exam 2- polarity and simple embryo development

Question 1

name the 3 major body axes

Answer 1

1- dorsal-ventral (dorsal top, ventral bottom)
2- anterior-posterior (anterior head, posterior tail)
3- lateral/left-right

Question 2

briefly describe axis formation and polarity

Answer 2

axis formation is an orchestration of several events

• breaking the radial symmetry of the egg (entry of sperm)
• establishment of the future dorsal/ventral axis

Question 3

describe a fucus (brown algae)

Answer 3

fucus produces free-floating eggs, fertilized by motile sperm
- after fertilization, the zygote attaches itself to a rock and begins embryogenesis
- the fucus egg has no cell wall and is apolar

Question 4

describe a fucus zygote

Answer 4

egg is fertilized (usually additional signal from outside, ex: light) –> starts to specify zygote as unique feature on one side of it (anterior & posterior) this polarity happens before cell division
- rhizoid is where sperm entered (rhizoid on bottom), thallus on top

• embryo has polarity (sperm entry establishes polarity)
• after fertilization –> cell division

Question 5

describe primary axis development in a fucus

Answer 5

the first sign of polarity in the zygote occurs within minutes of fertilization (as a patch of F-actin accumulates at the site of sperm entry)

• in the absence of polarized environmental cues, the sperm entry site will become the rhizoid pole of the zygote

Question 6

how do environmental cues alter polarity in fucus

Answer 6

environmental signals, such as light, initiate signaling allowing calcium to flow into cell –> calcium ions flow thru organism and set up another gradient of polarity –> change internal structures –> cell division

Question 7

while sperm entry entry establishes initial polarity in zygote fucus, what can change it?

Answer 7

environmental cues can change in
- these include directional light, gravity, water currents, and temp
- polarity is not fixed until the cell wall is laud down (cell wall then involved in maintaining polarity)

Question 8

polarity axis fixation in fucus

Answer 8

several hours after fertilization (sperm entry and environmental cues), the longitudinal axis (A-P) becomes fixed, the positions of the future thallus and rhizoid cannot be changed by external cues

• this involves interactions b/w cytoplasm and cell wall
• axis stabilizing complex, actin filaments, and substances in the cell wall

Question 9

cell fate can be switched by cell wall contact in fucus

Answer 9

if destroy rhizoid cell and cell wall –> cannot generate rhizoid because no signal (signal in cell wall) to make it anymore (already committed) –> continue to make a thallus (which is ok, but have no place to hold onto things)

Question 10

why is learning about fucus polarity traits impt?

Answer 10

• common polarity traits are found in many organisms
• sperm entry is often initial determinant of polarity (if all else fails, go back to this determination spot)
• environmental cues, including hormones, can and often do override that polarity to establish their own axis (usually erase initial polarity, but it’s there as default)
• polarity must be maintained until fixed

Question 11

benefits of looking at sea urchin embryo development

Answer 11

free living embryo
sequenced genome
can be easily treated and their development observed (not in utero)

Question 12

rapid events in early development of sea urchin

Answer 12

calcium as an early signal

• widespread role as an intracellular messenger
• functions by binding to and influencing activity of calcium binding proteins- troponin C, calmodulin

Question 13

___ and ___ are involved in sea urchin fertilization

Answer 13

G proteins and calcium ions

Question 14

gastrulation…

Answer 14

during gastrulation, cells are repositioned in relation to each other and the 3 primary germ layers are formed:
1- ectoderm
2- mesoderm
3- endoderm

Question 15

germ layer: ___, the innermost layer, will form the ___

Answer 15

endoderm
gut

Question 16

germ layer: ___, in the middle, will form the ___

Answer 16

mesoderm
muscles, circulatory system, blood, and many diff organs

Question 17

germ layer: ___, the outermost, will form the ___

Answer 17

ectoderm
skin and nervous system

Question 18

the 3 major events in gastrulation

Answer 18

1- 3 primary germ layers established
2- basic body plane established, including primary body axes
3- cell movement occurs allowing new interactions with neighboring cells that weren’t initially nearby (paves way for beginning of organogenesis)

Question 19

name 5 types of cell movement during gastrulation

Answer 19

1- invagination
2- involution
3- ingression
4- delamination
5- epiboly

Question 20

type of cell movement during gastrulation: ___, in-folding of a cell sheet into the embryo (sea urchin endoderm)

Answer 20

invagination

Question 21

type of cell movement during gastrulation: ___, in-turning of a cell sheet over the basal surface of an outer layer (amphibian mesoderm)

Answer 21

involution

Question 22

type of cell movement during gastrulation: ___, migration of individual cells into the embryo (sea urchin mesoderm)

Answer 22

ingression

Question 23

type of cell movement during gastrulation: ___, splitting or migration of one cell sheet into two sheets

Answer 23

delamination

Question 24

type of cell movement during gastrulation: ___, expansion of one cell sheet over other cells

Answer 24

epiboly

Question 25

sea urchin cleavage

Answer 25

lots of cell divisions

• in the vegetal tier, there is an unequal equatorial cleavage to produce 4 large cells (the macromeres), and 4 smaller micromeres at the vegetal pole

Question 26

cell lineage or fate map of sea urchin emrbyo: what do animal and vegetal poles become?

Answer 26

the early distinct animal pole cells become mesomeres and ectoderm

vegetal pole half is more complex- ex: veg1 tier gives rise to both ectodermal and endodermal lineages

Question 27

what is necessary for endoderm specification in sea urchin embryo?

Answer 27

micromeres, specifically beta-catenin in micromeres

• B-catenin appears to be responsible for specifying the micromeres (and their ability to induce neighboring cells)
• B-catenin is a TF activated by the Wnt pathway (beta-catenin functions through signal cascades)
• this works by NUCLEAR LOCALIZATION of B-catenin

Question 28

beta-catenin accumulates in the nuclei of cells fated to become ___ and ___

Answer 28

endoderm and mesoderm

Question 29

if nuclear localization of beta-catenin is prevented, what happens?

Answer 29

micromeres fail to become specified

Question 30

how can we track B-catenin expression in a sea urchin embryo?

Answer 30

by staining with fluorescently labeled antibody (protein labeling)

Question 31

what happens if there is overexpression of B-catenin during sea urchin embryo development?

Answer 31

beta-catenin in ALL nuclei –> too much endoderm and mesoderm

Question 32

what happens if under/non-expression of beta-catenin during sea urchin embryo development?

Answer 32

beta-catenin in NO nuclei –> all ectoderm