I found this paper on hydra desmosomes (if they can be called that, maybe better called intercellular junctions of the 3rd kind) just to compare with mammalian desmosomes. I pasted up a graphic from the diagram in this paper by Wood, in 1959, that shows the structure next to a mock up of the plasmalemma (about 4 nm thickness) and then estimated the thickness of the juxtaposed cell’s CADHERINS  ha ha… named such for lack of knowledge about what hydra intercellular adhesion molecules are — at this point –but i am looking). Interesting structure i think.  what is interesting to me is how closely the intercellular space (here about 20-30nm) matches the intercellular space of mammalian desmosomes. Another similarity of the hydra desmosomal structure is that the densities between cells are perpendicular to the direction of the plasmalemma, AND each layer of the trilaminar membrane of the plasmalemma is of a different density, this was called a plaque.

The distance between the “cadherins” is about 25nm, the intercellular distance in a non-desmosomal area of two cell connection he reports is about 20-30nm. This would be a difference with mammalian desmosomes since the intercellular space in mammalian desmosomes appears to be just a little less wide than when there is no junctional complexing going on.

I like the justification for many and for increased length of desmosomes, as this is a multicellular organism in which the need for protection from the extracellular milieu is critical.

Wood seems to suggest (which is in sync with mammalian desmosomes) that the “lamellae” as he calls them that are perpendicular to the 2 plasmalemmae of adjacent cells) are penetrated by elements of those lamella (similar to the way a transmembrane portion of a protein might behave).

Micrographs at bottom are interesting, and show the layered grid of these septate desmosomes, and while not mentioned by this author, a very close mitochondrion is present (though not touching) the desmosome in the very bottom micrograph.