Electron micrographs showing special associations of LDs and ER within briefly AIM-stimulated human preadipocytes.
(a) Small and characteristic LDs can be found by short-time adipose conversion showing droplets with several flat layers and cisternae of smooth endoplasmic reticulum (LDER, white double arrows; simultaneous fixation method). Occasionally, very regularly spaced dots of vimentin filaments can be seen at the surface of such droplets sandwiched between droplets and ER (black arrowheads). In addition, smaller LDs of 0.2– 0.5 µm in diameter without such surrounding ER layers - but with interaction of vimentin IFs (marked by black arrows; left side) – and larger, fully accomplished LDs of several µm in diameter with mitochondria (M) in the neighborhood can be seen. (b) The regular pattern of vimentin arrays at the surface of LDs can be recognized best where fewer and less densely packed ER sheaths surround the droplets (black arrowheads), which might constitute a specific stage of adipogenesis. In this situation LDs seem to be matured, i.e. grown enough in size and the multiple layers of ER cisternae are just about to be released. Note, such EM pictures suggest that different stages and types of LDs in differentiating adipocytes can be pictured and characterized. Bars: 0.50 µm.