Cleavage (embryo) 1892841 212575845 2008-05-15T10:42:20Z SuW 2866009 /* Holoblastic */ link found In [[embryology]], '''cleavage''' is the division of [[cell (biology)|cell]]s in the early [[embryo]]. The [[zygote]]s of many species undergo rapid [[cell cycle]]s with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called [[blastomere]]s and form a compact mass called the [[morula]]. Cleavage ends with the formation of the [[blastula]]. Depending mostly on the amount of [[yolk]] in the egg, the cleavage can be '''holoblastic''' (total or entire cleavage) or '''meroblastic''' (partial cleavage). The pole of the egg with the highest concentration of yolk is referred to as the [[vegetal pole]] while the opposite is referred to as the [[animal pole]]. ==Mechanism== The rapid cell cycles are facilitated by maintaining high levels of proteins that control cell cycle progression such as the cyclins and their associated [[cyclin-dependent kinase]]s (cdk). The complex CyclinB/cdc2 a.k.a. MPF ([[maturation promoting factor]]) promotes entry into mitosis. The processes of [[karyokinesis]] (mitosis) and [[cytokinesis]] work together to result in cleavage. The mitotic apparatus is made up of a [[central spindle]] and polar [[asters]] made up of polymers of [[tubulin]] protein called [[microtubules]]. The asters are nucleated by [[centrosomes]] and the centrosomes are organized by centrioles brought into the egg by the sperm as basal bodies. Cytokinesis is mediated by the [[contractile ring]] made up of polymers of [[actin]] protein called [[microfilaments]]. Karyokinesis and cytokinesis are independent but spatially and temporally coordinated processes. While mitosis can occur in the absence of cytokinesis, cytokinesis requires the mitotic apparatus. The end of cleavage coincides with the beginning of zygotic transcription. This point is referred to as the [[mid-blastula transition]] and appears to be controlled by the nuclear/cytoplasmic ratio (about 1/6). ==Types of cleavage== ===Determinate=== Determinate is the form of cleavage in most [[protostomes]]. It results in the developmental fate of the [[cell]]s being set early in the [[embryo]] [[morphogenesis|development]]. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete [[embryo]]. ===Indeterminate=== A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is a characteristic of [[deuterostomes]] - when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism. ===Holoblastic=== In the absence of a large concentration of yolk, four major cleavage types can be observed in [[isolecithal]] cells (cells with a small even distribution of yolk) or in [[Centrolecithal|mesolecithal]] cells (moderate amount of yolk in a gradient) - '''bilateral''' holoblastic, '''radial''' holoblastic, '''rotational''' holoblastic, and '''spiral''' holoblastic, cleavage.<ref>{{cite web |url=http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio.section.1767 |title=Early Development of the Nematode Caenorhabditis elegans |accessdate=2007-09-17 |format= |work=}}</ref> These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging. In holoblastic eggs the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms. *Bilateral :The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate; compared with bilateral meroblastic cleavage, in which the blastomeres stay partially connected. *Radial :Radial cleavage is characteristic of the [[deuterostomes]], which include some [[vertebrate]]s and [[echinoderm]]s, in which the spindle axes are parallel or at right angles to the polar axis of the [[oocyte]]. *Rotational :[[Mammal]]s display rotational cleavage, and an [[isolecithal]] distribution of yolk (sparsely and evenly distributed). Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients. :Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells. The way in which this cleavage differs is that one of the daughter cells divides meridionally, whilst the other divides equatorially. *Spiral :In spiral cleavage, the cleavage planes are oriented obliquely to the polar axis of the oocyte. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet. All groups showing spiral cleavage are [[protostomia]], such as [[annelid]]s and [[mollusk]]s. ===Meroblastic=== In the presence of a large amount of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are '''discoidal''' and '''superficial'''.<ref>{{cite web |url=http://www.ucalgary.ca/UofC/eduweb/virtualembryo/currentnotes.html |title=Current Notes |accessdate=2007-09-17 |format= |work=}}</ref> *Discoidal :In discoidal cleavage, the cleavage furrows do not penetrate the yolk. The embryo forms a disc of cells, called a blastodisc, on top of the yolk. Discoidal cleavage is commonly found in [[birds]], [[reptiles]], and [[fish]] which have [[telolecithal]] egg cells (egg cells with the yolk concentrated at one end). *Superficial :In superficial cleavage, [[mitosis]] occurs but not [[cytokinesis]], resulting in a polynuclear cell. With the yolk positioned in the center of the egg cell, the nuclei migrate to the periphery of the egg, and the plasma membrane grows inward, partitioning the nuclei into individual cells. Superficial cleavage occurs in [[arthropods]] which have [[centrolecithal]] egg cells (egg cells with the yolk located in the center of the cell). {| align="center" class="toccolours" border="1" style="border:1px solid gray; border-collapse:collapse;" |+'''Cleavage patterns followed by holoblastic and meroblastic eggs''' |- !Holoblastic !Meroblastic |- valign="top" | *Bilateral ([[tunicate]]s, [[amphibian]]s) *Radial ([[sea urchin]], [[amphioxus]]) *Rotational ([[mammal]]s) *Spiral ([[annelid]]s, [[mollusk]]s) | *Discoidal ([[fish]], [[bird]]s, [[reptile]]s) *Superficial ([[insect]]s) |- valign="top" |} ==Mammals== There are several differences between the cleavage in mammals and the cleavage in other animals. Mammals have a slow rate of division that is between 12 and 24 hours. These cellular division are asynchronous. Zygotic transcription starts at the two, four, or eight-cell stage. Cleavage is holoblastic and rotational. At the eight-cell stage, the embryo goes through a process called '''compaction'''. Most of the blastomeres in this stage become polarized and develop tight junctions with the other blastomeres. This process leads to the development of two different populations of cells: polar cells on the outside and apolar cells on the inside. The outer cells, called the '''trophoblast''' cells, secrete fluid on their basal (inner) surface to form a blastocoel cavity through the process of '''cavitation'''. These trophoblast cells will eventually give rise to the embryonic contribution to the placenta called the '''chorion'''. The inner cells adhere to one side of the cavity to form the '''inner cell mass''' (ICM) and will give rise to the embryo and some extraembryonic membranes. At this stage, the embryo is called a '''blastocyst'''. == References == <references/> *{{cite book| author=Wilt, F. & Hake, S.| year=2004| title=Principles of Developmental Biology}} *{{cite book| author=Scott F. Gilbert| year=2003| title=Developmental Biology}} ==See also== * [[Embryogenesis]] ==External links== *[http://www.pnas.org/cgi/content/full/94/15/8001 Cleavage patterns and the topology of the metazoan tree of life], [[James W. Valentine]] *'[http://www.madsci.org/posts/archives/feb99/918160589.Dv.r.html What are the 'advantages' of developing a deuterostome pattern of embryonic]' on [[MadSci Network]] *Videos of initial cleavages of [[Xenopus laevis]] embryos ([http://www.blackwell-synergy.com/doi/suppl/10.1111/j.1432-0436.2006.00114.x/suppl_file/Movie+S1.mov side view] and [http://www.blackwell-synergy.com/doi/suppl/10.1111/j.1432-0436.2006.00114.x/suppl_file/Movie+S2.mov top view]), acquired by [[MRI]] ([http://dx.doi.org/10.1111/j.1432-0436.2006.00114.x DOI of paper]) {{embryology}} [[Category:Developmental biology]] [[ca:Segmentació (zoologia)]] [[de:Furchung]] [[es:Segmentación (zoología)]] [[fr:Segmentation (biologie)]] [[ja:卵割]] [[oc:Segmentacion (zoologia)]] [[pl:Bruzdkowanie]] [[pt:Clivagem (embriologia)]] [[ru:Дробление]] [[sr:Браздање]] [[fi:Vakoutuminen]]