Mostly, if not all, hepatocytes from a variety of species. Some of these images have the species designation in a lower case letter as follows: r=rhesus, st=stub-tail, m=mouse, oops, i might have put r for rat somewhere, f=ferret, gp=guinea pig. May not all be present but must a few are marked. I am looking for species differences but these are not ideal images for such detail and are just archived samples collected in 40 years of microscopy.
There are certainly differences in size, and also number of mitochondrial tethers that can be seen, and the biggest nicest desmosomes I have seen are in tongue (image below is bovine i believe — thank you to whomever’s image that is), and the most numerous of course in skin. I havn’t bothered to add micron markers…. it seems to be quite variable depending upon where in the “disc, or spot” the section occurs. The mitochondria and cristi give relevance to magnification anyway.
I found a nice review article the title is “Intermediate Filaments as Organizers of Cellular Space: How They Affect Mitochondrial Structure and Function by Nicole Schwarz * and Rudolf E. Leube” free online, about intermediate filaments and mitochondria, which is part of my reference list as I am trying to figure out where there are species differences in the ultrastructure of desmosomal-intermediate filament-mitochondrial tethers. So this is a nice diagram but i think it is too primitive, or perhaps too limited, or maybe just has not been taken to the next step.
This image is their Figure 1 which shows three possible intermediate filament – mitochondrial arrangements. My exception to it is the following : there are areas around each mitochondria where all these types of bonding-binding-exchange take place, at least it seems to me. For instance, the binding of intermediate filaments to the outer mitochondrial membrane (OMM) probably doesn’t occur (at least in hepatocytes) in a complete manner surrounding the entirety of the outer mitochondria membrane, but rather definitely is “focal” as it looks when it is tethered to another cytoplasmic organelle such as the outer edges of the nuclear pore, or the inner plaque of the desmosome. There is a definite change in the structure of the outer mitochondrial membrane, gross enough that it is seen with TEM, and looks like a thickening of the OMM and a “flattening” out of that area which is engaged with intermediate filaments. There is also a shape change to the mitochondria when they are bound (i prefer the word tethered) to the filaments. (so still talking hepatocytes here). Intermediate filaments positioned remotely from the OMM and called “signaling” is something that I don’t believe would be seen physically. As for confinement, there are certainly areas of confinement of organelles, and they might be called “no fly” zones, I also need to find an example where the mitochondria are almost repelled by intermediate filaments.
In a couple of mouse models i have looked at, intermediate filaments themselves can be repelled (or it looks that way), of which one is the central confinement of intermediate filaments in small intestine microvilli where the gastric HKatpase is missing). There are a dozen, at least, intermediate filament x OMM sites that can be seen hepatocytes where there is obvious “tethering”, and i bet signaling as well. So these categories might have a lot of crossover.
I am thinking that in order to signal, the intermediate filaments need to be physically closer to the mitochondria than this diagram projects…. I wonder how that could be determined with TEM.
This particular electron micrograph of a desmosome with mitochondria tethered to either side shows some nice orientation and detail. Particularly the intercellular space has the zipper lines that are the desmocollins and desmogleins. These lines have some regularity, but owing to the enormous numbers of possible orientation that one could get in TEM, it is not that likely that a perfect one will ever arise. I even consider the roundness of the “spot” desmosme and the possibility that the organization is radial, wouldn’t that be fun. Someone out there with 3D imaging skills could certainly test this with the molecular models that do exist. I think it would be just as fun as looking at the tomographs of thicker sections. Brown dots are likely areas where desmocollins and desmogleins are intersecting-interacting, and these represent the intercellular central dense line of the desmosome. Black lines are areas where the 5-repeats in desmocollin and desmoglein (i suspect) are spanning the intercellular space. The black dots are some kind of periodicity visible on the outer lamina of the trilaminar plasmalemma. I didn’t find any good cross sections of intermediate filaments up near the mitochondria…. though I though they were there as dense elongated areas, not nice round cross sections. (BTW… love the two eyes — aka intramitochondrial granules… these actually are very likely arranged strategically within the mitochondria near places of tethering… would love to know where and why). Red circles are around little interesting radial symmetries… that showed up…
Anyway, this micrograph and inset are from a Stub tail monkey, which was, for all intent and purposes, a control, thought it did receive a tiny test dose of artificial blood.
I have no clue, nor any record of how and why I ended up with a couple of blocks of human liver (most notably taken while i worked at childrens hospital) no way to trace the origina at all. It is kind of interesting, this sample had lots of glycogen, also very dense mitochondria (might have preserved in a different laboratory — actually definitely by someone else since I never did any tissue exams on human tissue). The desmosome (which is not quite a double-mitochondrial tether (only one clearly tethered but the mitochondria in the adjacent cell is obviously tethered just down (or up) in the same block. This desmosomal mitochodnria tether is, like many, just adjacent to the bile canaliculus.
I measured the periodicity that I saw (looking like a little zipper) on the outer plasmalemmal membranes of two adjacent hepatocytes at a desmosome (green dots). They seemed to be spaced about 19nm apart, and were small (maybe 7 or 8 nm densities). In addition the intercellular space in this desmosme really had a nice alternating linear look where the desmocollins and desmogleins would lie. Micrograph on the left, unretouched mouse hepatocytes and desmosome (mitochondrial tethered on the upper left, but somewhat inconspicuous), green dots, zipper densities, red spots, ribosomes, lines, likely to be intermediate filaments.
This particular micrograph is too tangential to the desmosome to show a lot of detail but it does show some cross sections of what I presume to be (because the size is about 10-11nm) intermediate filaments. These crossectional “dots” of IF are in the right position above the inner desmosomal plaque. Red dots = approximate ribosomal size (27nm) , blue dots = approximate IT cross section (about 11nm). Micron marker is 100nm for both images. Image on the left is unretouched, image on the right has dots over ribosomes and intermediate filaments for comparison with image on the left. Rhesus monkey, control biopsy, before the administration of perfluorochemical blood substitute, #71 female, fixative=modified Karnovsky’s (isoosmolar Chick-fix), Millonigs uffer, 2% osmium tetroxide, EPON 812.
There are some ultrastructural characteristics of this desmosome which are interesting. Having a desmosome sectioned opportunely is hit and miss. This particular section has some structures that look to be the correct size of intermediate filaments that are trailing off the the upper right under the mitochondrion, and there are definite light-dark-and dotted portions of the desmosome (in and including the plasmalemma portion) which have a distinct vertical repeating pattern.
I squeezed the micrograph bottom to top to determine if this was just a shifting of the image during photography or whether these vertical striations would still be visible when i removed (in photoshop) that kind of vertical shift. The answer was that even then the spacing and the effect of the alternating light and dark areas in the cell membrane portion of the desmosome remained. Red dot=27nm as determined by ribosome size, and thus the alternating dark and light vertical areas in the section are about 10nm in width.
In addtion, there is a clear darkening of the outer mitochondrial membrane, and a slight darkening of the inner mitochondria membrane at the point of the desmosomal-mitochondrial tethering. One can suggest that there is a relationship between this patterning and the various protein families that comprise the full structure of the desmosome, intermediate filaments and mitochondrial connection.
Control animal (neg 5936 block 8830 baboon, f, #23, right lobe of the liver, control bx, modified Karnovsky’s fix, 1% osmium tetroxide, Epon 812, (9 28 1977)). There are a lot of smooth vesicles in this sample and just a little bit of RER, three lipid droplets, and part of a bile canaliculus, possible the space of Disse in the very upper most right part of the image, mitochondria look pretty normal) I was looking for desmosomal-mitochondrial tethers and found this pretty nice desmosome and I pseudocolored the plasmalemma from the two adjacent hepatocytes present at the desmosome purple. There was a clear the “outer dense layer” of the desmosome (which i think is so unfortunately named “outer” and should have been named “inner dense layer” of the desmosomal complex). It had a particularly “loopy” appearance that I highlighted in orange. Top image is the original micrograph (two hepatocytes) and box is the area enlarged in the figure below with a desmosome).
Here is a gallery sheet of desmosomal mitochondrial tethers, and some are double, some are double-doubles (haha) and some have three tethers. I can’t overemphasize how interesting these little structures are and how critical they must be for cell function. Just considering how the desmosome itself has become a known mechanism of cell signaling, then the energy provided (mitochondria) and the movement of proteins and mitochondria and signaling proteins via the intermediate filaments, and the latter involved also in cell shape change…. makes these structures very very important. This gallery (making the gallery itself) caused me great grief… ha ha.. my little desk top computer just didn’t want to handle composite images of over a gig. Would that I had real equipment… wonder of wonders. Enjoy the image(s) of whatever these DES-MiTes (desmosomal-mitochondrial-tethers) will ultimately be called (dumber names exist in science).
34 pix, 43 desmosomes, at least 56 mitochondria, numerous species represented here (ferret, rat, guinea pig, stub tail monkey, rhesus monkey, maybe others, and all images are from hepatocyte-hepatocyte junctions)
I havn’t read much (or found much to read) about the making – breaking – moving – adjusting of desmosomes, or the energy requirement for desmosome modification, but it seems too much a coincidence that mitochondria would be closely tethered to desmosomes and not provide this function. That said, they have been photographed by me in mouse, rat, neonatal rat, ferret, hamster, syrian hamster, dog, and guinea pig, just to name a few in the list that I have looked at in the last weeks. And they are so similar in all, that so far, if i list one image of a desmosomal-mitochondrial junction from one species I cannot tell it from another. This is good, i guess, but it would be more interesting if the structure showed some variation in structure….. that could perhaps be correlated with differences in the component proteins.
Here is one desmosomal-mitochondrial junction from ferret, which didn’t have a nice even pattern of cadherins in the middle, but did have a punctuated central line…with dots less than 20nm apart…. they seemed a little staggered. Here is the original micrograph (i removed a scratch in the negative not in the area of the desmosome using photoshop bandaid tool, but it otherwise just a scan of the print. 5940 18648 ferret hepatocyte.
The image below it is an enlargement of the box in top image. Ribosome-red dot for size, and the black lines overlie densities in the intercellular area of the desmosome.