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

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Abstract
The internalization of the yolk proteins has been investigated by EM and cytochemistry in the oocyte of the trout which stores up large quantities of yolk. The oocyte evolution as followed for 18 mo. in a homogenous group of animals. Anionic ferritin was injected during vitellogenesis. As in other oocytes the yolk proteins seem to be 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, i.e., 10 mo. earlier. During vitellogenesis yolk progressively accumulates in this lysosomal compartment. Injected anionic ferritin follows the same pathway of internalization. Apparently, 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.