Category Archives: Electron micrographs of liver

Desmosome-to-mitochondria: connections

More desmosomal mitochondria tethering from hamster hepatocytes. This particular view has two mitochondria, and one shows a line of “dots” or fibers or fibrils in cross section.

This image is from a syrian hamster hepatocyte, and it is just a routine TEM but I noticed (while making my gallery collection of desmosomal-mitochondrial images) that one of the mitochondria showed the repeating dots (cross sections likely of intermediate filaments (maybe types I and II).

Upper image is unretouched micrograph and lower image has been pseudocolored. The latter has green for the two mitochondria that are tethered to the desmosome  at their outer mitochondria membrane (which appears rigid where intermediate filaments are present, and a distinct and definitely separated inner mitochondrial membrane). Along with the intermediate filaments are connections to the desmoplakin molecules the latter being part of the outer dense plaque of the desmosome. The yellow is the cytoplasmic area of the two adjacent cells including the intercellular space and the spot desmosome. Red dot is appoximately the size of a ribosome (27nm) which makes these filaments cut in cross section about 13.5nm in diameter (orange dots in a row) bottom electron micrograph. This hastily made measurement is not that far off from the reported 10nm intermediate filament made up of four monomer filament tetramers (haha… figure that out).

According to Thomason et al,  desomosomal proteins are as follows (slightly rearranged to make more sense to me): Desmosomes have five major component proteins, the DCs (desmosomal cadherins) DSG (desmoglein) and DSC (desmocollin) –DSC and DSG are the desmosomal adhesion molecules. Plakin family cytolinker DP (desmoplakin), and the arm (armadillo) proteins PG (plakoglobin) and PKP (plakophilin).  DP links the desmosomal plaque to the IF (intermediate filament) cytoskeleton, and PG and PKP are adaptor proteins that link between the adhesion molecules and DP.

More on desmosome shape

“Desmosomes are a complex assembly of protein molecules that form at the cell surface and mediate cell–cell adhesion. Much is known about the composition of desmosomes and there is an established consensus for the location of and interactions between constituent proteins within the assembly. Furthermore, X-ray crystallography has determined atomic structures of isolated domains from several constituent proteins. Nevertheless, there is a lack of understanding about the architecture of the intact assembly and the physical principles behind the adhesive strength of desmosomes therefore remain vague. We have used electron tomography to address this problem. In previous work, we investigated the in situ structure of desmosomes from newborn mouse skin preserved by freeze-substitution and imaged in resin-embedded thin sections. In our present work, we have isolated desmosomes from cow snout and imaged them in the frozen unstained state. Although not definitive, the resulting images provide support for the irregular groupings of cadherin molecules seen previously in mouse skin.” ABSTRACT from Gethin Rh. Owen, Devrim Acehan, K.D. Derr, William J. Rice, David L. Stokes . Some things might be right, others not…. the last idea that cadherin molecules are grouped irregularly i don’t like. (This article i wont read as it is pay-per-view, which really galls me particularly because most research is paid for by tax-dollars and should be free).

” I found a dimension for the width “wide intercellular space (~240 A)” at 24nm and a measurement of the intermediate filament at 8nm (i presume diameter). The 22-24nm (could be less in some instances more like 15nm) as the width of the desmosomal intercellular space (which includes the densities on the outer leaflet of the plasmalemma from both cells and the central dense line of the desmosome.  That measurement looks about right. From this I estimate the width of the electron lucent annulus of the desmosome (which is much more evident when there is a tight junction on either side of the desmosome) to be dependent upon where the section sectioned the spot desmosome, and whether the cut was equatorial or tangent to the spot itself. On one micrograph (stub tail monkey hepatocyte) in which one area of the desmosome is close to a tight junction, shows this lucent area (the outer annulus) to be about 35nm in its radius – that dimension applies to the edge of the central dense line extending to the tight junction. The width of that particular desmosome (where the central line was apparent) was @265nm.  This makes the ratio of the dense desmosomal central line density distance to lucent annulus distance in cross section about 3.5%.

That lucent area may not be a uniform annulus, but if it were even, this assumes pretty much a “round” desmosomal spot weld. In measuring the radius of the annulus (just in the part ring part) I understand that the central dense line of the desmosome will appear in any view with the spot perpendicular to the plane of section, and this doesn’t diminish the fact that closer cuts to the outer edge of the desmosome will have a greater annulus to spot ratio. Fuzzing of the central dense line in the desmosome occurs when the angle of the section is not perpendicular to the spot, and can happen either at an equatorial section or anywere past that equator. So. any question I can ask, someone has an answer to…. so here is the definition of calculating the area of an annulus.. ha ha “An annulus (Latin word for ring) is a two-dimensional region of space bounded by two concentric circles. The area of an annulus is the difference in the areas of the larger circle of radius R, and the smaller circle of radius r. Mean radius (ρ) is the average of the exterior (R) and interior (r) radii. Breadth (δ) is the width of the annulus. “ So I just have to figure out how to use the online calculator.

The ratio of lucent annulus to dense desmosome is not too apparent when there is no tight junction visible right next to it. But that said, I can see a slight increase in plasmalemmal density for a short distance (same type of annulus arrangement as if there were a tight junction there) on each end of the desomosome cross section.

Begin here: “empty annulus” outer ring around the desmosomal protein core”

How funny this is, the perimeter of a desmosome with this very precise and quite abrupt “beginning”. It seems that adjacent cells (with a concentric area of pretty well adhered plasmalemmae (two adjacent cells involved)) make the abrupt 45 degree bend just to (coincident with?)  the widened intercellular space at the core of the desmosome to its typical width of about 25nm (up from the space between two cells at its closest… more around 17nm).

What is most interesting is what signals the 45 degree spread (bend) on the plasmalemma just before the desmosomal disk, and why is it so electron lucent? or free or proteins?.  This is a “free” zone before the electron dense desmosomal complex proteins that make their junctional adherent  weld of about 150nm. the width of the annulus. The area of the annulus would vary depending upon where in the spot desmosome the section happened to fall, and a cut in the annulus completely would just look like a wider two-cell junction, showing no desmosome at all.  Whether the width of the annulus around the desmosome is consistent from one species, or even one cell type to another is likely not known.  I will google to see.

Stub tail monkey hepatocyte desomosomal-mitochondrial tether: more

There are several nice things about this micrograph of a desmosomal mitochondrial tether (i.e. tethered via the intermediate filaments of the desmosome). One nice thing is the very rigid, and parallel membrane-leaflet quality of the plasmalemma where there is the desmosomal protein complex, and it also exhibits a very even and well defined separation of the outer and inner leafelets of the plasmalemma (at least this is what i think is shown in the micrograph below).  I have marked the two leaflets on the cell membranes on the adjacent cells with black lines. There are obvious periodicities within the inner and outer membrane leaflets too… not withstanding the density of the lead citrate and uranyl acetate stains and the quality of fixation) which seems to present overall patterning in the entire micrograph. I have marked the extracellular lines (which are not likely to be random as they appear, but more likely to be organized attached wishbone shapes (mirrored vertically and copied horizontally) shown in an earlier post but the lines here have been drown over actual densities in the extracellular (intercellular) space of the desmosome. Green blobs mark periodicities within the two leaflets of the plasmalemma, and the pink blobs mark the densities which are extracellular but likely adjacent, maybe even part of (like the cadherins in desmosomes) those proteins. (green blobs are INTERcellular, the pink blobs are within the plasmalemma (transmembrane). Electron micrograph on left is unretouched, on right, contrast and brightness enhanced and vector overlays show densities which can be matched to image on the left. This stub-tail monkey hepatocyte came from an animal that received two test doses of perfluorodecalin (a very small amount) a year prior, which in my experience has nothing to do with these tethers. Shown immediately below the outer leaflet of the plasmalemma is what appears to be a regular compartmentalization (shown as rectangles) likely representing a highly structure arrangement of plakophilin and plakglobin and maybe some desmoplakin and a bit of vimentin.  The number of densities (green or pink, and likely connected parts of the cadherin dimers, turn out in this micrograph to be about 9 nm…  a little close than found in mouse micrographs so far. 

Stub tail monkey hepatocyte desomosomal-mitochondrial tether

These tetherings between desmosomes and mitochondria are pretty plentiful. Here is another example of one such from a hepatocyte from a stub tail monkey. This one shows some very distinct banding…. a little more demarcated than liver, I wonder if this is just opportune sectioning or some molecular difference in the components.  Enlargement of the desmosome below top image.

I looks in this image like the plasmalemma of each cell has a very rigid separation of the two leafelets.

Desmosomes and mitochondria tethered together

Looking still for some molecular models which fit the images I see for desmosomal mitochondrial junctions (adjacent structures tethered by intermediate filaments). These junctions are so prevalent that I cannot understand why they are not part of the everyday histology lessons. This particular mitochondrion and desmosome are from a mouse liver (control for other studies) . The sectioning and orientation are no too bad though it would be nice to have even more detail (haha as much detail as one might get from electron tomography, which isnt going to happen for me so i will do the best with what i have).

The molecular models of desmosomal proteins, which are pretty widely published, may have relevance to the structures here. The first think I am pretty convinced about is the stretchy hair-pin zigzag like connection of the cadherin molecules within the center of the desomosomal structure, and the little periodicity that is apparent (10-14nm spacing gets calculated from this image).  I found a nm marker only on the molecular model for intermediate filaments which was 5nm (a very helpful marker indeed) and it comes close to confirming what I have as the magnification on the micrographs as judged by a 27nm ribosome standard. The proteins overlying the TEM here are just forced to size, as there were no nm markers on those models…  and the position of the one large molecule (desmoplakin) is off to the side waiting to be figured out).  There are big gaps between what I see with TEM and what the molecules say…. it is fun to try to figure it out.

Upper micrograph, unretouched, also with rectangle for enlarged and pseudocolored image below.  The bottom image has the intercellular space colored green, and you can see the cadherin springy wishbones above it, and also the plasmalemma and likely other proteins (plakophilin and plakoglobin) sort of carelessly put in place, and the desmocolin off to the side in an “unknown” placement. Red dots are ribosomes (for size reference).

Stub tail monkey liver: desmosomal mitochondrial tethering

This is just a micrograph to justify the idea that the desmosomal mitochondrial tethering, or junction or des-mites as i have aptly named them (as opposed to pore-mites) for nuclear pore mitochondrial tethering, are pretty universal. Here a single tether is in a stub tail monkey hepatocyte, from tissue taken back in the 1970s while studying the effects of infusing artificial blood emulsions (perfluorochemical based blood substitutes) . This particular monkey (Maccaca speciosa, probably female) did receive two test doses of a perfuorodecaline emulsion, just a tiny amount, 0.05cc/kg of a 10% emulsion PP5ct and 5% F68) (Lee Clark Jr named all his emulsions, this was EM#750428). no perfluorochemical droplets were seen in this hepatocyte. Sac date was 5 10 76.

So desmosomal mitochondrial junctions are here. Mitochondrion is in top part of micrograph, desmosome is attached to the down pointing portion.


More pseudocolored desmosomes and mitochondria – tethered

Here is another pseudocolored desmosome with portions of two mitochondria top and bottom parts of this electron micrograph. It was difficult to determine exactly where the plasmalemma from each of these two cells went at the point of the desmosome, but I thought long and hard about which part of the structure they were. It seemed to me that there was a slight electron lucency just under the plasmalemma on each side, therefore this is the way I pseudocolored (with pink) the cell membranes.  The densities within the desmosome itself looks like there are three rows… the central dotted (periodic) line where the cadherin molecules knot together (my guess is this is a totally symmetric arrangement, not at all random like suggested by some) and perhaps another periodicity (well not perhaps…. it is pretty striking) which lies a little separated from the plasmalemma.  I don’t know if any of the models of the cadherins show a “lump” structure before the transmembrane part… ? That will take some searching.

Top image is unretouched transmission electron micrograph of a desmosome, as mentioned, mitochondria portion seen top and bottom.  The box in this image is what is enlarged in the second image. There are two very prominent intramitochondrial granules, especially the one in the mitochondria at the bottom of the micrograph.
This image is from the boxed area above, thus enlarged, pink is the plasmalemma, orange is the area just under the plasmalemma of each cell and into the space of the desmosome. Blue is what I see as the possible densities of cadherin molecules.

In this inset the periodicity of the outer part of the desmosome (probably still cadherin molecules) is a spacing about 1/15nm  not too different from that found in a previous post at something around 1 density for each 13-14nm spacing.  The periodicities on the central dense line of the desmosome in this micrograph might be something around 18nm spacing… I would have preferred if the densities came out one-to-one, but anticipate that in other assessments that it might do just that. But for now, i just count what shows up. 6102_5070_mouse_female_control.