Oogenesis:A Collaborative Effort

The amount and distribution of yolk varies considerably in the animal kingdom. You should be able to distinguish among oligolecithal, telolecithal and centrolecithal eggs and name examples of each. How is an amphibian egg classified?

Yolk is formed in one of two ways: It is either synthesized within the oocyte (autosynthesis) or exogenously and transported into the oocyte (heterosynthesis). Vertebrate vitellogenesis is predominantly heterosynthetic. The yolk precursor (vitellogenin), which is synthesized in the liver, is incorporated via receptor-mediated endocytosis. Study this process in your textbook. A similar process occurs in Drosophila. Yolk is synthesized in the fat body and ovaries.

(See Browder et al., Figures 3.17-3.20; Gilbert, Fig. 22.22; Kalthoff, Fig. 3.20; Shostak, Fig. 7.32)

Yolk is distributed in the amphibian oocyte in an asymmetrical pattern, with most platelets and the largest ones located in the vegetal hemisphere. The other hemisphere is the animal hemisphere. This polarity foreshadows the polarity of the embryo itself. The establishment of this polarity has been studied by monitoring the deposition of fluorescent-labeled vitellogenin.

(See Browder et al., Fig. 3.22)

Young oocytes have a centrally located germinal vesicle (what is a germinal vesicle?) and uniformly-distributed yolk platelets; the latter are localized in the subcortical cytoplasm. Asymmetry results from a displacement of the yolk platelets from the animal hemisphere to the vegetal hemisphere (at a rate of 50 �m/day). The displacement process presumably involves the cytoskeleton.

The cortex is a semi-rigid gel (as distinct from the endoplasm, which is fluid). The cortex resists centrifugation. The cortex may contain specialized organelles, such as cortical granules and/or pigment granules

(See Browder et al., Figs. 3.11 and 3.17)