2. 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.
3. Characteristic
Yolks is the 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
4. Types of yolk
In animals, the yolk is of 3 types
1. Granular yolk
2. Yolk platelets
3. Yolk sphere
5. Granular yolk
It is 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.
6. The yolk Platelets contain 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.
7.
8. Yolk sphere
The yolk 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.
9. Function
It store food 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
