7: Xenopus

Question 1

Xenopus is what

Answer 1

a tetraploid… no genetics
not genetic models

cell tissue transplantation and grafting is easily done

commmonly known as african clawed toes frogs

fully aquatic, easy to keep

large frog eggs, produced in large quantities and easy to manipulate

Egg production stimulated by injection of chorionic gonadotropin (HcG), used to be a simple pregnancy test since the urine of pregnant women contains chroionic gonadotropin

Question 2

since fro embryogs develops externally

Answer 2

allows experiments to be performed prior to or folloiwing fertilization
rapid embryo growth means that the tadpole hs fully functioning organs within a few days

Question 3

Does mature xenopus eggs have distinct polarity?What distinguishes it

Answer 3

yes
Animal region: Dark : cells here during embryogenesis divide rapidly and become actively mobile, sperm enters egg in animal region
Vegetal region: Pale yolky: bottom portion, serves as food for the embryo, divide more slowly and undergo less movement during embryogensis

Question 4

What is the egg enclosed in

Answer 4

vitelline membrane, embedded in gelatinous coat

Question 5

Where on the egg does fertilization generally occur and what significance
does to site of sperm entry have to development?

Answer 5

occurs 45 degrees from animal pole
-point where the sperm entry will become ventral (sperm centriole breaks radial symmetry that was present in unfertilized eggg and determines the dorsal ventral axis

Question 6

What is a slow block to polyspermy

Answer 6

the formation of the fertilization envelop (occurs when fertilization triggers cortical granule reaction)… involves release of ca from er triggered by IP3

Question 7

When does the egg reorganize its cytoplasmic contents

Answer 7

45-90 mins after fertilization in a g2 like period of the FIRST CELL CYCLE

Question 8

Cortical rotation

Answer 8

-when fertilization occurs, the outs layer (cortex) of the egg loosens from the inner dense yolky core
-midway through the first division, the coretex begins to rotate relative to the core (CORTICAL ROTATION) continues to the eind of the 1st cycle
Result: 30 degree displacement of the vegetal cortex away from sperm entry towards the future dorsal region
-cortical rotation coicincides with the translocation of maternal dorsalizing activity, from the vegetal pole to the furture dorsal side of embryi

-dependent on parallel arrays of microtubules, plus ends need to be aawy from site of sperm entry
-dorsal activity and cortex move towards plus end (dorsal end)

NO Microtubules means that embryos lack dorsal anterior structures

Question 9

What does the cortex include

Answer 9

is the outer layer
include: plasma membrane of the egg, cytoskeletal componenets, rough er and other components

Question 10

What is the shear zone

Answer 10

following loosening, there is a yolknfree area between the core and cortex calked the shear zone

Question 11

What is the gray crescent and what is the process by which it is formed?
What is the destiny of the gray crescent?

Answer 11

Gray crescent: inner gray cytoplasm that appears following a rotation of the cortical cytoplasm with respect to the internal cytoplasm in the marginal region of the 1-cell amphibian embryo
-Gastrulation starts here

Microtubules appear in the vegetal shear zone (mid thru 1st cycle)
-both cortical rotation and the translocation of dorsal determinants are dependent on PARALLEL ARRAYS OF MICROTUBULES
-as a result of cortical rotation, a band of inner gray cytoplasm can be observed

Question 12

What is responsible for dorsal anterior structures

Answer 12

parallel arrays of microtubules that cause cortical rotation and translocation of dorsal determinants

if mts are destroyed, embryos lack dorsal anterior structures

where is the plus end of the mt: away from site of sperm entry (dorsal side) since sperm enter ventral side

Dorsal specifciation requires not just the activity but the translocation of the dorsalizing activity towards the marginal zone from vegetal region… it is only activated when relocated

Question 13

What breaks the radial symmetry of the ambphian egg

Answer 13

cortical rotation, specifying the orientation of the embryonic body axis

this is because when the sperm enters through the animal hemisphere, the outer cortex rotates in 30 degree angle perpendicular to the animal vegetal axis, and this causes dorsal determinants to move from vegetal pole to more dorsal posiition

Question 14

Is there a cortical granule reaction

Answer 14

yes, it is triggered by fertilization and the vitillene envelope is changed to a fertilization envelop because of this

Question 15

What pattern of cleavage is observed in the Xenopus embryo?

Answer 15

Radially Symmetric and Holoblastic

cleavage slower in the vegetal hemisphere of since lost of yolk (mesolecithal so yolk is centered in one region, unevenly distributed)

second cleave in animal may happen before 1st cleavage in vegetal is finished

first two cleaveage planes are MERIDONIAL (Vertical) 3rd is equitoral (horizontal) but displaced more twrds animal pole

many smaller cells in animal region, and fewer yold enriched macromeres in vegetal region

Question 16

What purposes might we attribute to the Xenopus blastocoel?

Answer 16

1) Permits cell migration during gastrulation
2) Prevents early cell-cell interactions

blastula (128 cells)

Question 17

When does the mid-blastula transition occur and what molecular events are
associated with zygotic gene activation in Xenopus?

Answer 17

TIming affects by nuclear/cytoplasmic ratio (titration of some unknown factor from the egg cytoplasm by increasing chromatin content

Molecular events:
-DNA demethylation of promoters that control early zygotic genes
-histone methylation assist zygotic gene transcription
-degredatiion of maternal molecules
-blastomeres become motile

Question 18

What are the very first signs of gastrulation in Xenopus that can be observed
externally? What event is happening internally that also signals the initiation
of gastrulation?

Answer 18

Endoderm suorrounded by mesoderm, covered by exctoderm
-epidermis, neural plate, blastopore, endoderm

Archaentron formation signals the initiaiton of gastrulaton internally

Gatrulation is intiated by two movements:
1) Epiboly
2) Vegetal rotation

Question 19

What is the mariginal zone? What is vegetal rotation

Answer 19

marginal zone:
in amphibians, is theplace where gastrulation begins. Region surrounding the equator of the blastula, vehere the animal and vegetal hemisphere meet

Vegetal rotation: During gastrulation, the internal cell rearrangement place the pharyngeal endoderm cells adjacent to the blastocoel and immediately above the involuting mesoderm

Question 20

Epiboly and Radial intervalation

Answer 20

: refers to the omvement of cells over another layer/ surface of embryo
:driven by radial cell intercalation
ectoderm thins expand ventrally over the endoderm
: Driven by chemoattractant that pulls DEEP cells towards Superficial layer

radial intercalation:
Radial intercalation is a type of cell movement during embryonic development in which cells from different layers of a tissue merge into a single, continuous sheet, thinning and spreading the tissue in the process. It plays an important role in processes like epiboly and gastrulation to expand and reshape tissues.

Question 21

What is gross tissue movement =

Answer 21

sum of all cell movement