There are planes and types in embryology.
The large volume egg cytoplasm is divided by the stereotyped pattern of early mitotic divisions.The early zygote is large.There is a period of growth for most cells, but not for early stage blastomeres.The cells get smaller with each division.The mid-blastula transition is where the zygotic nucleus takes control of the cell cycle after a rapid pattern of cell division without growth.
There is evidence that a maternal factor is to blame for the rapid pattern of cleavage divisions.The time of the midblastula transition can be changed by altering the cytoplasmic to nuclear DNA ratio.The time when the major switch from expression of maternal to zygotic genes takes place is referred to as the midblastula transition.
Fertilization in some species leads to radical cytoplasmic movements that are essential for the proper location of the cytoplasmic determinants.The pattern of the embryo's growth is determined by its position and amount of yolk.Yolk has an effect on cleavage.It slows it down or prevents it completely.Yolk is an adaptation of animals that go through more or less embryogenesis isolated from a food supply.Sea urchins have relatively little yolk because they rapidly develop into a free swimming form that gets its nutrition from their environment.A premature baby can be given food in a parental pouch.Placental mammals have a specialized organ through which the embryo is nourished throughout development.
The types of eggs that are described are: sparse evenly distributed yolk, eg., sea urchin, mouse, and Telolecithal: dense yolk concentrated at one end.Many fly animal eggs have a yolk rich pole and a poor pole.The nucleus of the animal is usually displaced.The zygotes have relatively little yolk.All the way through the egg is the cleavage furrow.There is a MEROBLASTIC cleavage where the plane extends only to the accumulated yolk.There are several types of symmetry seen in nature.Two figures show examples of cleavage symmetries.
Sea urchins have some interesting differences.The first and second cleavages are meridional.While 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, giving 4 large MACROMERES and 4 much smaller MICRO.The fifth division of the MESOMERES divide into two parts, one for an 1 and another for a 2, forming a tier of eight cells below the veg.The veg2 layer is given by the sixth divisions.The seventh divisions have a 128 cell blastula.What makes these patterns happen?Are they dependent on the previous cleavage or are they determined by a clock?Horstadius found that the appearance of micromeres occurred at the right time regardless of the history of cleavages.
The conclusion from these experiments is that there is a factor in the pole of the egg that determines the formation of micromeres.The clock is not related to the event.The 128 cell blastula is a ball of cells.The ball has a thick layer of cells in contact with the external hyaline layer and the internal fluid of the blastocoel.The cells begin to form tight junctions at this stage in development.The central blastocoel is no longer in contact with the outside world.The blastomeres continue to divide with their axis parallel to the hyaline layer.The blastocoel is getting bigger.There are two theories to account for the enlargement of the blastocyst 1.The osmotic theory suggests that the blastocoel has something to do with the flow of water.The pressure would be responsible for aligning the axis of the blastomeres.2.The alternate theory suggests that it is between the blastomeres and the hyaline layer that the axis is aligned.The interaction with the blastocoel wall is the least important.The expansion of the blastocyst and blastocoel is caused by the dominant adhesion with the hyaline layer.The cells of the blastula become free swimming when they grow cilia on their outer surface.The large volume of yolk can interfere with the cleavage of Amphibians.At the animal pole, the first cleavage proceeds at 1mm/min, while through the vegetal pole it is 100 times slower.The second meridional cleavage begins to take place while the first one is still incomplete.The nuclei and asters are displaced away from the animal axis and not equally divide the blastomeres.There are four smaller animal blastomeres and four large vegetal pole blastmeres.The animal pole made up of smaller micromeres and a slower dividing vegetal pole are the result of this unequal holoblastic cleavage.The animal pole is composed of many small micromeres and a few yolk filled large macromere.The formation of the blastocoel is not obvious until the 128 cell stage.What is the function of the BLASTOCOEL?The blastocel separates cells so they don't touch each other.The cells on the roof of the blastocoel become ectoderm.You can transplant cells from the roof of the blastocoel to the yolky cells at the base.The cells that make the smedermal derivatives are adjacent to the endodermal precursors.One possibility that we will explore is that the cells in the same area become similar in appearance.The formation of the blastocoel may be necessary to prevent inductive interactions among early cells.Cells migrate into the interior of the blastocoel during the next stage of development, called GASTRULATION.The egg is released from the ovary into the oviduct where it is fertilized.After fertilization, the first cleavage begins.It can be very slow in mammals.
The cleavage planes are different from other animals.The first cleavage is similar to sea urchin and frog.One of the blastomeres divides meridionally and the other equatorially in the second cleavage division.There is a type of cleavage calledROTATIONAL HOLOBLA.
The blastomere cleavages are asynchronous.The synchrony of sea urchin and frog is not as high as it is with the midblastula transition.The embryo is regulated by the zyotic nucleus from the beginning.The blastomeres form a ball of cells just like the other animals we have studied.The cells of the blastula change their behavior before the fourth cleavage.They try to maximize their contacts with the other blastomeres in order to cause the blastula to compact.
The formation of tight junctions between outer cells is what seals off the interior of the blastula from the outside.The cells form gap junctions among themselves that allow the passage of small molecule.The morula has an outer rind of cells and a few cells inside.The TROBLASTIC or TROPHECTODERMAL CELLS are the result of most of the external cells.These cells don't contribute to the embryo proper, but they are needed to form the tissues of the baby in the uterus, an essential component of a placenta.
The embryo's cells come from the inner few cells of the 16 cell stage blastula.The inner cell mass of the embryo is generated by these cells.The trophoblastic layer is separate from the inner cell mass by the 6th cleavage.The blastocoel is created by the secret fluid inside the trophoblasts.The embryo is called a blastocyst.
How are the inner cell mass cells created?Is it possible for certain blastomeres to become inner cell mass progenitors?The answer seems to be no.The early blastomeres seem to be totipotent and the determination of which cells will contribute to the trophoblastic layer is a matter of chance.Cells from a 4 cell stage embryo, which will normally give rise to both inner cell mass and trophectoderm cells, are only used to transplant to the outside of a 32 cell embryo.They don't contribute to the embryo proper.The fusion of two 8 cell stage mouse embryo results in a normal embryo, suggesting that all the cells at that stage are totipotent.
There is a large dense yolk in telolecithal eggs.Centrolecithal eggs are similar to insects and are characteristic of birds, fishes, and reptiles.Eggs result in meroblastic discoidal cleavage.The blastodisc is at the animal pole of the egg.The blastomeres are continuous at their vegetal margins at early cleavages.The movie by Rolf Karlstrom is excellent.The movie was written and directed by Paul Myers.
The blastoderm does not separate from the yolk until the equatorial cleavages.The blastoderm is three or four cells thick.
There is a space between the blastoderm and the yolk in birds.Cells from the blastoderm migrate into the subgerminal cavity to form a second layer.The outer and inner layers are called EpibLAST and HYPOBLAST.When we discuss bird and mammal gastrulation, we will study this in more detail.There is a large mass of yolk in the egg.There is a variation in insects.The nuclei divide.The nucleus undergoes karyokinesis without cytokinesis, the division of the cell.The nude nuclei are called EnERGIDS.Every 8 minutes, the nuclei divide at an amazing rate.
The naked nuclei migrate to the center of the egg after a few rounds of karyokinesis.The nucleus share the same cytoplasm so it's called the SYNCYTIAL BLASTODERM.The CELLULAR BLASTODERM is created by cellularzation at the 14th nuclear division.Cells divide after this time.The midblastula transition is related to the frog and sea urchins.The midblastula transition was thought to be triggered by the ratio of chromatin to cytoplasm.There is evidence for this mechanism in flies.The midblastula transition and cellularization takes place on the 15th.You can speed up cellularization by ligating the egg and reducing the volume of the cytoplasm.Although the syncytial blastoderm stage suggests that all the nuclei are equipotent, in fact the cytoplasm is very regionalized and the nucleus have highly organized cytoplasmic domains around them.
The blastomeres go directly from M to S without the need for G1 or G2 stages.Both animals have G1 and G2.Experiments show that the cytoplasm regulates both karyokinesis and cytokinesis.If nuclei from dividing cells are transferred into oocytes, they immediately stop dividing.
If non-dividing cells are put into fertilized eggs they start dividing.Eggs with no centrioles will undergo cortical contractions reminiscent of cleavage.The factors that regulate cell division in the early embryo have been identified.The oocyte is arrested in the second meiotic metaphase after the first one.Meiosis is completed after fertilization when the Ca inactivates the CSF.M phase is caused by MITOSIS PROMOTING FACTOR.Nuclear envelope breakdown by hyperphosphorylation of 3 nuclear lamins is one of the causes of MPF activation.It is possible to shut down transcription with the help of RNA polymerase inhibition.Myosin regulatory subunits are involved in the production of cytokinesis.There is a model for regulation of cell cycle.MPF causes cell to move from S to M.The cell proceeds through M to S and the cycle is repeated when the cell remains in M. Ca.Cyclin B and cdc2 make up the MPF.cyclin B is a cell cycle specific synthesis and degradation that is regulated by the cells nucleus.During oogenesis, the egg is loaded with "regulators" of cyclin B and ZYME so that it can be regulated by maternal factors.It is not untill the maternal components run out that the normal cell cycle returns.
It is a general and basic mechanism for early patterning.When and how cell fates are determined during development is a major question of developmental biology.The question of how pattern formation occurs during development is related to this.The embryo needs to generate the right number of differentiated cells and organize them in a way that will allow them to form a functional animal.There are two possibilities of cell fate determination.Cell fate could be determined by placing factors into the egg during oogenesis and then parceling them out to blastomeres during cleavage.The signals provided by the embryo's environment could be used to regulate cell fate.Most complex organisms use a combination of signals to regulate cell fate and pattern formation.
"regulative" development suggests that a cell's fate is determined by external signals from other cells, whereas "arbitrary" cell fate specification by the cytoplasmic determinants suggest that the fate of the cell is dependent on its lineage.There are two mechanisms of cell specification that can be distinguished from each other.If a blastmere is isolated from an embryo and is still in its normal position in the embryo, we can say that it is doomed.It can be said that its cell fate is dependent on external signals.If we remove the blastomere from an embryo and it develops without all the cells fates that normally arise from it, we say that development is cell independent.The blastomeres can regulate their cell fate if the embryo develops normally.If a transplant cell maintains its original position, then we say its fate has been determined, if it takes on a new position and is regulated by nearby cells.
The tunicate egg has a clearly distinguished animal pole at the end of oogenesis.The grey yolky inner cytoplasm is surrounded by a yellow cortical cytoplasm.The nucleus is moving towards the animal pole.The egg is fertilized by sperm entry in the vegetal hemisphere.There is a dramatic change in the egg's cytoplasm after fertilization that seems to correlate with subsequent blastomere fates.
The fate map shows the different colored cells of the embryo.There are two figures with different colors.The Yolky (yellow) cytoplasm is correlated with muscle fates.There is a correlation between the grey and yellow crescents.
The invariant linage correlation with blastomeres parceled particular colored cytoplasms is shown on the map.There are invariant cleavages and lineages that do not prove cell specification.
It is necessary to test regulative versus cell autonomously determination of cell fate.In the next three figures, the classic isolation experiments attempt to show that cell fate is determined by the cytoplasmic determinants that they acquire.The B4.1 pair of blastomeres are separated from the rest of the embryo using a glass needle.The yellow crecent cytoplasm is correlated with muscle cell fate.
The results of the isolation experiments can be seen here.In each case, the isolated blastomeres give rise to a subset of cell fates normally produced in the intact embryo.The blastomeres don't compensate for their missing neighbors.There are two animal pole blastomeres, a 4.2 and b 4.2.A 4.1 and B 4.1 give rise to muscles and cells in the body.None of the isolated blastomeres can give rise to a normal embryo.
The pole animal blastomeres, b 4.2, acquire some of the "yellow crescent" cytoplasm after using a needle to manipulate the equatorial cleavage plane.Some muscle cells are created when these blastomeres are isolated.The "yellow crescent" cytoplasm can determine muscle cell fate in a cell autonomously.
The location of the animal pole is determined by a canal.The early pattern of cleavages is determined by thesymmetry of the egg, not the site of sperm entry.Boveri described a band of color to the animal-vegetal axis.The location of the archenteron's cytoplasm is indicated by these granules.Horstadius showed that only the vegetal blastomere would give rise to micromeres, gastrulate, and form skeleton.His conclusion was that the factors located in half are necessary for micromeres, gastrulation and archenteron fromation.The pattern of early breasts.The micromeres arise from an equal division of the blastomeres.
The map shows the fate of the sea urchin blastula.The primary mesenchyme cells are the micromeres.
If the blastomeres are isolated from each other, they will be able to regulate their fate and give rise to 4 small pluteus stage larvae.
If you isolated animal half blastomeres you found that they only produced an animalized dauerblastula that did not express any cell fates.The fate of the cells can be regulated with the help of isolated half vegetal blastomeres.Isolated micromeres are always on schedule and undergo the correct number of cell divisions.Micromeres are the progenitors of the skeletogenic mesenchyme cells when they first appear at the 16 cell stage.Micromeres could "induce" new cell fates in the animal pole blastomeres, despite the fact that micromere fate was "fixed or determined" at the time of their birth.The micromeres were able to induce fates in the animal pole blastomeres.The late experiment in "C" shows that when micromeres are added to an animal half blastula you can induce the formation of a recognizable larva.