the main results of fertilization 1. restoration of the …classification of ova according to yolk...
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The main results of fertilization
• 1. Restoration of the diploid number of chromosomes
• 2. Determination of the sex
• 3. Initiation of cleavage
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Cleavage
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Definition of Cleavage
• The repeated division of a fertilized ovum, producing a cluster of cells with the same size as the original zygote
• OR
• Succession of rapid mitotic cell divisions that followed fertilization up to blastula stage.
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Differences between Cleavage and Normal mitosis
• 1. Site of occurrence:
• Cleavage: In zygote or parthenogenetic egg.
• Normal mitosis: In most of somatic cells.
• 2. Interphase:
• Cleavage: shorter period.
• Normal mitosis: longer period.
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• During cleavage, the cell undergo the S (DNA synthesis) and M
(mitosis) phases of the cell cycle but often virtually skip the G1
and G2 phases.
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• 3. Growth:
• Cleavage: Does not occur. The total cellular volume of the early blastula is about the same size as the original zygote; just very small cells.
• Normal mitosis: Occurs during interphase
• 4. Oxygen consumption:
• Cleavage: High as is very rapid process
• Normal mitosis: Low as is slow process
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• 5. Size of daughter cells:
• Cleavage: Decreases.
• Normal mitosis: Remains same after growth.
• 6. DNA synthesis:
• Cleavage: Faster.
• Normal mitosis: Slower
• 7. Nuclear-cytoplasmic ratio:
• Cleavage: Increases.
• Normal mitosis: Remains same.
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Yolk in the Ovum
• Yolk defined as the region of the ovum that richer with lipoprotein and mRNA
• Yolk plays a role in cell division( it retards cell division)
• Amount of yolk determines the pattern of cleavage
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Classification of ova according to yolk amount
• 1. Microlecithal: having little yolk e.g. , sea urchin, star fish and mammals eggs
• 2. Mesolecithal: an egg that has a yolk of intermediate size and strongly concentrated in one hemisphere e.g., amphibian eggs
• 3. Macrolecithal: Enormous amount of yolk is present e.g., birds, lizards and fish eggs
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Classification of ova according to yolk distribution
• 1. Isolecithal: having the yolk small in amount and nearly uniformly distributed, e.g., star fish, sea urchin and mammalian eggs.
• 2. Telolecithal: Yolk becomes more abundant and tends to concentrate in one hemisphere of the egg e.g., fish, amphibian, reptiles and birds
• 3. Centrolecithal: Yolk is concentrated in the interior of the egg and the cytoplasm is distributed as a thin layer on the outside of the yolk, as in insects and many other arthropods
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Patterns of Cleavage
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Rotational cleavage in Mammals
• This pattern of cleavage is characterized by slowness of cell division.
• Another feature that distinguish mammalian cleavage , the first cleavage is a normal meridional(vertical); however, in the second cleavage, one of the two blastomeres divides meridionally and the other divides equatorially (horizontally). This is called Rotational Cleavage
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• As a result of that pattern of cleavage (Rotational) blastomeres
don’t divide at the same time (Asynchrony), so mammalian
embryos don’t increase exponentially, but frequently contain odd numbers of cells.
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Cleavage in Human
• Once the zygote has reached the two-cell stage after 30 hours of fertilization, it undergoes a series of mitotic divisions, increasing the number of cells. These cells, which become smaller with each cleavage division, are known as Blastomeres.
• Until the eight-cell stage, they form a loosely arranged clump.
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Two- cell stage
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Four-cell stage
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Eight-cell stage
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• Each cell (blastomere) up to 8-cell stage is totipotent.
• Totipotency means, each cell able to form everything, this mean if a one cell has been separated from the others, it will be able to develop into an embryo
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Compaction
• Blastomeres up to 8-cell stage are loosely arranged with plenty of space between. After the third cleavage, blastomeres undergo spectacular change in their behavior.
• Cell adhesion proteins such as Ecadherin are expressed, cells become tightly adhered(huddle) forming a compact ball of cells held together by tight junctions.
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Morula( 16-32 cells )
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• At morula stage and as a result of compaction. Inner cells of morula constitute the inner cell mass (ICM) small in size, and surrounding cells compose the outer cell mass (OCM) large in size.
• ICM: gives rise to tissues of the embryo proper
• OCM: forms the trophoblast, which later contributes to the placenta.
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ICM
OCM
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Conclusion
•Cells after the third cleavage become pluripotent (capable to form many things)
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Blastula Stage of different organisms
• The blastula (from Greek βλαστός (blastos), meaning "sprout") is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the blastocoele formed during an early stage of embryonic development in animals
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Blastulation
• The mechanism of cavitation(blastocoel formation) in blastula stage.
• Sodium ions pumping to inside( between the inner cells in the compact ball) , as a result of that water uptake will be, cells will be displaced forming the balstoderm
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Types of Blastulae
• 1. Coeloblastula:
• A. centric blastocoel in sea urchin & star fish
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• B. Eccentric blastocoel: in Amphibian
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• 2. Blastocyst blastula: in mammals
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3. Superficial blastula: in insects
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4. Stereoblastula
• It is a solid blastula because there is no blastocoel cavity,
• The type is formed due to spiral cleavage.
• E.g. Molluscs
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5. Discoblastula in birds, fish and reptiles