Lecture 10: Body Plan Development II

Question 1

Characterize the location of and fate of each of the three areas of the mesoderm

Answer 1

Paraxial mesoderm- becomes segmented into somites
Intermediate mesoderm- gives rise to urogenital system
Lateral plate mesoderm- internal organs

Question 2

Distinguish between somitomeres and somites

Answer 2

Somitomeres are initial pairs of mesenchyme on either side of the neural plate in paraxial mesoderm

Somites are dense blocks of mesoderm that form along the notochord, transformed from somitomeres

Question 3

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

Answer 3

Forms behind/caudal to the 7th pair of somitomeres at the expense of the 8th pair of somitomeres

Question 4

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

Answer 4

ephrin B: on posterior border, prevent two adjacent somites from mixing (ephA anterior)
Wnt-6: stimulates expression of TF paraxis
Snail downregulated & expression of paraxis leads to transformation of mesenchymal cells into epithelial cells

Question 5

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

Answer 5

The wavefront is also referred to as the determination front because there is a balance between FGF-8 and retinoic acid that results in cellular determination toward somitogenesis– balance of opposing gradients

Determination front is a developmental threshold that prepares the RA and FGF concentrations for segmentation/somitogenesis

Question 6

What role does the downregulation of snail play in somitogenesis?

Answer 6

dec. in Snail leads to mesenchymal cell to epithelial cell conversion

Question 7

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

Answer 7

Cells at the anterior border of a somite express Eph A (receptor) and cells on the posterior border of the anterior somite will express Eph B (ligand), resulting in a fissure between the two adjacent somites

Question 8

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

Answer 8

Under the influence of secreted products of Wnt genes produced by the dorsal neural tube and Shh signaling from the notochord, the dorsal half of the epithelial somite becomes transformed into the dermomyotome and expresses its own characteristic genes (Pax3, Pax7, paraxis)

Question 9

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

Answer 9

Noggin inhibits BMP-4, which would normally inhibit myogenesis

BMP-4 produced by the lateral plate suppresses myogenesis in the ventrolateral dermomyotome and stimulates cells from this area to migrate from the somite into the limb bud

FGF from the myotome signals the sclerotome to produce scleraxis, which causes the anterior and posterior borders of each somite to form the syndetome which is the precursor of tendons

Question 10

List the derivatives from the final subdivisions of the somites

Answer 10

Ventral Sclerotome: vertebral bodies and their intervertebral disks
Lateral Sclerotome: distal ribs, some tendons
Dorsal Sclerotome: dorsal part of neural arch, spinous process
Central Sclerotome: pedicles and ventral parts of neural arches, proximal ribs, or transverse processes of vertebrae
Medial (meningotome) Sclerotome: meninges and blood vessels of meninges

Arthrotome: Intervertebral disks, vertebral joint surfaces, and proximal ribs

Dermatome: dermis, blade of scapula

Dorsomedial Myotome: intrinsic back muscles (epaxial)
Ventrolateral Myotome: limb muscles or muscles of ventrolateral body wall (hypaxial)

Neurotome: Endoneurial and perineurial cells

Syndetome: Tendons of epaxial musculature

Question 11

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

Answer 11

BMP from lateral ectoderm
Activin from paraxial mesoderm
- both cause expression of Pax-2 TF
Hox-4 - expressed cranially
Hox-11 - expressed caudally

Question 12

Differentiate between the intraembryonic coelom and the extraembryonic coelom

Answer 12

Initially, the intraembryonic coelom is continuous with the extraembryonic coelom, but as folding is completed in a given segment of the embryo, the two coelomic spaces are separated.
The extraembryonic mesoderm that lines the inner surface of the cytotrophoblast ultimately becomes the mesenchymal component of the placenta.

Question 13

Distinguish between somatic and splanchnic mesoderm

Answer 13

Somatic mesoderm- dorsal layer, associated with ectoderm

Splanchnic mesoderm- ventral layer, associated with endoderm and specified by TF Foxf-1

Question 14

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

Answer 14

Somatopleure- combination of somatic mesoderm and ectoderm

Splanchonopleure- combination of splanchnic mesoderm and endoderm

The lateral plate mesoderm divides into two layers with a body cavity b/w the two (somatic mesoderm and splanchnic mesoderm)… makes a lateral folding process

Question 15

Describe the cardiac crescent and distinguish from the cardiogenic mesoderm

Answer 15

cardiac crescent: U-shaped region of cardiac mesoderm that forms after leaving the primitive streak, these cells are committed to heart-forming pathway

Question 16

List the factors that induce the formation of the cardiac crescent

Answer 16

BMP and FGF families

Question 17

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

Answer 17

Migration of heart-forming cells arising in the epiblast through the primitive streak in a well-defined anteroposterior order
Cells passing through the streak closest to the primitive node form the outflow tract, cells passing through the midstreak form the ventricles, and cells that form the atria enter the streak most posteriorly
After leaving primitive streak, cells become the cardiac crescent

Question 18

List the genes important in early heart formation

Answer 18

Nkx2-5, MEF2, GATA4

Question 19

What is the secondary heart field?

Answer 19

Located in that splanchnic mesoderm on the posteromedial side of the cardiac crescent
Cells anterior part of the second heart field form most of the outflow tract and the right ventricle
Cells from posterior part of the field contribute to the formation of the atria

In contrast, cells derived from the cardiac crescent form the left ventricle and most of the atria and make minor contributions to the outflow tract and right ventricle.

Question 20

Describe the derivatives of the cardiogenic mesodermal cells

Answer 20

From the cardiogenic mesoderm, the heart and great vessels form from bilaterally paired tubes that fuse in the midline beneath the foregut to produce a single tube
Forms cardiogenic plate in splanchnic mesoderm
Forms pericardial cavity b/w the two layers of mesoderm