Seborrheic Keratosis: 5-Minute Pathology Pearls

Seborrheic Keratosis: 5-Minute Pathology Pearls


So the first case that we have here this is
a seborrheic keratosis. These are kind of benign keratinocyte proliferations
that usually occur in older people and have this kind of stuck on appearance. Usually they’re dark in color. And microscopically there’s a variety of different
patterns you can see. This was kind of an acanthotic pattern. The epidermis is really thick. Look here’s the normal epidermis. You can see the thickness of it right there. And then here’s the lesion, look at how thick
and how elongated the rete ridges are there, really thick and fat and filled with cells. And the rete ridges all kind of interconnect
together as they come down. Let’s find our arrow here. See they come down, kind of interconnect and
kind of spread along merging with each other. And what is left behind are these cystic spaces
that are filled with usually with loose orthokeratin. So this is dead keratin like you would have
up here in the corneal layer. It’s the same stuff that you have it’s accumulating
down here. And these are not really cysts, these are
pseudocysts because if you cut deep but of what will happen is these cysts actually connect
up to the surface. So this is kind of a funny artifact of the
way we cut through and section the tissue. So these are called horn pseudocysts. So you get a thickening of the epidermis and
it’s made of these kind of small some people say basaloid, I don’t really like that term
personally because I think these look a quite a bit different basal cells, but they’re kind
of small keratinocytes, they’re usually very uniform and monotonous. And they often have pigment in them, and that’s
why these lesions look dark brown clinically. You can see that pigment there, that’s melanin
pigment. Now remember that melanocytes make melanin
and then kind of feed it to their neighboring keratinocytes. So just because he’s a brown and it doesn’t
mean that they’re actually melanocytes, it’s actually keratinocytes. And you can see this little halo that they
have. If you have a little halo with the naked nucleus
in the middle, that’s a good sign that you’re probably done the keratinocyte not a melanocyte. Alright, so this is a seborrheic keratosis. And these are benign sometimes they can clinically
look like melanoma or other melanocytic lesions. Here’s another cut from the same lesion, look,
again there’s a thickening of the epidermis and these horn pseudocysts that are filled
with that orthokeratin. Now if you start seeing a bunch of dense pink
keratin with retain nuclei, parakeratosis, in the middle of these pseudocysts, then you
have to stop and think about up the possibility of a subtle squamous cell carcinoma. Because squamous cells they tend to make little
keratin pearls but instead of it being loose orthokeratin like this, it usually has parakeratosis
in the middle. When seborrheic keratoses get inflamed or
irritated they can also make parakeratin down in those horn pseudocysts. So you don’t want to use that as a hard and
fast rule, but it is a thing that’s worth considering. And another thing I think is useful about
seborrheic keratosis, let me get in focus here, is that they usually kind of grow up
from the surface of the skin. They kind of push up. And that actually if you can kind of draw
a straight line across the bottom of the lesion, most the time. See if we go to the top piece, it gives you
the same thing. The lesion looks like it’s growing down, but
it’s actually not. It’s actually pushed up like a little dome
above the skin. And you can draw a straight line across the
bottom. So seborrheic keratosis usually do not infiltrate
down the skin, they kind of push up. See again the same thing on the third piece
here, you can draw a straight line underneath. So thickening, a thickened lesion that’s kind
of a rising for the epidermis, made of bland small keratinocytes, often with melanin pigment
and then horn pseudocysts. That’s a seborrheic keratosis. Let’s do the next case. We’ll make it right side up. This lesion is so big that we can’t actually
even put the whole thing underneath the microscope, but let’s look and see if we can figure out
what this says. It’s a lot bigger than the previous case and
all that bright red stuff, that’s keratin, just like the last case. It’s just a little more dense, that’s why
it looks so red and also I think this slide I may have just picked up some more of the
eosin stain. But look if you see the epidermis is thickened,
the rete ridges are really elongated and fusing together, and what you’re left with are these
horn pseudocysts again. These kind of pseudocystic areas in between. And look, when you get a certain cut here,
you can see those cysts aren’t really cysts at all. They open up to the surface, that’s why we
call them pseudo cysts. And here it almost looks a little bit warty. So you can sometimes think about a verruca
or a wart with this lesion. But look right here, you get kind of a straight
line across the bottom. I think this is another example of seborrheic
keratosis. This one is very irritated and inflamed. It’s got an area over here that you can see
is really thick. And has got a lot of lymphocytes, a lot of
inflammation. So sometimes these lesions get really inflamed
over time and when they do they can sometimes mimic squamous carcinoma clinically. And look at all that pigment there, this is
why they look so dark. Not only is the pigment in the keratinocytes,
which you can see up here, there’s pigment in keratinocytes, but there’s also these little
branchy thin dendritic cells. Dendritic means as little branches arms. Sorry just let me adjust the light a little
bit. And those are actually melanocytes, those
are pigmented melanocytes that are kind of dendritic, you can tell because of their little
branching processes. So melanocytes usually aren’t very darkly
pigmented, but sometimes they are. So there’s always exceptions to every rule
in dermatopathology, and sometimes dendritic melanocytes get caught up in the middle of
a seborrheic keratosis, and they further contribute to the dark color. And also when seborrheic keratosis s get inflamed,
they often will do this, they will drop melanin pigment out of the epidermis and into the
dermis, and then macrophages come and eat that up. And so those are called melanophages. So if you see real dark cells in the dermis,
usually they’re going to be melanophages, which are pigmented macrophages that are eating
up the melanin. Usually they’re not melanocytes. Again, exceptions exist for all these rules,
but that’s kind of a good rule of thumb if you see real dark cells in the dermis, most
the time they’re melanophages. So go back to lower power here again, even
in this area that’s more inflamed, you can see it’s a thickened lesion arising from the
epidermis. You got horn pseudocysts which are really
useful clue for low power. And again, when they look kind of darker blue
or purple, sometimes squamous cell carcinomas can have the same kind of pattern. So go a little closer and look, but I still
think of the cells here are very uniform and monotonous looking. And I think that’s good. And sometimes they can get a little bit of
atypia and some mitotic activity when a seborrheic keratosis is gets really inflamed. And you see when you get irritation or inflammation
in a seborrheic keratosis, sometimes the horn pseudocysts do begin to produce parakeratin. And so again I always look a little closer
when I see that just to make sure I’m not missing a squamous cell carcinoma, but in
this case I think what we have is just a very big robustly irritated and inflamed seborrheic
keratosis. This is a very large one. And I can imagine that if I had that on my
skin, I would want it to be removed as well. Probably very uncomfortable.

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