Question

Vitellogenesis is the process of
A) Formation of vitelline membrane around the developing egg cell.
B) Formation of yolk sac membrane around the yolk mass during the development of reptiles and birds.
C) Formation of yolk and its deposition within the growing oocytes in amphibians
D) None of the above.

Answer 1

Hint: Vitellogenesis process is also known as yolk deposition. Chemically, yolk is a lipoprotein made of proteins, phospholipids and neutral fats with a small amount of glycogen. It is synthesized in the liver of the female parent in soluble form.

Complete answer:
Vitellogenesis is the process of formation of yolk through nutrients being deposited in the oocyte, or female germ cell involved in the reproduction of lecithotrophic organisms like reptiles, birds and amphibians. Other than mammals, it occurs in all groups of animals. Hence, the correct option is C, ‘Formation of yolk and its deposition within the growing oocytes in amphibians’.

The maturing ovum is surrounded by the follicle cells, which receive the yolk and are deposited as yolk platelets and granules in the ooplasm. It is believed that mitochondria and golgi complexes are responsible for converting the soluble form of yolk into insoluble granules or platelets.

Yolk accumulation in animal embryology:
>Isolecithal- Yolk is sparse and evenly distributed.
>Mesolecithal- moderate vegetal yolk disposition.
>Telolecithal- Yolk is dense and present throughout most of the cell.
>Centrolecithal- Yolk is in the center of the egg.

Amphibian vitellogenin is a sex-limited protein found in the serum of normal females. Vitellogenin is the protein from which the substance of egg yolk is derived. It is synthesized by the liver and induced by estrogen. The uptake of vitellogenin is stimulated by gonadotrophins and is highly selective. Once incorporated into the oocyte, vitellogenin is converted within the yolk platelets into the yolk proteins.

Hence the correct answer is option ‘C’.

Note:In amphibians, mesolecithal type yolk is present and displaced radial cleavage occurs. Vitellogenesis, or the production and packaging of the nutrients, yolk proteins, into the developing oocyte in the ovary is important in providing the egg with the nutrient resources to complete the process of embryogenesis.