vittelogenesis-220726044328-5b7c4706.pdf

vittelogenesis-220726044328-5b7c4706.pdf

• 1.
  Development Biology Vittelogenesis ByMiss Iram
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  Definition of vittelogenesis from Latin vitellus-yolk, and genero-produce  Vitellogenesis (also known as yolk deposition) is the process of yolk formation via nutrients being deposited in the oocyte, or female germ cell involved in reproduction of lecithotrophic organisms. In insects, it starts when the fat body stimulates the release of juvenile hormones and produces vitellogenin protein.
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  Characteristic  Yolks isthe most usual form of food storage in the egg.  Yolks appear in the oocyte in the secondary period of their growth called vittelogenesis.  Thus,the formation and deposition of yolks is known as vittelogenesis  Yolks is a complex variable assembled component.  The principle component are protein,phospholipid and fats in different combination.  Depending upon these component yolks is distinguished into protein yolks and fatty acid  For eg- the avian contain 48.19% water , 16.6 % protein, 32.6% phospholipids and fats and 1% carbohydrates
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  Types of yolk In animals, the yolk is of 3 types  1. Granular yolk  2. Yolk platelets  3. Yolk sphere
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  Granular yolk  Itis the characteristic feature of invertebrates and lower chordates likes amphooxus and tunicates.  The protein yolk is composed of fine granules which is evenly distributed into the cytoplasm of oocyte Yolk Platelets. 1. It is found in most of vertebrates like cyclostomes, lung fishes, amphibians, reptiles and birds 2. The yolk occur in the form of large granules Known as yolk platelets. 3. The yolk platelets are oval and flattened in one plane.
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  The yolk Plateletscontain 2 main proteinaceous substances.  1. Phosphovitin - it is highly phosphorylated protein  Its molecular weight is 35000 D , 8.5% of phosphorus content is present  2. Lipovittelin- it is a protein major bound to lipid .  Its molecular weight is 4000D . 17.5% of lipid content is present.  In yolk Platelets, 2 molecules of phosphoviti are associated with one molecule of lipovittelin a structural unit  The units are being arranged in Platelets in a crystalline lattice with a regular hexagonal packing.  The yolk Platelets of bony fish have fat droplets inside its mass.
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  Yolk sphere  Theyolk of birds, reptile and bony fishes lies in a compact mass in the interior of a egg.  The cytoplasm is restricted to their layer on the surface of the egg.
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  Function  It storefood in the egg.  It influence the differentiation of neoplasm pattern of cleavage.  The size of egg is determined by amount if yolk present in it.  It is responsible for morphogenetic movement of elastomer during gastrulation.  The mature of development of an organism is governed by amount of yolk present in the egg