Ultrastructural indications for autosynthetic proteinaceous yolk formation in amphibian oocytes

The formation of proteinaceous yolk is a main feature during amphibian oogenesis. The main bulk is built up by a process called heterosynthesis. The precursor complex vitellogenin is synthesized in the liver, transported by the bloodstream to the ovary, where the oocytes sequester the material by means of endocytosis. This pathway has been described in detail by many authors. The ultrastructural study of amphibian oocytes indicates on the other hand a small but distinct contribution of the oocyte itself towards yolk formation. This process has been called autosynthesis and starts before the onset of heterosynthetic activities. The cell organelles possibly involved in yolk-recursor and yolk-platelet formation are the nuclear envelope, annulate lamellae, endoplasmic reticulum, Golgi complex, GERL and mitochondria. The aim of this paper is to discuss the data, mainly of ultrastructural nature, so far accumulated during the study of autosynthesis. It is hoped to stimulate more biochemically orientated research in this field.