Early Vert Development (2)

Question 1

Why might test results say that a mother’s kids are not biologically hers even if they are?

Answer 1

• mother could be a chimera (organism with two distinct cell types)
• father had halotype 1 and 2
• mother had halotype 3 and 4
• the mother was derived from two eggs (3 and 4) and two sperm (1 and 2)
• would typically give rise to fraternal twins but the eggs fused together
• blood shows (1 and 3) but germline show (2 and 4) making all offspring (2 and 4)

Question 2

What is an interesting case of twinning?

Answer 2

• twins: a phenotypic male and a hermaphrodite
• both shared 100% maternal alleles and 50% paternal alleles
• this means they came from 1 egg and 2 sperm
• pathenogenetic activation and cleavage of ovum then fertilization by two sperm or 3n egg that later split to 2 2n eggs

Question 3

What study was conducted by Wilhelm Roux in the late 1800’s?

Answer 3

• at the 2-cell stage, a hot needle was put into one cell to kill it
• the cell died and no longer divided or developed, however the other cell continued to develop normally according to fate map
• he believed this to be evidence towards mosaic development

Question 4

What is a problem with Wilhelm Roux’s experiment?

Answer 4

• he left the dead cell there and it could be having an effect on the living cell
• if the dead cell is removed, you get a proper left and right axis forming
• this provides evidence towards regulative development

Question 5

What occurs if you split cells of mice at the 2-cell stage? At the 4-cell stage?

Answer 5

• 2-cell: both cells can form a new embryo

- 4-cell: do not develop

Question 6

What are the 3 cleavages of an amphibian embryo like?

Answer 6

• mesolecithal: displace radial cleavage
• first two cleavages are longitudinal
• the third cleavage is equatorial (splitting the embryo so that the vegetal pole is larger)

Question 7

How were frog late blastula fate maps formed?

Answer 7

• generated using different dyes and markers

Question 8

What are different layers where the blastopore will form?

Answer 8

• chorda-mesoderm: notochord
• head mesoderm: prechordal plate
• pharyngeal endoderm: pharynx, tongue

Question 9

What occurs during xenopus gastrulation?

Answer 9

• bottle cells form and involution starts which forms the blastopore
• cells crawl to the animal pole

Question 10

Where do the axis lie in the gastrula of the xenopus?

Answer 10

• blastopore is posterior (anus) opposite to pharyngeal endoderm that is now anterior
• animal pole is dorsal and vegetal pole is ventral

Question 11

How is ectoderm regionalized after gastrulation?

Answer 11

• ventral: epidermis

- dorsal: neural plate

Question 12

What occurs during neurulation?

Answer 12

• neural plate stage to neural tube formation
• neural groove forms and neural fold brings two sides together
• brings us to the neurula stage

Question 13

Does a fate map tell us what cells are at an earlier stage?

Answer 13

• no, it tells us what they will become at the end point

- fate map is more of a guideline since cells can be influenced

Question 14

What type of experiment can we do to see if a cell is specified and determined?

Answer 14

• transplant to see if it changes

- separate cells from embryo and simply observe

Question 15

What was the golden age of embryology?

Answer 15

• early 1900’s
• addressed fundamental questions underlying vertebrate body plan formation
• “simple” questions and “simple” experiments
• used large embryos and simple tools
• amphibian embryos great for physical manipulation (big and heal fast)

Question 16

What experiment did Hans Spemann do?

Answer 16

• transplant neural tissue to epidermis during early gastrula stage: became epidermis (also happens if grown in isolation)
• transplanted neural tissue to epidermis during late gastrula stage: became neural tissue
• sometime between these two times the neural tissue fate became determined

Question 17

Following Hans Spemann experiment, what hypothesis might one come up with?

Answer 17

• the underlying mesoderm has an instructive role in the formation of neural tissue

Question 18

What is Spemann/Mangold’s dorsal lip transplantation experiment?

Answer 18

• remove the dorsal lip (fated to become dorsal mesoderm) from one embryo and transplant it to where epidermis is likely to form (opposite to dorsal lip)
• results in embryo with a twinned axis
• some cells of host changed their fate to become neural tissue

Question 19

What was the dorsal lip named?

Answer 19

• Spemann’s organizer

- dorsal lip-axis inducing property

Question 20

What might the molecular mechanism underlying the Spemann Organizer be?

Answer 20

• neural inducing factor

- without the neural inducing factor, the default state of the ectoderm is epidermis

Question 21

What conclusions can be made about the neural induction/ectoderm regionalization?

Answer 21

• neural tissue is not determined until late gastrulation
• default state of the ectoderm is skin (epidermis)
• the dorsal lip of the blastopore is sufficient to induce secondary axis formation and neural tissue from ectoderm that had been fated to become epidermis

Question 22

What study did Nieuwkoop conduct in 1963?

Answer 22

• he removed the animal cap and cultured it in isolation: this resulted in epidermis
• he also removed the animal cap and dissociated it, then reaggregated it which resulted in neural tissue

Question 23

What molecular mechanism combines Nieuwkoop’s results and Spemann’s organizer experiments? **

Answer 23

• if you add LiCl to an blastula, you get dorsalized embryos (only neural ectoderm, no epidermis)
• used subtractive hybridization to identify mRNAs enriched in dorsal tissue
• noggin

Question 24

Where is noggin mRNA localized?

Answer 24

• dorsolateral region
• prechordal mesoderm
• notochord

Question 25

What does noggin do?

Answer 25

• inhibits bone morphogenetic protein BMP signalling

- antagonist: prevents BMP from binding to receptors

Question 26

What is BMP?

Answer 26

• member of the TGF-beta super family of signalling molecules

Question 27

What is the TGF-B pathway?

Answer 27

• Activin or a TGF-B ligand attaches to receptors II and I
• activates Smad
• dimerization of Smad (Smad can now enter nucleus)
• transcription occurs