In situ architecture of the intercellular organelle reservoir between epididymal epithelial cells by volume electron microscopy

Mammalian epididymal epithelial cells are crucial for sperm maturation. Historically,
vacuole-like ultrastructures in epididymal epithelial cells were
observed via transmission electron microscopy but were undefined. Here, we
utilize volume electron microscopy (vEM) to generate 3D reconstructions of
epididymal epithelial cells and identify these vacuoles as intercellular organelle
reservoirs (IORs) in the lateral intercellular space (LIS), which contains
protein aggregates, autophagosomes, lysosome-related organelles and mitochondrial
residues. Immunolabelling of organelle markers such as P62, LC3,
LAMP1 and TOMM20 confirm these findings. The IOR size or number varies
across four epididymal regions and decreases with age. Rab27a mutant mice
exhibit reduced IORs in the caput epididymis and a subfertility phenotype,
suggesting the involvement of Rab27a in the formation of IORs. Furthermore,
we observe the presence of IORs between intestinal epithelial cells besides
epididymis. Amino acid transporters at IOR edges suggest dynamic protein
recycling. Our findings reveal that the IOR is an important structure critical for
organelle turnover and recycling outside epithelial cells with limited selfdegradation
capabilities.