3. Frog Development

Question 1

Amniotes

Answer 1

can survive on land but fetilization and devlopent happens in water

Question 2

Why frogs as a model

Answer 2

Advantages
• Cells and eggs are large.
• Rapid initial development.
• Transplantation Experiments.

Disadvantages
• Long life cycle before maturation.
• Mutagenesis difficult due to multiple genes

Question 3

frog egg type and clevage type

Answer 3

mesolithical

Unequal radial, holoblas:c cleavage

Question 4

Where does sperm enter and why (hemisphere and axis)

Answer 4

at aminal hem (ventral axis) bc there;s less yolk to impede movement

Question 5

Mixing after sperm entry

Answer 5

• inner cytoplasm rotates by 30º always
• mixing of cytoplasm
• results in formation of ‘gray crescent’
• this is cause the sperm enters and shifts the cytoplasm
• due to shifting of pigment in A.Hem
• Rotation is microtubule dependent

Question 6

Experminent:

grey crescent

Answer 6

• cleavage –> 2 cells
• gray crescent distribution equally between 2 blastomeres
• develops into 2 tadpoles
• if you force grey crescent so its only in 1 blastomere
• one with grey cres. forms tadpole
• no grey cres only forms tissue of belly piece (ventral piece)
• therefore grey cres. drives process for dorsal processes that’s missing in the belly piece

Question 7

Experiment:

UV radiation

Answer 7

• expose v.hem (yellow) to UV light
• as reult you cntrl coritcal rotation depending on UV intensity
• stronger UV = no/less rotation of cortical
• only forms belly piece (ventral tissue)
• some rotation 10º some doarsal sturcture formation
• only complete formation if 30º attainined

Question 8

Experiment:
UV radiation problems:
1) is cortical rotation inhibited and there’s no/improper mixing of the cytoplasm, or
2) uV is destroying the contents of the V.hem

Answer 8

• put in gelatin
• gelatin polymerized tends to freeze (solidify) cortical cytoplasm
• tilted egg B of slope, Egg A not tilted
• take out embyo’s and allow to grow
• egg A no development, egg B normal development
• therefore, UV isn’t degrading contents of V.hem
• UV just prevents mixing

Question 9

Cleavage in frog: overview from start to blastocoel

Answer 9

• holoblastic radial cleavage
• before 1st meriodnal clevage ends the 2nd meriodonal starts 90º away.
• 3rd is equatorial displaced to A.hem due to less yolk
• forms morula ‘mulberry’
• absorbs water –>blastocoel in A.hem

Question 10

The role of blastocoel is to

Answer 10

• Permit the movement of cells within the developing
embryo
• Prevent the exposure of vegetal cells to ectoderm (could be secreting conditional molecules with conditional specification

Question 11

Where is dorsal side of embyo

Answer 11

•180º from sperm entry (in the vegetal cytoplasm,
below the grey crescent, and on the putative midline of the embryo)
•called Nieuwkoop center –> “dorsal determining” centre
•effects tissue above to form dorsal blastopore lip (will form future anus)
•dorsal side is NB! (notochord, etc.)
•if transplant N. center to ventral side, it will change the cells around it and form a new dorsal axis

Question 12

Experiment:
4-celled embryo is separated into halves such that only one half contains the Nieuwkoop centre
OR
transplant N. center to ventral side

Answer 12

the half without the Nieuwkoop centre forms ventralized embryo, and the other half with the entire Nieuwkoop centre, is dorsalized embryo.

If transplant N. centre to ventral side, it will change the cells around it and form a new dorsal axis. therefore 2 dorsal sides.

Transplantation of “dorsal” vegetal blastomeres can
“rescue” irradiated embryos, or induce a twin embryo with an entire new body axis

Question 13

Start of gastuation, where, cell movements

Answer 13

Initiates in the Grey Crescent (marginal cells of the Vegetal and the Animal
hemisphere opposite to the sperm entry)
•cells change shape, oblong form bottle cells
•start pinching and moving inside (invag)
•invag is very breif, just enough to form lip
•dorsal blastopore formed then cells start involuting inside
•Bottle cells push pharyngeal endoderm first then precursor for head mesoderm and
chordamesoderm cells into the blastocoel.

Question 14

Frog Mid-Gastrula to late

Answer 14

Mid
•Endodermal cells involute from the blastopore lip and an archentron is formed. Sift in the location of AP
•Outer layer of cells from the ectoderm move via epiboly displacing the Blastopore lip towards the vegetal region.
•Aquires ventral and lateral lips as well
•yolk plug, is eventually covered by ectodermal cells and gets internalized

Late
•archenteron is formed and gets rid of blastocoel
•now mesodermal cells start involution
•Blastocoel is obliterated and the three germ layers are formed (via massive rearrangement of cells, only a small bit if left)