Endosomes transfer yolk proteins to lysosomes in the vitellogenic oocyte of the trout.

The internalization of the yolk proteins has been investigated by electron microscopy and cytochemistry in the oocyte of the trout which stores up large quantities of yolk. The oocyte evolution has been followed for 18 months in a homogeneous group of animals. Anionic ferritin has been injected during vitellogenesis. The results indicate that as in other oocytes the yolk proteins are absorbed by coated vesicles during vitellogenesis. But unlike most other oocytes the yolk proteins are then transferred via typical endosomes to a conspicuous lysosomal compartment built up very early at the onset of the cytoplasmic differentiation of the oocyte e.i. 10 months earlier. During vitellogenesis yolk progressively accumulates in this lysosomal compartment. Injected anionic ferritin follows the same pathway of internalization. These findings indicate that in this oocyte, the whole yolk cycle presumably represents an adaptation of a general cellular activity, the receptor-mediated endocytosis, largely amplified, sequenced and spread over several months.