Q. I wanted to inquire as to whether you will have histology slides or pictures on the exams.  If so, I want to ensure that I can identify pseudostratified columnar epithelium.  If we see epithelium that looks columnar and looks stratified will it always be pseuostratified?  Is there stratified columnar epithelium or only pseudostratified for the columnar cells?  Or will you present a picture where we can always see that the cells touch the basement membrane?

A. Our exams won’t contain histologic images. But it’s a good idea to get the concepts straight, though, like you are – because our exam questions often refer to histologic findings.

There is both stratified and pseudostratified columnar epithelium – and as you mentioned, in stratified epithelium only the bottom layer of cells touches the basement membrane, but in pseudostratified epithelium, every cell touches it.

This is a perfect example of something that I think would be fair game for an exam question (in text format) – but unfair for a photo on an exam (because stratified and pseudostratified can look very similar!). So to test you on this concept, I might ask something like: “Which of the following characteristics is true of pseudostratified epithelium?” And the correct answer would be: “Every cell touches the basement membrane.”

Q. I just had a quick question regarding one of the 5 glandular epithelia cells that we discussed in lecture today. With Ion transporting cells, the figure (to me) shows sodium being absorbed. Do these cells end up working in both directions, by transporting another ion out of the apical end of the cell?

A. Yes! Usually there are ions going in both directions – for some reason, this diagram just shows sodium entering the cell. Often, the movement of ions requires ATP-dependent pumps – so that’s why there are so many mitochondria in these cells.

Q. We talked about how the basement membrane can be highlighted using the PAS stain, is that because it contains sugars in the perlecan layer of the lamina densa in the basal lamina? I know in past lectures that was one of the characteristics of PAS staining, so I wanted to double check!

A. Yes! That is a big part of it (and it is really the only sugary thing we talked about in the basement membrane!). There are also some additional sugar-containing substances in the reticular lamina – but I didn’t want to make you start memorizing all of them. In addition, basement membranes in different parts of the body may have additional components (the basement membrane in the glomerulus of the kidney, for example, is insane).

But back to your question: yes – the perlecan that we talked about in the lamina densa would stain nice and magenta with the PAS stain because it’s a proteoglycan, and therefore has nice sugar residues.

Q. In lecture we talked about apical structures such as microvilli and cilia. It says that cilia are inserted into basal bodies, what are those? They aren’t part of the basal lamina even though it has the word basal correct? I am a bit confused because they are apical structures.

A. Great question! The term basal means “bottom” – and we throw it around a lot! In “basal bodies” basal refers to the bottom of the cilium! So basal bodies are little structures at the bottom (base) of each cilium. Basal bodies are the places where axonemes (the central structures of cilia) start growing; they also serve as anchoring structures for the completed axonemes.

So yes: they are apical structures, and have nothing to do with the basal lamina.

Q. Why are the 2 central microtubules not a pair but the 9 pairs on the outside are considered pairs (instead of 18 microtubules) in the cilia? Is that because they are of a different type?

A. Thank you for bringing this up! It is confusing that we call this a “9+2” arrangement when it looks like there are 9 pairs of microtubules around the outside, and one pair of microtubules in the middle! Seems like it should be either “9+1” or “18+2.” There’s a good explanation, though. Here’s a diagram of an axoneme – we’ll talk about the “2” part and the “9” part separately below.

The “2” part of the axoneme
The two microtubules in the center of the axoneme are totally separate from each other. So they can be considered to be 2 separate, independent units.

The “9” part of the axoneme
The 18 microtubules around the outside are bound together, two at a time – so they can be thought of as 9 separate, independent units.

It turns out that in each of these 9 units, one of the microtubules is incomplete. If you look at the microtubules circled in red, you can kind of see that the top one looks round, and the bottom one looks like it’s not quite round, and has sort of just latched on to the top one. The bottom one is actually not a fully-formed microtubule (if you pulled the two apart, the top one would be round, and the bottom one would look like a C-shaped structure). So we really shouldn’t even call these guys “pairs” since they don’t consist of two fully-formed microtubules. The official name is “doublet” – and that is a little better than “pair,” I guess.