evidence from which this concept evolved. Stein
and Stein localized radioautographic grains over
subcellular structures after the intravascular injection
of [SH]glycerol and palmitic acid. Although
the interpretation was made initially that grains
were localized over the rough endoplasmic reticulum
by 2 and 5 min, the published images show
grains only in the vicinity of the ends of parallel arrays
of RER or of single RER cisternae, and no
grains were clearly associated with flattened cisternae
containing ribosomes (49). Although the
grain size was too large for precise localization,
these investigators have recently stated that
grains were seen often in the regions of transition
between the rough and smooth ER (50). Thus,
contrary to previous interpretations, the radioautographic
studies suggest to us that the ribosome-
studded, flattened cisternae of the rough
endoplasmic reticulum do not synthesize triglycerides.
The presence of label clearly localized in
regions of transition between smooth and rough
endoplasmic reticulum, particularly at the earliest
times of sampling of liver (2-5 min after injection),
suggests that triglyceride synthesis occurs
either in this transitional compartment or in
nearby smooth ER cisternae (and is then transported