Re: Do all cells contain the same organelles?

Thanks for your question, Jen.  To be honest, I had to give it a fair bit
of thought before submitting my response because when it comes to cell
biology, there are almost always exceptions to any rule you can think of!

Based on the way you framed your question, I'm fairly certain you're
already aware of the fact that the vast majority of cells from
multicellular organisms (both plant and animal) share a certain complement
of "standard" organelles: nuclei, ribosomes, endoplasmic reticulum, Golgi
bodies, and mitochondria.  When comparing plants and animals, there are
certain organelles possessed by one and not the other.  For example, only
plant cells contain chloroplasts, while only animal cells would be expected
to possess phagosomes (plant cells are not phagocytic) and centrioles.  For
more information regarding the differences between plant and animal cells,
there is another MadSci article on the subject that you might find helpful:
 http://www.madsci.org/posts/archives/oct2001/1002388734.Cb.r.html

I have a feeling you're most interested in knowing whether or not there are
different "specialized" organelles that only certain cell types within a
given organism possess.  First of all, there are certainly cell types that
may possess more or less of a particular type of organelle than another. 
Liver cells, for example, contain copious amounts of lysosomes and
peroxisomes relative to other cell types because the liver is largely
responsible for detoxifying many compounds in the blood.  Muscle cells
would be expected to contain large numbers of mitochondria relative to,
say, a skin cell, owing to their significant energy requirements. 
Erythrocytes (red blood cells), unlike most cells in our bodies, lack a
nucleus entirely.  So when it comes to organelle quantity, there are
significant cell-to-cell dissimilarities depending on function.  

There are also some specialized organelles that probably show up in only a
specific subset of cells.  The sarcoplasmic reticulum, which has so far
been found only in muscle cells, is a modified version of the endoplasmic
reticulum which serves mainly to store and quickly release calcium at
designated times.  There are several known specialized organelles that play
roles in protein secretion.  For the most part, these are different
variations on the basic vesicle theme.  Melanosomes, for example, are
produced only by melanocytes in response to ultraviolet light exposure and
contain melanin, an important pigment in the skin, eyes and hair.

Finally, there are other specialized structures that only certain cell
types possess that probably would not fall under the heading of
"organelles" but are nonetheless functionally important.  The first example
that comes to mind is the synapse of a neuron.

Concerning the second part of your question regarding where cell/organelle
differentiation takes place, I would say most of the differentiation
process probably takes place during embryogenesis when all of our necessary
cell types are derived from the endoderm, mesoderm and ectoderm.  These
differentiation programs are, of course, laid out within our genetic code.
 The specifics probably differ markedly from cell type to cell type.

While the generic animal and plant cells we see in student textbooks do not
describe the incredible structural and compositional diversity across cell
types, they do a pretty good job of giving students an idea of what makes a
eukaryotic cell different from a prokaryotic one.  And though there are
certainly differences, there are probably many more similarities!

Thanks for your question, and good luck with your students and the rest of
the school year!