Cleavage (embryo)
Encyclopedia
In embryology
, cleavage is the division of 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
.
Cleavage differs from other forms of cell division
in that it increases the number of cells without increasing the mass. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases.
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 aster
s 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 midblastula transition
and appears to be controlled by the nuclear:cytoplasmic ratio
(about 1/6).
being set early in the embryo
development
. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo
.
cells (cells with a small even distribution of yolk) or in mesolecithal
cells (moderate amount of yolk in a gradient) - bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. 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.
At the eight-cell stage, the embryo goes through some changes. 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, pump sodium in from the outside, which automatically brings water in with it to the basal (inner) surface to form a blastocoel cavity in a process called compaction. The embryo is now called a blastula or early blastocyst. The trophoblast cells will eventually give rise to the embryonic contribution to the placenta called the chorion
. The inner cells are pushed to one side of the cavity (because the embryo isn't getting any bigger) 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
.
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
, cleavage is the division of cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s in the early embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. The zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
s of many species undergo rapid cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
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
Blastomere
A blastomere is a type of cell produced by division of the egg after fertilization.- References :* "Blastomere." Stedman's Medical Dictionary, 27th ed. . ISBN 0-683-40007-X...
s and form a compact mass called the morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....
. Cleavage ends with the formation of the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
.
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
Animal pole
In developmental biology, a blastula embryo is divided into two hemispheres: the animal pole and the vegetal pole.The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. The animal pole draws its name from its liveliness relative to the...
.
Cleavage differs from other forms of cell division
Cell division
Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
in that it increases the number of cells without increasing the mass. This means that with each successive subdivision, the ratio of nuclear to cytoplasmic material increases.
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 kinaseCyclin-dependent kinase
thumb|350px|Schematic of the cell cycle. outer ring: I=[[Interphase]], M=[[Mitosis]]; inner ring: M=Mitosis; G1=[[G1 phase|Gap phase 1]]; S=[[S phase|Synthesis]]; G2=[[G2 phase|Gap phase 2]]...
s (cdk). The complex CyclinB/cdc2 a.k.a. MPF (maturation promoting factor
Maturation promoting factor
Maturation-promoting factor is a heterodimeric protein composed of cyclin B and cyclin-dependent kinase that stimulates the mitotic and meiotic cell cycles...
) promotes entry into mitosis.
The processes of karyokinesis (mitosis) and cytokinesis
Cytokinesis
Cytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
work together to result in cleavage. The mitotic apparatus is made up of a central spindle and polar aster
Aster (cell biology)
An aster is a cellular structure shaped like a star, formed around each centrosome during mitosis in an animal cell. Astral rays, composed of microtubules, radiate from the centrosphere and look like a cloud....
s made up of polymers of tubulin
Tubulin
Tubulin is one of several members of a small family of globular proteins. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of α- and...
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
Actin
Actin is a globular, roughly 42-kDa moonlighting protein found in all eukaryotic cells where it may be present at concentrations of over 100 μM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans...
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 midblastula transition
Midblastula
In developmental biology, midblastula or midblastula transition is a stage during embryonic development in which zygotic gene transcription is activated. There are three major characteristics of pre-MBT embryos. Firstly, all of the embryonic cells undergo cell division at the same time...
and appears to be controlled by the nuclear:cytoplasmic ratio
NC ratio
The nuclear-cytoplasmic ratio is a measurement used in cell biology...
(about 1/6).
Determinate
Determinate is the form of cleavage in most protostomes. It results in the developmental fate of the cellsCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
being set early in the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
development
Morphogenesis
Morphogenesis , is the biological process that causes an organism to develop its shape...
. Each cell produced by early embryonic cleavage does not have the capacity to develop into a complete embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
.
Indeterminate
A cell can only be indeterminate if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is 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 isolecithalIsolecithal
Isolecithal refers to the even distribution of yolk in the cytoplasm of ovums of mammals and other vertebrates—notably Petromyzontidae,Amiidae, and Lepisosteidae. Isolecithal cells have two equal hemispheres of yolk...
cells (cells with a small even distribution of yolk) or in mesolecithal
Centrolecithal
Centrolecithal is the placement of the yolk in the centre of the cytoplasm of ovums. Many arthropod eggs are centrolecithal....
cells (moderate amount of yolk in a gradient) - bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. 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 vertebrateVertebrateVertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s and echinodermEchinodermEchinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s, in which the spindle axes are parallel or at right angles to the polar axis of the oocyteOocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
.
- Rotational
- MammalMammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s display rotational cleavage, and an isolecithalIsolecithalIsolecithal refers to the even distribution of yolk in the cytoplasm of ovums of mammals and other vertebrates—notably Petromyzontidae,Amiidae, and Lepisosteidae. Isolecithal cells have two equal hemispheres of yolk...
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
- Spiral cleavage is conserved between many members of the lophotrochozoaLophotrochozoaThe Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
n taxa, referred to as SpiraliaSpiraliaSpiralia is a grouping of animals."Lophotrochozoan" and "spiralian" are sometimes considered equivalent.It receives its name from the spiral cleavage found in most members....
. Most spiralians undergo equal spiral cleavage, although some undergfo unequal cleavage (see below). This group includes annelidAnnelidThe annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, molluscs, and sipunculaSipunculaThe Sipuncula or Sipunculida is a group containing 144-320 species of bilaterally symmetrical, unsegmented marine worms...
. Spiral cleavage can vary between species, but generally the first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo. These first two cleavages are oriented in planes that occur at right angles parallel to the animal-vegetal axis of the zygoteZygoteA zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
. At the 4-cell stage, the A and C macromeres meet at the animal pole, creating the animal cross-furrow, while the B and D macromeres meet at the vegetal pole, creating the vegetal cross-furrow. With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole . The divisions that produce these quartets occur at an oblique angle, an angle that is not a multiple of 90o, to the animal-vegetal axis. Each quartet of micromeres is rotated relative to their parent macromere, and the chirality of this rotation differs between odd and even numbered quartets, meaning that there is alternating symmetry between the odd and even quartets. In other words, the orientation of divisions that produces each quartet alternates between being clockwise and counterclockwise with respect to the animal pole. The alternating cleavage pattern that occurs as the quartets are generated produces quartets of micromeres that reside in the cleavage furrows of the four macromeres. When viewed from the animal pole, this arrangement of cells displays a spiral pattern.
- Specification of the D macromere and is an important aspect of spiralian development. Although the primary axis, animal-vegetal, is determined during oogenesisOogenesisOogenesis, ovogenesis or oögenesis is the creation of an ovum . It is the female form of gametogenesis. The male equivalent is spermatogenesis...
, the secondary axis, dorsal-ventral, is determined by the specification of the D quadrant. The D macromere facilitates cell divisions that differ from those produced by the other three macromeres. Cells of the D quadrant give rise to dorsal and posterior structures of the spiralian. Two known mechanisms exist to specify the D quadrant. These mechanisms include equal cleavage and unequal cleavage.
- In equal cleavage, the first two cell divisions produce four macromeres that are indistinguishable from one another. Each macromere has the potential of becoming the D macromere. After the formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo . This contact is required to distinguish one macromere as the official D quadrant blastomere. In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become the future D blastomere. Once specified, the D blastomere signals to surrounding micromeres to lay out their cell fates.
- In unequal cleavage, the first two cell divisions are unequal producing four cells in which one cell is bigger than the other three. This larger cell is specified as the D macromere . Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage. Unequal cleavage can occur in two ways. One method involves asymmetric positioning of the cleavage spindle. This occurs when the asterAsterAster may refer to:* Aster , a genus of flowering plants in the family Asteraceae* Callistephus, another genus of flowering plants in the family Asteraceae, commonly called Aster or Chinese Aster...
at one pole attaches to the cell membrane, causing it to be much smaller than the aster at the other pole. This results in an unequal cytokinesisCytokinesisCytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
, in which both macromeres inherit part of the animal region of the egg, but only the bigger macromere inherits the vegetal region. The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called a polar lobe. This polar lobe forms at the vegetal pole during cleavage, and then gets shunted to the D blastomere . The polar lobe contains vegetal cytoplasm, which becomes inherited by the future D macromere.
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.- 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 monotremes, birds, reptiles, and fishFishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
that have telolecithalTelolecithalTelolecithal , also known as macrolecithal, refers to the uneven distribution of yolk in the cytoplasm of ovums found in birds, reptiles, and fish...
egg cells (egg cells with the yolk concentrated at one end).
- Superficial
- In superficial cleavage, mitosisMitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
occurs but not cytokinesisCytokinesisCytokinesis is the process in which the cytoplasm of a single eukaryotic cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, and sometimes meiosis, splitting a binucleate cell in two, to ensure that chromosome number is maintained from one generation...
, 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 that have centrolecithalCentrolecithalCentrolecithal is the placement of the yolk in the centre of the cytoplasm of ovums. Many arthropod eggs are centrolecithal....
egg cells (egg cells with the yolk located in the center of the cell).
| Holoblastic | Meroblastic |
|---|---|
|
Fish Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups... , bird Bird Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from... s, reptile Reptile Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors... s, monotreme Monotreme Monotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals... s) Insect Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae... s) |
Mammals
There are several differences between the cleavage in placental mammals and the cleavage in other animals. Mammals have a slow rate of division that is between 12 and 24 hours. These cellular divisions 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 some changes. 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
Trophoblast
Trophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...
cells, pump sodium in from the outside, which automatically brings water in with it to the basal (inner) surface to form a blastocoel cavity in a process called compaction. The embryo is now called a blastula or early blastocyst. The trophoblast cells will eventually give rise to the embryonic contribution to the placenta called the chorion
Chorion
The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...
. The inner cells are pushed to one side of the cavity (because the embryo isn't getting any bigger) to form the inner cell mass
Inner cell mass
In early embryogenesis of most eutherian mammals, the inner cell mass is the mass of cells inside the primordial embryo that will eventually give rise to the definitive structures of the fetus...
(ICM) and will give rise to the embryo and some extraembryonic membranes. At this stage, the embryo is called a blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
.
External links
- Cleavage patterns and the topology of the metazoan tree of life, James W. ValentineJames W. ValentineJames W. Valentine is an American evolutionary biologist and Professor Emeritus in the Department of Integrative Biology at the University of California, Berkeley...
- 'What are the 'advantages' of developing a deuterostome pattern of embryonic' on MadSci NetworkMadSci NetworkThe Madsci Network is a website known primarily for its Ask-A-Scientist forum where users can ask questions to a panel of volunteer scientists. Each question, submitted via a Web interface, is reviewed by a volunteer moderator...
- Videos of initial cleavages of Xenopus laevis embryos (side view and top view), acquired by MRI (DOI of paper)