Amphibian Vitellogenesis

Amphibian oocytes undergo an extensive period of growth primarily due to the deposition of yolk protein, vitellogenin, into the oocyte. The pituitary gland releases FSH which travels via the bloodstream to the ovary and stimulates the somatic follicle cells surrounding small oocytes (stage III to stage V). The follicle cells secrete estrogen into the bloodstream which travels to the liver nuclei and stimulates vitellogenin gene transcription. The liver cells translate the mRNA into vitellogenin protein and release the yolk protein back into the bloodstream. FSH stimulates endocytosis of the yolk protein into the oocyte cyoplasm. The membrane-bound vesicles fuse to form the yolk platelets that concentrate primarily in the vegetal hemisphere of the oocyte.