A three-dimensional study of organelle interrelationships in regenerating rat liver. 2. Transient connections with luminal continuity between thin- and thick-membraned organelles.

Luminal continuities can be established between thin- and thick-membraned structures, and serve the transfer of their contents. The abrupt change in membrane thickness at the site of connection suggests that exchange of membrane constituents is very limited. This enables retrograde separation of the connected organelles. sER (endoplasmic space) makes up such continuities with exoplasmic structures (mature Golgi cisterna, GERL-system, occasionally MVB). Transfer tubules, possessing a thiny membrane but nevertheless belonging to the exoplasmic space, take up endocytized material from large coated vesicles or from coated plasma membrane loops and carry it to MVB. They offer the possibility to recycle the internalized plasma membrane.