THE ENDOPLASMIC 
RETICULUM 
In 1945, as the electron microscope 
was becoming a useful research tool, 
Albert Claude of Belgium and Keith 
Porter, who was then at The 
Rockefeller Institute, used it to discover 
a vast network of channels bounded 
by membranes in the cytoplasm of 
chick embryo cells. At times this net- 
work looked like the concentric circles 
of a slice of onion. Porter called this 
network the endoplasmic reticulum 
because it was more concentrated in 
the inner (endoplasmic) region of the 
cell than in the peripheral (ectoplas- 
mic) region. Similar networks were 
later found in all eukaryotic cells, 
except mammalian red blood cells. 
It was discovered that the mem- 
branes of this endoplasmic reticulum 
all interconnect, forming a system of 
tubes and flattened sacs that is con- 
tinuous with the nuclear membrane. 
In effect, this system divides the cyto- 
plasm into two main regions, one 
enclosed within the "plumbing" and 
the other forming the outer region, or 
cytoplasmic matrix. 
Some parts of this membrane look 
smooth, while others appear "rough" 
because they are dotted with ribo- 
somes that form granules on their 
outer surfaces. These ribosomes de- 
posit newly formed proteins into the 
lumen, or inner space, of the endo- 
plasmic reticulum. The endoplasmic 
reticulum then segregates the proteins 
into those that will be needed in the 
cytoplasm and those that will be trans- 
ported to other organelles or secreted 
from the cell. 
In the mid- 1 950's, George Palade, 
then of The Rockefeller Institute, con- 
cluded that the amount of rough endo- 
plasmic reticulum in a cell corresponds 
closely to the quantity of protein the 
cell exports. Those white blood cells 
that produce infection-fighting immune 
system proteins called antibodies have 
highly developed rough endoplasmic 
reticula, for example. These antibod- 
ies are found mainly in cellular storage 
areas, from which they go forth to 
combat infections. The smooth endo- 
plasmic reticulum, on the other hand, 
is particularly well developed in cells 
where it takes on some extra function 
— for example, in liver cells, where it 
breaks down drugs by making them' 
water soluble. 
In addition to its role in protein seg- 
regation, the endoplasmic reticulum is 
the cell's membrane factory. Phospho- 
lipids and cholesterol, the main com- 
ponents of membranes throughout the 
cell, are synthesized in the smooth 
portion of the endoplasmic reticulum. 
These compounds form the coating of 
protein-filled sacs, called vesicles, that 
"bud off" from the endoplasmic reticu- 
lum, migrate to another organelle, 
fuse with it, and then deposit the pro- 
tein cargo. 
Most of the proteins leaving the 
endoplasmic reticulum are still not 
mature; they must undergo further 
processing in another organelle, the 
Golgi apparatus, before they are 
ready to perform their functions within 
or outside the cell. 
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