Lecture 11: Establishment of Body Plan I

Question 1

Formation of Neural Tube:

Answer 1

• Formation of thickened neural plate:
• Columnarization of midline ectodermal cells overlying notochord:
• Involves microtubules
• Shaping of neural plate:
• Explain the process of convergent extension.
• What role does planar polarity play?

Question 2

Folding of neural plate:

Answer 2

• Median hinge point:
• Inducedbynotochord
• Involves changes in shape of columnar cells to pyramidal-shaped
cells
• Lateral hinge point:
• Elevation of neural folds:
• Duetopushinginwardbyexpandingnon-neuralepithelium

Question 3

Formation of neural tube:

Answer 3

• Due to fusion of apical surfaces of neural folds
• Mediated by glycoconjugates
• Begins midway along neural tube (21-22 days)
• Extends cranially and caudally
• Anterior and posterior neuropores at day 23

Question 4

Tripartite Brain, Pentapartite brain

Answer 4

see physiology lectures, or your notes. :)

Question 5

Three organizing (signaling) centers:

Answer 5

• Isthmicorganizer: 
• Wnt-1 and FGF-8
• Anterior neural ridge:
• Important in organizing telencephalon, parts of diencephalon,
olfactory area, and pituitary gland 
• ShhandFGF-8
• Zonalimitans:
• Organizes border between dorsal and ventral thalamus • Shh

Question 6

Segmentation genes:

Answer 6

• Setupbasicpatternofsegmentationinhindbrain: • Krox20→r3andr5
• Kreisler and Hoxa-1 → r5
• Retinoic acid gradient → r4-r7 • Gbx-2→r1-r3

Question 7

Pattern of Hox gene expression determines cranial nerves and pharyngeal arch derivatives (covered in later chapter):

Answer 7

Note that expression of these genes is stimulated by retinoic acid gradient which initiates expression of Hoxa-1 and Hoxb-1.

Question 8

Review text (page 96) and explain what is meant by the statement that the caudalmost part of the neural plate possesses the properties of a stem cell zone.

Answer 8

…

Question 9

What roles do retinoic acid and FGF-8 play in the segmentation of the spinal cord?

Answer 9

… already answered in questions section

Question 10

Zones of mesoderm

Answer 10

• Paraxial (segmental plate) mesoderm (closest to notochord)
• Intermediate mesoderm
• Lateral plate mesoderm

Question 11

Somitomeres are

Answer 11

initial pairs of segments of mesenchyme (barely discernable as segments) that begin to develop along and on either side of the neural plate in the paraxial mesoderm.

Question 12

New somitomeres are

Answer 12

added caudally as the primitive streak regresses.

Question 13

After 20 pairs of somitomeres have formed (about day 20)

Answer 13

the first pair of somites forms behind (caudal to) the 7th pair of somitomeres.

Question 14

Somites are more dense

Answer 14

blocks of mesoderm that form along the notochord.

Question 15

In a caudal direction, a pair of somitomeres

Answer 15

is transformed into a pair of somites every few hours.

Question 16

As pairs of somites are formed, pairs of somitomeres are

Answer 16

also formed at the anterior end of the primitive node.

Question 17

Because of this transformation and the formation of somites,

Answer 17

11 pairs of somitomeres are kept constant at the causal end of the paraxial mesoderm

Question 18

Mechanisms involved in somitogenesis:

Answer 18

• Wavefront:
• Note balance between the opposing gradients of retinoic acid and
FGF-8 which results in a cessation of somitogenesis. • What is the determination front?
• Segmentation clock:
• Refer to Figure 6.9 and note the roles of oscillating molecules in the Notch pathway that lead to a defining of the anterior and posterior borders of each somite.
• Also, note the role of ephrins in maintaining the intersomitic spaces.

Question 19

History of a somite

Answer 19

• Epithelialstage:
• Epitheliomesenchymal transformation stage
• Separation of the myotome
• Break up of epithelial dermatome in to derma lfibroblasts

Question 20

Dermomyotome:

Answer 20

• This is the dorso-lateral part of a somite.
• Shh signaling from the notochord and Wnt signaling from the dorsal neural tube create a balance that commits the myotome portion to commit to the myogenic lineage.
• 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 sclerotometo produce scleraxis, which causes the anterior and posterior borders of the each somite to form the syndetome which is the precuror of tendons.

Question 21

Sclerotomal breakup

Answer 21

• Sclerotomal portion of each somite breaks up into an anterior and a posterior portion.
• Posterior portion of one somite joins the anterior portion of an adjacent somite, etc.
• This creates gaps that allow nerves to grow out from the neural tube into the epaxial segmental musculature derived from the myotomes.

Question 22

Intermediate mesoderm

Answer 22

The future intermediate mesoderm responds to BMP (from lateral ectoderm) and activin (from paraxial mesoderm) and becomes intermediate mesoderm.
• Intermediate mesoderm is associated with the formation of the pronephros and, later, the mesonephros.

Question 23

Heart Formation

Answer 23

• Examine Figure 6.14(A) and note the movements of future heart-forming cells through the primitive streak.
• Note the shape and position of the cardiac crescent in Figure 6.14(B).
• Figures 6.14(C), 6.15, 6.16, 6.17 and 6.18 will be used in class to demonstrate the formation of the heart through the 4-somite stage.
• Figure 6.19 will be used in class to demonstrate the formation of the blood islands.