cleavage

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Biological terminology

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Cleavage refers to Zygote Early fast mitosis 。 One cell or nucleus Continuously and rapidly split cytoplasm Split into many smaller Nucleated cell This process is called cleavage.

Chinese name: cleavage
Foreign name: cleavage

Introduction: Zygote Early division of
Features: cell proliferation Acellular growth

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six biosynthesis
seven Ontogeny

Zygote

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Rapid progress of fertilized eggs mitosis , called cleavage, when the cell does not grow, it is only divided into many small cells, which are hollow and composed of small cells Spheroid be called blastula （blastula）。 because Egg cell Of Fertilization Can occur at any stage of maturity (e.g Primary oocyte . Medium term I Secondary oocyte , mid-term II, or Meiosis End), if the egg cell is fertilized before it is fully mature, the fertilization will trigger the completion of meiosis, and then the cleavage will be carried out after the completion of meiosis.

Cleavage characteristics

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Cleavage: The fertilized egg undergoes several successive rapid cell division The development process of forming many small cells. Produced by each cleavage Daughter cell call Blastomere （blastomeres）。

Cleaving mitosis And body Cell mitosis The comparison has the following three characteristics: ① accompanied by a certain degree of redistribution of substances in the egg; ② Due to the first characteristic Nucleoplasm The proportion is increasing; ③ Cells interval Short, fast splitting, rapid formation blastula 。

The main characteristics of cleavage are cell cycle Short, Phase G1 Very short or not; Each cleavage, Nuclear matter Resynthesis and growth, the cytoplasm does not grow, but its Physical and chemical properties Changes occur, so the blastomere almost does not grow, and the next division is carried out rapidly. The size of the blastomere is getting smaller and smaller. Once the nuclear cytoplasm ratio reaches balance, the cell division speed starts to slow down. A large number of small cells formed after cleavage are convenient for later Morphogenesis Middle cell rearrangement Blastoderm and Organ primordium Formation and Embryoid body Shape shaping. The cleavage speed of various animals varies greatly, Sea urchin The eggs form thousands of cells within a few hours after fertilization, and the frogs form thousands of cells within one day Mammals The first cleavage has not been completed in one day, and each subsequent cleavage takes 10-12 hours vertebrate The middle is the slowest. The difference of animal cleavage pattern and the content in eggs yolk The quantity and distribution of egg yolk are closely related. The yolk can block the speed of cleavage, or even prevent Cleavage surface Completely separate the cells. Cleavage is divided into Complete cleavage and Incomplete cleavage 。

Complete cleavage

cleavage

The cleavage surface completely separates the fertilized egg, Blastomere The size difference is not much, average Oligoyolk egg （ Isabelline egg ）Are Holoschisis 。 Medium Telangium egg It is also cracked completely, but Plant pole There are many differences in the size of blastomere, such as most amphibians Lungfish 。 Cleavage is regularly carried out in a certain form, in 32 cells Precell More than doubled, and then gradually irregular. There are many types of complete cracks, according to Blastomere The arrangement mainly includes: radial cleavage, such as Echinoderma 、 amphioxus ； Spiral cleavage , such as some Molluscs 、 Polychaete Annelida ； Bilateral symmetry Type cleavage, such as Ascidian ； Irregular cleavage, some eggs are initially Spiral Later, it was changed to bilateral symmetry type, such as some annelids; Some cleavage procedures are irregular, and the initial cleavage of blastomeres is not synchronous, with 3 Cell phase , such as most mammals. Mammalian cleavage to 16-64 cell stage, forming multicellular The solid sphere is a typical morula.

Incomplete fissure

The yolk rich eggs undergo this cleavage because they contain a large amount of yolk, Cleavage surface It cannot pass through the whole egg, and cleavage occurs only in the cytoplasm of the egg. yes Discoid cleavage , cleavage occurs in Animal pole Blastoderm The yolk beneath the blastoderm does not divide, as in teleost fish 、 Reptiles and birds ； Surface cleavage ， Insect egg Of yolk It is concentrated in the center of the egg, the nucleus and a small amount of cytoplasm are located in the center, and most of the cytoplasm is distributed around the egg. After multiple divisions, the cleavage ring is finally located on the surface of the egg.

A fertilized egg of an animal, or an egg that develops parthenogenously without fertilization, without interval , continuous cell division without increasing the total volume.

Generations generated by cleavage Daughter cell , called Split ball 。 At each cleavage, the expression of the nucleus is mitosis Similar, but the schizoid does not grow after division. Therefore, after continuous division, the volume of the cytoplasm of each division ball gradually decreases while the volume of the nucleus remains unchanged. Therefore, the ratio of nucleus to mass increases from small to large. Until the ratio of nucleus to cytoplasm is the same as that of somatic cells of this animal, the mitosphere will start to grow and carry out general cell division. as Sea urchin The ratio of nucleus to cytoplasm of the fertilized egg is 1 ∶ 550 Blastocyst stage The nucleus to cytoplasm ratio of the mitochondrion is 1:6.

effect

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In the process of cleavage, not only DNA synthesis Fast, and it is known that in some animals, there is no cleavage Phase G1 。 Xenopus laevis Except for G1, Phase G2 It is also very short, so that the whole Fission cycle Short. Therefore, the time between two divisions is much shorter than that of adult cells (see cell cycle )。

cleavage

Although the rate of cleavage is related to the temperature of the environment, which is higher and faster, it is mainly determined by genetic factors and is related to the egg quality. If the Sea urchin The eggs are divided into two halves, nucleated and non nucleated, which are fertilized with the same sperm respectively. Then the two halves of the eggs divide at the same time and at the same speed as the normal fertilized eggs, or when hybridizing two sea urchins with different cleavage speeds, remove the nucleus of the fast splitting eggs first, and then fertilize them with the sperm of the slow splitting species Speed ratio The slow rate of planting is fast, which indicates that the cleavage rate is determined by the egg quality.

Another characteristic of cleavage is the relative spatial position of the substance in the egg plasma, that is, the distribution of the egg substance before cleavage and the distribution of the egg substance after cleavage Split ball The overall material distribution is basically consistent.

Direction of cleavage and egg content yolk Quantity matters. Yolk also affects the rate of cleavage. The so-called Belfort rule is that the cleavage speed of any region of the embryo is inversely proportional to the amount of yolk it contains. Such as amphibians and Sturgeon Inferior fish Eggs, Plant hemisphere It splits slowly because it contains a lot of yolk. O. Hertwig believes that: ① The nucleus is always in the Protoplast To affect cleavage. ② Spindle And Protoplasm The long axis of the mass is consistent, and the cell is transversely cut into two according to the long axis of the protoplast during division. The general cleavage process starts from no or little yolk and sinks into the surface of the egg to form a cleavage surface. Generally, the first split plane is from Animal pole reach Plant pole , via Egg axis , with egg Equatorial surface Vertically, the egg is divided into two basically equal fission balls; This split is called Meridional fissure 。 The splitting surface of the second cleavage is also meridional cleavage, which is perpendicular to the first meridional cleavage surface, thus four Split ball 。 The direction of the fourth splitting surface is different from that of the first two, which is parallel to the equatorial surface and perpendicular to the first two. The existing four splitting spheres are divided into four upper, four (animal pole) and four lower (plant pole) splitting spheres. This splitting is called Weft cleft 。 After the fourth time, the split surface is not very regular, some are warp split, and some are weft split.

type

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The number and distribution of yolks in cleavage type eggs directly affect the type of cleavage. Eggs with few and evenly distributed yolks, Split surface The egg can be completely separated; Eggs with many and concentrated yolks cannot be completely separated by the cleavage surface. Other structural characteristics of the egg can also affect the type of cleavage, such as the egg mass density of some animals（ Ascidian (eggs of mollusks) and egg mass flow (eggs of mollusks) will affect the position of the metaplate.

The cleavage can be divided into two categories:

Holoschisis

The split surface will Split ball Completely split in two, Daughter cell Completely separate. The eggs of most animals, not only yolk It is evenly distributed, and the size of the split ball is generally not much different, which is full split; Some eggs with uneven distribution of yolks, such as those of amphibians and some lower fishes, are also holoschisis. However, the size of the third split ball is quite different Animal pole Is small in size, called a small split ball Plant pole It contains more yolks and has a large volume, which is called the big fissure ball.

Complete split can be divided into 6 types according to the arrangement of split balls:

① After the third cleavage of the radiation type, the blastomeres in the upper layer are arranged neatly above the lower layer in a radiation arrangement (Figure 1), such as echinoderms and coelenterate Etc.

② At the third spiral cleavage, Spindle Not and Equatorial surface The animal pole is vertical but inclined at an angle of 45 ° Split ball Located in two Plant pole The later cleavage between the cleavage balls is also inclined (Figure 2), press Clockwise Inclined, called dextral; Press counter-clockwise direction Inclined, called left-handed. They are usually arranged alternately in right and left directions, such as Polychaete Annelida , mollusk Gastropods and Lamellibranchia Etc.

③　 Bilateral symmetry The situation of type A cleavage is similar to that of radiation type, mainly because after the first cleavage, it has become bilaterally symmetric, and it has remained this form for many subsequent cleavage. larva It is consistent with the symmetrical plane on both sides of the adult and the first split plane (Figure 3), such as amphibians and Caudate animals Etc.

④ After the second cleavage of the biaxially symmetric type, the four mitospheres Left right symmetry , front and back are also symmetrical, and later cleavage always keeps this situation, so it is called biaxial symmetry (Figure 4), as shown in Ctenophora 。

⑤ The mixed type is spiral type at first, and then symmetrical type. Such as some Annelida 。

⑥ Irregular Split ball The arrangement and cleavage process are irregular, such as Ascaris lumbricoides The arrangement of the mitotic spheres after the second cleavage of (Fig. 5); Another example is the mammalian egg. From the second cleavage, the cleavage balls are irregularly arranged.

cleavage

Incomplete fissure

It is also called eccentric crack. A large amount of yolk is concentrated at one end of the egg or the center of the egg, so that the cleavage surface cannot divide the egg into two parts, only the egg cytoplasm is separated, and the parts containing yolk are still connected. Incomplete fissure It can also be divided into two types:

①　 Discoid cleavage Cleavage occurs only in Animal pole Of Blastoderm A large number of yolk parts under the blastoderm do not divide, as in mollusks Cephalopods (Figure 6), Cartilaginous fish 、 teleost fish , reptiles and birds.

②　 Surface cleavage 　 yolk Concentrated in the center of the egg, the first few cleavage is only the nucleus in the yolk Protoplasm In the island, the nucleus reaches a certain number, enters the egg cytoplasm around the yolk, and continues to divide on the surface, such as insects (Figure 7).

Cleavage receptor Genetic factor The distribution of substances in the cytoplasm of all kinds of animals is dominated by the distribution of substances, which has an important impact on the type of cleavage. Different types of material distribution lead to different types of cleavage. Such as Spiral cleavage Of Flatworm , the spiral case is generally twisted to the right; The direction of spiral shell twisting is determined by the direction of spiral cleavage（ Figure 8 ）。 The dextral cleavage of wild type is dominance (D) , left-handed is recessive (d). however Homozygous The eggs produced by a left-handed female, even if fertilized by the sperm of a right-handed male, still show left-handed characteristics. That is to say, the distribution of substances that affect the cleavage direction has been determined by the maternal genes during the formation of the maternal eggs genome This has no effect.

cleavage

mechanism

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It is generally believed that eggs Equatorial ring The contractile substances of the two groups play a major role in cleavage. From the measured polar and equatorial regions of the egg surface tension It is speculated that there is a strong surface tension Shrink ring 。 ultrastructural And found that a squid 、 Polychaete and Salamander Waiting Split ball There are 50~70 angstroms under the surface Microfilament , which is parallel to the equatorial surface and splitting surface beside the splitting trench. Cytochalasin B It can dissolve microfilaments. If treated with cytochalasin B before cleavage, the nucleus still divides but the cell does not divide. If the splitting ball that is starting to split is treated, the formation of the splitting groove stops or recovers to Trenchless State, indicating that microfilaments are the main material basis of cleavage.

The changes in the tenacity of the egg's cortex and the egg's surface tension are also Cleavage sulcus Formed necessary condition 。 from Star ray Constituent Mitotic apparatus The position in the cell directly affects the changes of surface tension and cortex toughness.

biosynthesis

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Cleavage stage Split ball Although it does not grow, some substances are still being synthesized. protein The synthesis of is always in progress. Histone It is the component of the nucleus, Sea urchin In the metaphase of embryo cleavage, 50% of the proteins in the nucleus are newly synthesized. The main substances involved in the cleavage mechanism - Microfilament , is by Actin This protein was also synthesized during this period. In addition, there are major enzymes in the cleavage stage, such as nucleotide reductase, DNA polymerase Is also newly synthesized. RNA The synthesis of is very inconsistent, generally only a very small amount, or lack of. There are very few new rRNA Appears, but on Frogs There was no rRNA synthesis during cleavage. mRNA and tRNA It is synthesized at least at the late stage of cleavage.

In many animals, many mRNA in cleavage stage are also in a "hidden" inactive state, as in unfertilized eggs. however mammal Of eggs, which is completely different from the above situation. RNA is synthesized early in cleavage. For example, in the two cell stage of mouse embryo, it can be found that there are newly synthesized RNA similar to mRNA, rRNA and tRNA are synthesized slightly later, and there is obvious rRNA synthesis in the four cell stage.

Ontogeny

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Cleavage and Ontogeny During the development of eggs, except yolk In addition, a large amount of material was accumulated in the egg, and there was a certain distribution. Cleavage distributes these substances to different Split ball In later development, these substances - Ascidian E.G. Conklin called it organogenic substance - determining the Split ball What tissues and organs will be formed respectively in the future. In the development of some animals, if one or several mitospheres are damaged, some tissues or organs will be caused Hypoplasia , or even completely lacking. This kind of egg can not adjust itself during development, which is called Mosaic egg 。 The eggs of other animals, such as Amphibian or Sea urchin Of eggs, affected by After injury It can adjust itself during development. Sea urchin Zygote When developing to two cells and four cells, each blastomere is separated, and each separated blastomere can develop into a normal embryo. This egg is called Accommodative egg 。 There are also various transitions between the two types. Some animals have poor egg adjustment ability, which is close to mosaic type, and some have strong ability, which is close to adjustment type.

But no matter what type of egg, some substances are distributed to a certain level through cleavage Split ball , each mitosphere contains different substances, and will develop different Tissue and organ 。 Such differentiation is simply due to cytoplasm The difference is due to the different cytoplasm nucleus There are also differences, resulting in different structures of each split ball. Experiments have proved that the nucleus of the mitosphere is isoenergetic, which means that the development ability of the nucleus of the fertilized egg remains unchanged after several divisions. J. B. Gordon transplanted the nucleus of the intestinal epithelium of Xenopus tadpole into the enucleated egg, which can make the egg develop into a tadpole. It shows that the nucleus of a well differentiated cell still has the ability to develop into a normal individual and can be displayed in an appropriate environment.

Nuclear is omnipotent Split ball Develop into different structures, probably cytoplasm Play a leading role. T.H. Boveri It was found in horses in the early 20th century Ascaris lumbricoides Medium, somatic cell yes Chromatin depletion Phenomenon. In the first four cleavage cycles, some chromatin in somatic cells was reduced, while the primordial germ cells contained special Genitalia send Chromatin To remain intact.

It is generally believed that after cleavage, the cytoplasm of the egg is divided into different parts, and the isoenergetic nucleus is under the influence of different cytoplasm. Different kinds of cytoplasm stimulate different gene activities in the nucleus, and each mitosphere has different differentiation, so cleavage is the basis for the normal development of the embryo.

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