exam 2- polarity and simple embryo development Flashcards
name the 3 major body axes
1- dorsal-ventral (dorsal top, ventral bottom)
2- anterior-posterior (anterior head, posterior tail)
3- lateral/left-right
briefly describe axis formation and polarity
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
describe a fucus (brown algae)
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
describe a fucus zygote
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
describe primary axis development in a fucus
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
how do environmental cues alter polarity in fucus
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
while sperm entry entry establishes initial polarity in zygote fucus, what can change it?
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)
polarity axis fixation in fucus
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
cell fate can be switched by cell wall contact in fucus
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)
why is learning about fucus polarity traits impt?
- 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
benefits of looking at sea urchin embryo development
free living embryo
sequenced genome
can be easily treated and their development observed (not in utero)
rapid events in early development of sea urchin
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
___ and ___ are involved in sea urchin fertilization
G proteins and calcium ions
gastrulation…
during gastrulation, cells are repositioned in relation to each other and the 3 primary germ layers are formed:
1- ectoderm
2- mesoderm
3- endoderm
germ layer: ___, the innermost layer, will form the ___
endoderm
gut
germ layer: ___, in the middle, will form the ___
mesoderm
muscles, circulatory system, blood, and many diff organs
germ layer: ___, the outermost, will form the ___
ectoderm
skin and nervous system
the 3 major events in gastrulation
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)
name 5 types of cell movement during gastrulation
1- invagination
2- involution
3- ingression
4- delamination
5- epiboly
type of cell movement during gastrulation: ___, in-folding of a cell sheet into the embryo (sea urchin endoderm)
invagination
type of cell movement during gastrulation: ___, in-turning of a cell sheet over the basal surface of an outer layer (amphibian mesoderm)
involution
type of cell movement during gastrulation: ___, migration of individual cells into the embryo (sea urchin mesoderm)
ingression
type of cell movement during gastrulation: ___, splitting or migration of one cell sheet into two sheets
delamination
type of cell movement during gastrulation: ___, expansion of one cell sheet over other cells
epiboly
