Cleavage pt II (start for Exam II)

Question 1

describe first and second cleavages of amphibians (mesolecithal yolk) - what is the effect of the dense yolk?

Answer 1

1st/2nd cleavages are meridional, forrow meets resistance from yolk-rich cytoplasm in veg. hem.

divisions in veg. hem lag behind those in an. hem.

ex: 2nd cleavage starts at A pole before first finishes cutting thru V hem.

actin/myosin contractile ring complex needs to spend more time cutting through the yolk as it is much more dense than the animal hem.

Question 2

describe third and fourth cleavages for mesolecithal yolk eggs

Answer 2

3rd is equatorial, but unequal such that 4 small animal blastomeres + 4 large vegetal blastomeres result.

4th is meridional = 16 cells, embryo is in the “morula” stage (looks like a mulberry), will be called this for next several cleavages

Question 3

is the fifth cleavage meridional or equatorial?

Answer 3

fifth is equatorial (cleavages continues on like this for a while)

Question 4

when is the blastula formed? where is the blastocoel with respect to the hemispheres?

Answer 4

blastula forms at 128 cell stage. blastocoel is displaced towards animal pole bc V blastomeres are so large

Question 5

what are the blastomeres of the animal and vegetal hemispheres referred to during this period (think size)?

Answer 5

more numerous + smaller blastomeres in A hem are micromeres

less numerous + larger blastomeres in V hem are macromeres

Question 6

describe the features of the blastula (i.e. relative “structure” of the embryo)

Answer 6

consists of “epithelial-type” layer, outermost cells are polarized (different on one end vs. other), have tight junctions between them connecting lateral mems + toward the outside.

also have interconnecting desmosomes.

Question 7

what are the roles of tight/gap junctions and desomosomes?

Answer 7

tight junctions - really pinch membrane so that nothing can get into the embryo (located along top of embryo) - also helps to create a differential for the internal embryonic enviro vs. external areas.

gap junctions - allow for the flow of ions (and have been shown to allow flow of oligonucleotides and possibly small RNAs as a result)

desmosomes - keep structural integrity of embryo and allow blastomeres to resist mechanical forces pushing against embryo.

Question 8

describe the first two cleavages of holoblastic spiral cleavages - in which organisms + what yolk distribution does this occur in?

Answer 8

1st two cleavages run parallel to A-V axis, first 4 blastomeres that result are tightly interconnected (to expose minimized surface area), forms tetrahedron

subsequent blastomeres have same tendency

occurs in isolecithal egg animals like snails and certain worms

Question 9

what is a result of the next cleavages in holoblastic spiral?

Answer 9

4 large blastomeres (macromeres) will bud off quartets of smaller blastomeres (micromeres) toward animal pole.

first set of micromeres is rotated (e.g. clockwise) w/ respect to macromeres (contributed to by rotation and offsetting of mitotic spindles)

Question 10

what happens to the next set of micromeres? are divisions equal?

Answer 10

next set of micromeres is budded off in a counterclockwise orientation. no, divisions are not equal.

Question 11

what is one reason spiral cleavage is termed “spiral”? describe the adherence of cells and its relation to the blastula.

Answer 11

can trace lineage up + wrapping around embryo in a spiral pattern (as a result of offsetting cleavages)

cells are very tightly adhered to each other to minimize exposed surface area - as a result, blastocoel is very minimal (or sometimes there simply isnt one at all, due to dense packing of cells)