Lecture 10: Establishment of Body Plan II

Question 1

Characterize the location of and fate of each of the three areas of mesoderm in the early embryo.

Answer 1

1. Paraxial- closest to the notochord and becomes segmented into somites
2. Intermediate-urogenital system
3. Lateral Plate-internal organs and limbs

Question 2

Distinguish between somitomeres and somites.

Answer 2

somitomeres are segments of the mesenchyme that develop on either side of the neural plate

somites come after the somitomeres and are dense blocks of mesoderm

somitomeres become somites?

Question 3

In relation to the somitomeres, at what position does the first pair of somites form?

Answer 3

The first pair of somites forms behind the 7th pair of somitomeres at the expense of the eighth somitomere

Question 4

What is the relationship of ephrin B, Wnt-6, snail, and paraxis to somite formation?

Answer 4

Ephrin B: cells at the posterior border fo the anterior somite will express which results in the tissue between the two adjacent somites

Wnt-6: expressed by ectoderm dorsal to somites which leads to paraxis in somite

paraxis plus decrease in snail leads to epithelial cell conversion

Question 5

Relate Shh, noggin, Pax1 and Pax9 to the formation of the sclerotome.

Answer 5

The notochord secretes Shh+noggin which leads to am increase in Pax1 and Pax9 in the ventral part of the somite which leads to the sclerotome

Question 6

In the wavefront step of somitogenesis what is the determination front?

Answer 6

The determination front is a balance between FGF-8 and retinoid acid that results in cellular determination

Question 7

What role does the downregulation of Snail play in somitogenesis?

Answer 7

the down regulation of snail causes the mesenchymal cells to convert to epithelial cells

Question 8

In the segmentation clock of somitogenesis describe the role of ephrins in maintaining the intersomitic spaces.

Answer 8

Eph A and B act as a receptor and a ligand that allow the fissure between the two adjacent somites

Question 9

Describe the formation of the dermomyotome and the factors and pathways involved.

Answer 9

Wnt genes are in the dorsal neural tube and transform the dorsal half of the somite into the dermomyotome

Question 10

Describe the separation of the dermomyotome into myotome and dermatome and the
factors involved.

Answer 10

dermomyotome leads to an increased Pax3 Pax7 and paraxis which lead to the separation of the dermomytome into dermatome and ventral myotome

Question 11

List the derivatives from the final subdivisions of the somites.

Answer 11

???

Question 12

List the factors involved in the formation of the intermediate mesoderm and list the derivatives.

Answer 12

Lateral ectoderm expresses BMP and the paraxial mesoderm expresses activin and becomes the intermediate mesoderm that expresses Pax-2

cranial and caudal extent is dependent on Hox4 and Hox11

Question 13

Differentiate between the intraembryonic coelom and the extraembryonic coelom.

Answer 13

intraembryonic coelom separates the mesoderm into two layers

extraembryonic coelom is the gestational sac portion

Question 14

Distinguish between somatic and splanchnic mesoderm.

Answer 14

somatic mesoderm- associates with the ectoderm and contributes to the connective tissues

splanchnic mesoderm- overlies the endoderm and is continuous with the mesoderm; gives rise to the mesothelial covering the organs

Question 15

Describe the formation of the lateral plate mesoderm and distinguish between the somatopleure and the splanchnopleure.

Answer 15

lateral plate splits into the two layers and the portion that was underneath the ectoderm forms the somatopleure

the ventral layer is the splanchnic (visceral) mesoderm and overlies the endoderm forming the splanchnopleure

Question 16

Describe the cardiac crescent and distinguish from the cardiogenic mesoderm; list the factors that induce the formation of the cardiac crescent.

Answer 16

cardiac crescent comes from the cells that are moving through or entering the primitive streak

BMPs and FGFs induce the formation of the cardiac crescent

Question 17

Describe the early formation of the heart, including the pericardial cavity and mesocardium

Answer 17

comes from the proepicardial primordium

Question 18

List the genes important in early heart formation

Answer 18

(Nkx2-5, MEF2, GATA4)

BMPs and FGFs

Question 19

What is the secondary heart field?

Answer 19

splanchnic mesoderm caudal to the outflow tract

Question 20

Describe the derivatives of the cardiogenic mesodermal cells.

Answer 20

splanchnic becomes the heart and the blood vessels and the somatic mesoderm becomes the bones, ligaments, vessels, and connective tissues