesoderm differentiate into?

Answer 27

1) In cervical and upper thoracic regions (forms segmental cell clusters)
2) More caudally (forms an unsegmented mass of tissue)

Question 28

What is the lateral plate mesoderm?

Answer 28

Appearance and coalescence of intercellular cavities in the lateral plate

Question 29

What are the two layers of the lateral plate mesoderm?

Answer 29

1) Somatic or parietal mesoderm layer (continuous with mesoderm covering the amnion)
2) Splanchnic or visceral mesoderm layer (continuous with mesoderm covering yolk sac)

Question 30

What is the intraembryonic cavity?

Answer 30

Formed after dividing lateral plate mesoderm into two layers

Question 31

Where is the somatic/parietal layer located?

Answer 31

Lines the intraembryonic cavity and together with the overlying ectoderm it forms the lateral body wall folds

Question 32

What does the somatic or parietal layer form?

Answer 32

1) Dermis of the skin in the body wall and limbs
2) Bones of the limbs
3) Connective tissue of the limbs
4) Sternum

Question 33

What are the two cells that the somatic or parietal layer receives?

Answer 33

1) Sclerotome cells from costal cartilages
2) Muscle precursor cells from limb muscles and most of the body wall muscles

Question 34

Where is the visceral or splanchnic layer of the lateral plate mesoderm located?

Answer 34

Covers the yolk sac (primitive gut)

Question 35

What does the visceral/splanchnic layer form?

Answer 35

Together with embryonic endoderm forms the wall of the gut tube. Thin serous membrane around each organ.

Question 36

Which germ layer gives rise to blood and blood vessels?

Answer 36

Mesoderm

Question 37

Through which two ways do blood vessels form?

Answer 37

1) Vasculogenesis - vessels arise from blood islands
2) Angiogenesis - vessels entails sprouting from existing vessels

Question 38

Where do the first blood islands appear?

Answer 38

In the mesoderm surrounding the wall of the yolk sac at 3 weeks development. Slightly later in lateral plate mesoderm and other regions.

Question 39

Where are definitive hematopoietic stem cells formed?

Answer 39

1) Aortagonad-mesonephros region (AGM)
2) Liver
3) Bone marrow