Early Vert Development (1)

Question 1

How can asymmetry be established with the egg? with the sperm?

Answer 1

• egg may have asymmetrical distribution of factors (like tunicates)
• sperm makes a point of contact with the egg which is an asymmetrical interaction

Question 2

What is “Nature’s gift to science”?

Answer 2

• C. elegans
• Sydney Brenner
• won Nobel prize in 2002 for his role in establishing C.elegans as a novel experimental model organism

Question 3

What is an important asymmetric characteristic of the nematode?

Answer 3

• 1st cleavage is asymmetric (one cell bigger than the other)

Question 4

What experiment can be done to examine why the 1st cleavage in the nematode is asymmetric?

Answer 4

• genetic screen
• generate a bunch of genetic mutations
• assay: look for loss of asymmetry at 2-cell stage

Question 5

What exactly was done to examine the asymmetric 1st cleavage of nematodes?

Answer 5

• mutagenize: fed chemical N-ethyl-N-nitrosourea in diet
• breed the mutated worm with a normal worm
• screen eggs and see how they divide after fertilization

Question 6

What was found in the genetic screen to examine asymmetry of the 1st cleavae of nematodes?

Answer 6

• several genes were identified and named Par
• Par-1, Par-2, Par-3, all had loss of asymmetry
• Par-4 had a reversal of asymmetry

Question 7

What happens at fertilization in nematodes?

Answer 7

• sperm entry demarkates the posterior region
• Par3 is distributed evenly around the surface
• Mex5 is evenly distributed in the cytoplasm

Question 8

What happens after fertilization in nematodes?

Answer 8

• cytoskeleton rearrangements occur (asters initiated by sperm centrioles)
• pushes Mex5 away from sperm protonucleus
• par2 begins to be distributed around surface by sperm entry however still more par3 than par2, creating asymmetry in cell size

Question 9

What is one role of PAR proteins?

Answer 9

• cellular localization of p-granules (ribonucleoprotein complexes) which are essential for germ cell specification

Question 10

What has been identified for C. elegans?

Answer 10

• cell lineage map

- know that there is a fixed number of somatic cells (1031)

Question 11

Are par proteins conserved throughout evolution?

Answer 11

• yes, seen in humans as well

- they are associated with establishing asymmetry with cytoskeleton but differ from c elegans

Question 12

What is known about Par6?

Answer 12

• adaptor protein

- interacts with other Par proteins and other proteins implicated in cytoskeletal rearrangement

Question 13

What is known about Par6d?

Answer 13

• in mouse

- knockout studies indicate it is not involved in germ cell formation

Question 14

Why is the chick a good model organism?

Answer 14

• easily manipulated in ovo because of large embryos that grow well outside of egg
• similarly with frog

Question 15

Why is the frog a good model organism?

Answer 15

• eggs are synchronous and large
• acquire 1000’s at a time
• can dissect and graft

Question 16

What kind of holoblastic cleavage do frogs undergo?

Answer 16

• mesolecithal
• displaced radial cleavage
• the yolky material is divided into larger cells

Question 17

How long does it take Xenopus laevis to cleavage to blastula stage?

Answer 17

• about 7 hours at room temp

- quick

Question 18

What does the blastula of Xenopus laevis look like?

Answer 18

• animal pole and vegetal pole
• blastocoel (cavity) primarily in animal pole
• looks similar to sea urchin at this stage

Question 19

What happens during gastrulation in Xenopus laevis?

Answer 19

• bottle cells begin to crawl inwards forming the dorsal blastopore lip at a spot between the animal and vegetal poles
• the blastocoel is diplaced and the archenteron (primitive gut) forms
• 3 germ layers form

Question 20

What does the blastocoel become?

Answer 20

• space that internal organs are surrounded by

Question 21

What is formed after neurulation?

Answer 21

• notochord
• neural tube
• somite

Question 22

What is a neurula?

Answer 22

• stage where embryo has brain, gill area, somites, tailbud, and stomodeum

Question 23

What is known about the thyroid hormone?

Answer 23

• tri-iodothyrodine (T3)
• all changes are triggered by thyroid hormone
• can induce metamorphosis prematurely with T3
• or if removed will remain tadpole forever

Question 24

What is different about Eleutherodactylus coqui?

Answer 24

• “direct developers”
• they have a tail that quickly regresses but they bypass the larval stage (tadpole)
• they still undergo metamorphosis events and are dependent on thyroid hormone
• novel cell type not normally found in frogs: “nutritional endoderm” that does not integrate into the embryo

Question 25

What are some general characteristics of Eleutherodactylus coqui?

Answer 25

- large eggs ~3.5 mm | - small frogs

Question 26

What is different about cell division in Eleutherodactylus coqui?

Answer 26

- cell division does not progress through yolky division - leaves whole yolk to exist as its own cell type - "nutritional endoderm"

Question 27

What is the selective pressures/driving force that might have led to direct development in coqui?

Answer 27

- in amniotes: was move from water to terrestrial | - here: eggs laid in leaves

Question 28

What are some important parts of amniotes?

Answer 28

- chorion: outer most membrane, gas exchange, gives rise to placenta in mammals - yolk sac: nutrition (birds/reptiles) - allantois: stores waste, gas exchange - amnion: secretes fluid

Question 29

In humans, what is the yolk sac used for?

Answer 29

- not for nutrients | - immune cells are derived from yolk sac

Question 30

What does "mero" vs "holo" mean?

Answer 30

- mero: partial cleavage | - holo: complete cleavage

Question 31

What kind of cleavage do fish, reptiles and birds undergo?

Answer 31

- meroblastic cleavage, more specifically discoidal cleavage | - disc of cells on top of a yolk sac

Question 32

What is mammalian early cleavage like?

Answer 32

- asynchronous cleavage at the 8 cell stage: cells divide 'randomly' - "compaction": lose rounded appearance (may play role in blastocoel formation) - mediated by changes in cell adhesion properties

Question 33

What is needed for cell adhesion?

Answer 33

- diacyl glycerol and Ca++ --> PKC --> E-cadherin (cell junctions form)

Question 34

What controls compaction?

Answer 34

- cadherin-dependent filopodia control preimplantation embryo compaction

Question 35

What do you see if you label mCherry or e-cadherin with GFP?

Answer 35

- mCherry is membrane localized | - e-cadherin-GFP fusion protein is localized to cell junctions

Question 36

What is ICM?

Answer 36

- inner cell mass - within blastocyst (but not previous stage morula) - gives rise to all three layers and yolk sac?

Question 37

When does the early stage of implantation occur?

Answer 37

- mouse: 4 days after insemination - humans: 6-10 fays after ovulation - this means that early cleavage happens in the oviduct

Question 38

What are the two types of twinning?

Answer 38

- dizygous (fraternal): 2 eggs are released and fertilized | - monozygous (maternal): one egg

Question 39

What characterizes early monozygous twinning?

Answer 39

- ~25%: early separation leading to two separate embryos with their own inner cell mass - 2 chorions and 2 amnions

Question 40

What characterizes mid monozygous twinning?

Answer 40

- - ~60-70%: mid stage separation leading to one embryo with two inner cell masses that separate after the chorion is made - 1 chorion, 2 amnions

Question 41

What characterizes late monozygous twinning?

Answer 41

- ~1-2% - embryo with one inner cell mass - embryos separate after chorion and amnion are made - 1 chorion and 1 amnion

Question 42

What is interesting about Dasypus novemcintus?

Answer 42

- exhibit polyembryony: more than one embryo from one egg - always 4 embryos - share a single chorion and amnion - could be because each embryo takes up a lot of room with its shell and sharing a chorion and amnion allow for more offspring