Cleavage continued (Exam II)

Question 1

describe the basic characteristics of holoblastic rotational cleavage. in which animals does this occur?

Answer 1

occurs as egg is traveling down oviduct

cleavages ~ every 12-24 hrs

divisions are asynchronous (all blastomeres do not divide at exactly same time - not a neat exponential growth situation)

Question 2

describe the first 2 cleavages + 8 cell stage in mammals

Answer 2

cl 1 - meridional

cl 2 - often (not always) rotational - blastomeres divide in 2 different planes - results in cros-wise arrangement of cleavage furrows + blastomeres

8 cell stage - blastomeres undergo compaction, forms solid ball. caused by formation of tight junctions between outer cells, gap junctions form between inner cells.

Question 3

what is the “morula” stage? describe process of cavitation.

Answer 3

morula stage = 16 cell stage, 9-14 outer blastomeres surrounds 2-7 inside blastomeres. last stage before cavitation

cavitation is the process thru which outer blastomeres pump fluid endocytosed from uteris and exocytose it from basal membranes - forms fluid filled cavities.

cavities then merge to form blastocoel

Question 4

what hapens between the 32-64 cell stage? describe state of blastomeres.

Answer 4

embryo acquires blastocyst configuration (modified blastula that occurs in mammals): consists of:

a) outer layer of cells (trophoblast) - participates in formation of placenta
b) inner cell mass (ICM) which forms the embryo

Question 5

what is an important characteristic of ICM cells?

Answer 5

source of embryonic stem cells - are pluripotent (can become any cell type in body) - are NOT totipotent.

Question 6

define pluripotency and totipotency. what are the few cells that are totipotent?

Answer 6

pluripotency - having the potential to differentiate into any cell type in the human body (i.e. the three germ layers)

totipotency - can differentiate into any embryonic or extraembryonic stem cell (e.g. must be able to differentiate into placenta, etc.)

the zygote is the main example of a totipotent cell (as it gives rise to the entire embryo), but it’s believed up to the 4 cell stage that cells are also totipotent.

Question 7

what happens after blastocyst formation?

Answer 7

after 1-2 days of traveling down fallopian tube, blastocyst hatches from zona pellucida (via secretion of degradative enzymes by trophoblast cells)

Question 8

what is permitted by release from the ZP? describe the resulting process.

Answer 8

implantation @ abt 1 week post-fert:

1) contact of trophoblast cells w/ uterine wall cuases them to proliferate, forms 2 layers
2) inner layer = cytotrophoblast (cellular), outer layer = syncytiotrophoblast - fuse to form a syncitium
3) produces enzyme that digests hole in the uterine wall, erodes maternal blood vessels - blood is released, nourishes embryo by diffusion
4) tropho. cells secrete chorionic gonadotropin - disrupts menstrual cycle

Question 9

discoidal meroblastic cleavage - in which species does this occur? describe the initial state of the cytoplasm

Answer 9

species w/ telolecithal eggs (birds, reptiles, many fishes, etc.)

initially, cytoplasm may be uniformly distributed around yolk. upon fertilization, cytoplasm will stream to animal pole, form mound called blastodisc

Question 10

describe the first few cleavages of discoidal meroblastic

Answer 10

first cleavage is vertical (plane of div. perpindicular to surface) - cuts blastodisc in half. stops yolk

second is vertical and perpindicular to first, again is incomplete. result is 4 blastomeres that are continuous w/ yolk mass.

next few cleavages are shallow and vertical cuts (perpindicular to first).

Question 11

describe cleavage #6 and state of affairs after this cleavage

Answer 11

first horizontal cleavage is #6 - creates 2 layers of blastomeres

upper layer + marginal cells of lower layer = enveloping layer

rest of lower layer cells = deep cells

Question 12

what is a result of continued cleavage? what are characteristics of the blastodisc in birds/reptiles?

Answer 12

continued cleavages results in mound of cells atop uncleaved yolk mass.

in fish: no defined blastocoel - rather, small irregular extracellular spaces are formed between deep cells of blastodisc.

in birds + reptiles, blastodisc is quite small in relation to yolk mass. also, birds lose some of their deep cells under layer called area pellucida.

Question 13

describe production of blastoderm + nearby contents

Answer 13

further cleavage produces 10s of 1000s of cells - mass is now called blastoderm

between blastoderm + yolk is a fluid-filled space called subgerminal space

Question 14

describe seperation of layers of the blastoderm, resulting characteristics

Answer 14

separation partly due to detachment of internal cells from overlying cells (called delamination)

a) epiblast - upper germ layer, gives rise to embryo proper
b) hypoblast - lower germ layer, gives rise to extraembryonic endoderm which will later surround yolk.

layers are connected at edges, cavity between epiblast and hypoblast is the blastocoel

Question 15

in what animals does superficial meroblastic cleavage occur? list some basic terminology regarding yolk regions/dividing regions, describe the gist of it

Answer 15

occurs in insects/other arthopods

divisions limited to outermost layer of egg (yolk-free) = periplasm

inner yolk-rich cytoplasm = endoplasm

not true cleavage - cytokinesis is delayed until after multiple rounds of mitoses have occurred + occurs by process of membrane furrowing

Question 16

describe cycles 1-8, 9, 10-13

Answer 16

during first 8 cycles, nuclei are deep w/in endoplasm, not visible from outside

cycle 9 - a few nuclei have moved to posterior pole, become enclosed by plasma membranes. now called pole cells, are the primordial germ cells

cycle 10 - most of the nuclei have reached the periplasm

cycles 10-13: known as preblastoderm stage - nuclei exist at periphery, still a “syncytium” (technically, these result from fusion, but it’s close enough)

Question 17

describe period preceding cellularization and cycle 14

Answer 17

during preblastoderm period, yolk components become more concentrated toward center, periplasm thickens

pole cells divide by cytokinesis

cycle 14 - 5000 nuclei are in periplasm, now called syncytial blastoderm stage

Question 18

describe the process of cellularization

Answer 18

(proceeds until only a tiny cytoplasmic connection, cytoplasmic stalk, remains between blastoderm cells + endoplasm)

1) plasma membrane furrows cut down in between nuclei, nuclei elongate, become enclosed in a cage of MTs. membrane furrows (w/ actin), deepin in a MT-dependent process.
2) eventually, membrane furrows reach deeper than nuclei + broaden, thereby spreading the cells (except for persistence of cytoplasmic stalks).

Question 19

control of timing of cleavages - what causes transition from fert. to cleavage?

describe blastomere “cell cycle”

Answer 19

MPF activity causes transition (phosphorylates nuclear lamins, histones, etc)

blastomeres have a 2 step cell cycle - just M and S (some species there may be a short G2, but this is largely nonexistent)

MPF activity in cleaving cells is highest during M, undetectable during S, due to degradation of cyclin by proteasomes.

Question 20

what is the midblastula transition? describe the process. when is it thought to be triggered?

Answer 20

stage embryos go through when

1) G phases are added back into cell cycle
2) new mRNAs are transcribed - many encode needed proteins for gastrulation, specification
3) synchronicity of divisions is lost

when exactly this occurs (i.e. after n divisions) varies species to species

thought to be triggered @ certain chromatin-to-cytoplasm ratio, perhaps involving absorption of some cytoplasmic factor by the chromatin.

Question 21

how does cleavage occur? specifically, how does it occur in holoblastic vs. meroblastic cleavage?

Answer 21

carried out by actin/myosin molecules of the contractile ring - similar to muscle contraction

1) like tightening belts in holoblastic cleavages
2) in meroblastic, infoldings of plasma membranes advance and “cut like knives” thru cytoplasm - appears to involve an intricate interplay between actin and MTs

Question 22

describe how the plane of cytokinesis/spindle orientation affects cleavage in holoblastic cleavage.

Answer 22

contractile ring is parallel to plane of cytokinesis, in case of asymmetric divisions, often positioned closer to one end of mitotic spindle (closer to cell periphery) than the other

spindle orientation determines cleavage plane (will be perpendicular to its long axis).

*specific portions of the spindle that are most important = asters

Question 23

describe spindle orientation as an effect of cell shape

Answer 23

orientation can be influenced by shape - will often orient themselves w/ long axis parallel to long axis of a cell

in cases of asymmetric divisions, ill-defined molecules in cortex bind to one centrosome. spindle pulled to “eccentric” position (markedly towards one end of cell), leads to unequal div.

Question 24

using C. elegans as a model for polarity/asymmetric cell division - what proteins are involved, what is their distribution prior to gametic nuclear fusion

Answer 24

involves PAR proteins, found in cortex

prior to fusion of sperm and oocyte nuclear material (and when chromatin is still on opposite ends of egg), distribution of PAR proteins is relatively uniform

Question 25

what happens after sperm/egg membrane fusion?

Answer 25

cytoplasm internally starts flowing toward sperm pronucleus, away from it in cortex. Par-3 and Par-6 accumulate in future anterior end (opposite sperm MTOC)

Par-1 and Par-2 accumulate in future posterior

domain of Par-1 and Par-2 expands as sperm pronucleus moves toward center of cell

Question 26

how does the distribution of PAR proteins affect cleavage to occur? what interactions are important for cleavage as well?

Answer 26

relative distribution appears necessary for slightly asymmetric div. to occur (furrow occurs at interface between the two sets of Par proteins), such that the AB blastomere is slightly larger than the P1 blastomere.

interactions w/ dynein appear to be important for placement of spindle

may involve localizing protein complexes (including kinases) + perhaps also RNAs, action on cytoskeletal-associated proteins.