cleavage ch 6

amphibian ) two major embryonic region:

- A rapidly dividing region of micromeres near animal pole; - A more slowly dividing vegetal macromere area. As Cleavage progresses, the animal region becomes packed with numerous small cells, while the vegetal region contains only a small number of large, yolky macromeres

WHAT HAPPENS IF MICROMERES WERE PLACED IN THE ANIMAL POLE?

- Micromeres transplanted into the animal region of the blastula , will give rise to descendants that form skeletal structure. - Moreover, these transplanted micromeres alter the fates of nearby cells by inducing a secondary site for gastrulation. Cells that would normally produce ectodermal skin cells will be respecified as endoderm and will produce a segondary gut. almost like another embryo

5 blastomere types and their fates

- The animal half of the embryo gives rise to the ectoderm- the larval skin and its neurons. an 1 and an 2 oral ectoderm - The Veg1 layer aboral ectodetrm and endoderm The Veg2 layer gives rise to cells that can populate three different structures, the endoderm, the coelom (the internal body wall) and secondary mesenchyme (pigment cells, immunocytes, and muscle cells). The first tier of micromers produces the primary mesenchyme cells that form the larval skeleton, while the second tier contributes to the coelom.

hatching of embryo from zona pellucida

- blastocyst hatches from the zona by lysing a small hole in it and squeezing through that hole as the blastocyst expands. - Strypsin , a trypsin-like protease on the trophoblast cell membranes, lyses a hole in the zona.

5th division of sea urchin

- from the previous division we are left with 4 mesomeres in the animal pole and 4 macromeres in vegetal pole and 4 micromeres in vegetal pole the mesomeres divide equatorially to give two tiers of eight mesomeres an1 and an2 one staggered above the other. - The macromeres divide meridionally, forming a tier of eight cells below an2. The micromeres also divide later to produce a small cluster beneath the large tier.

MAMMALS embryo

- internal fertilization - NO MBT 1. Fertilization occurs in the ampulla of the oviduct (Meiosis is completed at this time, and first cleavage begins about a day later.) 2. Compaction of the human embryo occurs on day 4, when it is at the 10-cell stage. 3. The embryo hatches from the zona pellucida upon reaching the uterus. 4. implatation of blastula in endometrium

MPF (maturation/mitosis-promoting factor)

- is made up of 2 protein units and is activated only when these two units are attached to each other - exists in the oocyte as a protein and mRNA - MPF causes cells to enter M phase

implantation

- once blastocyst comes in contact with endometrium, the trophoblast integrins and pCadherin interact with the molecules on the endometrium. Once in contact with the endometrium, the trophoblast secretes a set of proteases (such as collagenase, stromelysin and plasminogen activators) that digest the extraxcellular matrix of the uterine tissue, enabling the blastocyst to bury itself within the uterine wall.

Compaction (Morula) phase of early embryo development

- only occurs in mammals - Before the fourth cleavage cells will undergo compaction mediated by the molecule Ecadherin. is stabilized by the formation of tight junctions between the outer cells - cells also form a gap junctions among themselves that allows the passage of small molecules, such as ions and some second messenger molecules such as Ca++ and cAMP. - The cells of the compacted 8-cell embryo divide to produce a 16-cell morula.

amphibian cleavage

- radially symmetrical and holoblastic. The amphibian egg contains much more yolk (concentrated in the vegetal hemisphere) which interferes with cleavage.' - division begins at the animal pole and slowly extends down into the vegetal region. At the animal pole, the first cleavage proceeds while division while at the vegetal pole proceeds 50-100 times slower - While the first cleavage is still incomplete in the yolky vegetal pole of the egg, the second meridional cleavage begins to take place. - The third cleavage is equatorial. It divides the embryo into four small animal blastomeres (micromeres) and four large blastomeres (macromeres) in the vegetal region.

experiment to determine what regulates cytokenisis during cleavage

- they added nuclei from dividing cells into a non fertilized oocyte ( oocyte will never divide unless fertilized) and that cell stoped dividing. - they added nuclei from non dividing cells into fertilized enucleated (no nucleus) eggs they start dividing - these Transplant experiments have demonstrated that it is the cytoplasm that regulates both karyokinesis and cytokinesis aka division of zygote. - the cytoplasm of zygote contains all the instructions necessary for cleavage in form of mRNs and proeins

mammalian cleavage pattern

-ROTATIONAL HOLOBLASTRIC CLEAVAGE. - cleavage in mammals can be very slow - cleavage planes are different - 2 types of cleavage can be occuring at the same time like one blastomere will have meridonal while the other will have equatorial - blastomere cleavages are asynchronous. Mammalian blastomeres do not all divide at the same time. Embryo often contain odd numbers of cells. - Cleavage of the mammalian embryo is regulated by the zygotic nucleus from the very start.

128 aka begining of blastula

-all cells same size including micromeres - blastocoel formation: the cavity surronded by cells - Tight junctions unite the blastomeres into epithelial sheet that completely encircles the blastocoel. - As the cells continue to divide, the blastula remains one cell layer thick, thining out as it expands.

MPF activation causes

1. Chromosome condensation by H1 histone phosphorylation [H1 histone is linker histone that is important for formation of chromosome] 2. Nuclear envelope breakdown by hyperphosphorylation of 3 nuclear lamins (lamina mainatin the structure of envelope once phosphorylated they disassemble) ; 3. RNA polymerase inhibition to shut down transcription. to initiate division

determinants of embryonic cleavage

1. The amount and distribution of yolk protein within the cytoplasm; 2. Factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation.

holoblastic cleavage

A type of cleavage in which there is complete division of the egg, as in eggs having little yolk (sea urchin) or a moderate amount of yolk (frog).

MICROMERES

AREEEE IRREVERSIBLEEEEEE - NO MATTER WHERE YOU PUT THEM THEY WILL DIFFERENTIATE INTO THEIR DESIGNATED CELLS

10th division

After the 10th divisions, the cells have become specified and they end develop cilia. The ciliated blastula begins to rotate within the fertilization envelope. Differences are seen in the cells. The cells at the vegetal pole begins to thicken, forming a vegetal plate.

Isolecithal

Blastula in which the yolk is uniformly distributed radial spiral bilateral or rotational cleavage

fate map determination of sea urchin

By the 60-cell stage, most of the embryonic cell fates are specified, but the cells are not irreversibly committed ( which means their fate can still be changed) with the exception .of micromeres. - if a particular blastomere is artificially placed in a different location in the embryo it will give rise to another type of specialized cel (change its fate)

equatorial cleavage

Cleavage plane through cell equator - perpendicular to animal vegetal axis

MPF experiment

Cleaving cells can be experimentally trapped in S phase by incubating them in a inhibitor (cyclobeximide which blockks translation ) of protein synthesis. - activation of MPF is dependent of protein synthesis and the transition to M phase is depnedent of MPF - When MPF is microinjected into these cells, they enter M. - After an hour, MPF is degraded and the chromosome return to S phase

Why is the zona pellucida still around embryo?

During its migration to the uterus, the zona prevents the embryo from prematurely adhering to the oviduct rather than traveling to the uterus. - preventing ectopic pregnancy

telocithal eggs

Eggs of birds and fishes have only one small area of the egg that is free of yolk are called telolecithal eggs. Cell divisions occur only in this small disc of cytoplasm, giving rise to the discoidal pattern of cleavage.

cell cycle stages

G1, S, G2, M

MPF activity

MPF undergoes cyclical changes y protein called cyclin in its level of activity in mitotic cells. The MPF activity of early blastomeres is highest during M and undetectable during S phase.

micromeres and fate of nearby cells

Micromeres appears to produce a signal that tells cells adjacent to them to become endoderm and induces them to invaginate into the embryo. -If isolated micromeres are recombined with an isolated animal cap (top two animal tiers), the animal cap cells will generate endoderm , and more or less normal larva develop.

blastomeres

Smaller cells produced by cleavage during mitotic cell division

yolk of an egg

The amount and distribution of yolk determines where cleavage can occur and the relative size of blastomeres. - yolk can slow down cleavage or hinder it When one pole of the egg is relatively yolk-free, cellular division occur there at a faster rate than at the opposite pole.

BLASTOCOEL FUNCTIONS

The blastocoel serves two major functions in frog embryos: 1.It permits cell migration during gastrulation; It prevents the cells beneath it from interacting prematurely with the cell above it.( INTERACTION LEADS TO FORMATION OF MESODERM AND SHOULD ONLY OCCUR DURING GASTRULATION)

chorion

The chorion enables the fetus to get oxygen and nourishment from the mother. It also secretes hormones that cause the mother's uterus to retain the fetus, and produce regulators of the immune response so that mother will not reject the embryo.

CYCLIN B

The cyclical nature of MPF is owed to this protein - cyclin b proteins get synthesized during S phase and once then when cell is done with m phase they get degraded - Cyclin B is encoded by mRNA stored in the oocyte cytoplasm, and if the translation of this message is specifically inhibited, the cell will not enter mitosis. - Cyclin B regulates the small subunit of MPF, the cyclin-dependent kinase

Blastocyst

The inner cell mass is positioned on one side of the ring of trophoblast cells. The resulting type of blastula, called the blastocyst.

vegetal pole

The portion of the egg where most yolk is concentrated; opposite of animal pole.

animal pole

The portion of the egg where the least yolk is concentrated; opposite of vegetal pole.The zygote nucleus is frequently displaced toward the animal pole

Centrolecithal

Type of egg, such as those of insects, which have yolk in the center and undergo superficial cleavage.

rapid division

cleavage before MBT stage is is completely under control of the proteins and mRNA stored in the oocyte provided by the mother.. during this rapid division G1 and G2 stages are abolished

morulla of amphibian embryo

containing 16 to 64 cells

cyclin-dependent kinase

enzyme to which cyclin binds during s phase inorder to initiate m phase. the second unti of the MPF - The presence of cyclin is controlled by several proteins that ensure its periodic synthesis and degradation. The regulators of cyclin B (MPF) are stored in the egg cytoplasm. The cell cycle is independent of the nuclear genome for numerous cell division.

G1 and G2 phase of cell cycle

gap phases where cell growth occurs. theses stages are abolished during cleavage because we need cells to become smaller so growth must be removed.

meroblastic cleavage

incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds

meridonal cleavage

longitudinal or vertical

mesolecithal

moderate amount of yolk unevely distributed like frog

sea urchin cleavage

radial holoblastic - First (2)and second(4) cleavage are meridional. Third (8) cleavage is equatorial. - Fourth (16) cleavage is meridional. the four animal pole cells split equally to give rise to eight equal sized animal blastomeres termed mesomeres. , the vegetal cells divide asymmetrically along the equatorial plane to give 4 large macromeres and 4 much smaller micromeres at the vegetal pole.

cleavage

series of mitotic divisions that occur to the zygote after fertilization. - they decrease the size and increase the number of cells

6th division in sea urchin

the animal hemisphere cells divide meridionally, while the vegetal cells divide equatorially, giving a veg1 and veg2 layers. the seventh division ithis pattern is reversed and we end up with 128 cells

128 cell stage of amphibians (BLASTULA )

the blastocoel becomes apparent, and the embryo is considered a blastula. -

external cells of morula

trophoblast (trophoectoderm) cells. These cells produce no embryonic structures, but instead are necessary for implantation of the embryo in the uterine wall and form tissues of the chorion, the embryonic portion of the placenta.

cavitation

trophoblast cells secrete a fluid into the morula to create a blastocoel.

activation of cleavage

where S phase moves directly into M phase - transition from fertilization to cleavage is caused by the activation of mitosis-promoting factor (MPF). MPF causes cells to enter M phase.

MBT stage

where the zygotic nucleus takes control of the cell cycle.and where cell cycle stages return to normal

ICM at 16 cell stage

which will give rise to the embryo ans its associated yolk sac, allantois, and amnion - the 64-cell stage, the inner cell mass (approximately 13 cells) and the trophoblast cells are completely separate. The distinction between trophoblast and inner cell mass blastomeres represents the first differentiation event in mammalian development.