Desmosomal junction

The desmosomal junction is a morphological entity that is very distinct from adherence junctions involving classical cadherins [44]. The plasma membrane domains of apposing cells are separated by a 20-30 nm thick layer, which in cross section in electron micrographs reveals a midline structure and electron-dense threads stretching laterally from the midline back to the plasma membrane. The cytoplasmic face of each membrane domain is covered with an electron-dense plaque to which bundles of intermediate filaments attach, rather than the actin filaments present in adherence junctions containing classical cadherins. In most epithelia the intermediate filaments are based on keratins but rarer examples of desmosomal junctions coupled into intermediate filaments of the vimentin or desmin type are known [5]. 

Classical and desmosomal cadherins are constituents of different types of intercellular junctions. E-cadherin, the classical cadherin of epithelial cells, is part of the adherens junction (zonula adherens), which is attached to a belt of actin via the catenins. As the name says, desmosomal cadherins are part of the desmosomes, which are rivet-like structures that make focal connections between cells. Desmosomes are characterized by a... 

Figure 6 An electron micrograph of intercellular junctions between two human colon Caco-2 cells in culture. D, desmosome LS, lateral space mv, microvilli ZA, zonula adherens ZO, zonula occludens (i.e., tight junction). Bar equals 200 nm.

Skeletal and cardiac muscles also have important differences. Skeletal muscle cells are elongated and run the length of the entire muscle furthermore, these cells have no electrical communication between them. Cardiac muscle cells, on the other hand, branch and interconnect with each other. Intercellular junctions found where adjoining cells meet end-to-end are referred to as intercalated discs. Two types of cell-to-cell junctions exist within these discs. Desmosomes hold the muscle cells together and provide the structural support needed when the heart beats and exerts a mechanical... 

FIGURE 1-11 An electro tonic synapse is seen at the surface of a motor neuron from the spinal cord of a toadfish. Between the neuronal soma (left) and the axonal termination (right), a gap junction flanked by desmosomes (arrows) is visible. (Photograph courtesy of Drs G. D. Pappas and J. S. Keeter.) X80,000. 

FIGURE 1-18 A typical desmosome (d) and gap junction (g) between two ependymal cells. Microvilli and coated pits (arrows) are seen along the cell surface. X35,000. 

The biochemical properties of these structures are known. Desmosomes display protease sensitivity, divalent cation dependency and osmotic insensitivity and their membranes are mainly of the smooth type. In direct contrast to desmosomes, the tight junctions as well as gap junctions and synapses display no protease sensitivity, divalent cation dependency or osmotic sensitivity, while their membranes are complex. These facts have been used in the development of techniques to isolate purified preparations of junctional complexes. 

A recent crystal structure based model [20] for the structure of C-cadherin postulates that the five extracellular domains EC1-EC5 protrude from the cell surface as a curved rod. The structural analysis of C-cadherin reveals that the molecules facing each other across apposed cell surfaces are antiparallel to one another, forming a dimeric interaction termed a strand dimer (Fig. 7-5). This forms the functional unit that is likely to mediate adhesion between cell surfaces. The structure from this recent paper allows the prediction of both cis and trans interfaces that together result in a lattice and not, as previously believed, an adhesion zipper. This new model allows for a mechanism by which adhesion plates or puncta might be generated, such as are formed at CNS synapses [21, 22], adherens junctions and desmosomes [23], all cadherin based organelles. 

The columnar, cuboidal basal cells are 18-20 xm in height and 8-10 iim in diameter. They form a monolayer adhered to a basement membrane by hemidesmosomes which are linked to anchoring fibrils of collagen type VII. At the lateral borders, basal cells are extensively interdigitated and joined together in places only by junctional complexes (zonula adherens), desmosomes, and gap junctions. 

Fig. 9. Distribution of the gap junctions, desmosomes and fascia adherens in an intercalated disk of a cardiomyocyte as assessed by electron microscopy of freeze-fractured rat and rabbit hearts according to Severs [1990]. 

From figure 9 it becomes clear that the fascia adherens is located transverse to the fiber axis on the cell processes and at the side walls of these processes gap junctions are located in clusters and desmosomes. According to Hoyt et al. [1989] the gap junctions are arranged in a more or less ribbon-like fashion... 

The surface of the intercalated disk is occupied to 5.7 + 0.6% by gap junctions (canine atrium) [Spira, 1971], 3.3% (right bundle, calf) [Arluk and Rhodin, 1974], or even 12.7-15.1% in canine left ventricular subepicardial myocardium [Hoyt et al., 1989]. The rest of the intercalated disk is made of fascia adherens and desmosomes. In the crista terminalis of the canine heart the gap junction profile length has been estimated to be in the order of 3.2-3.8 pm/100 pm intercalated disk length with 11-12 gap junctions/100 pm intercalated disk length and a mean gap junction profile length of about 0.3 pm [Saffitz et al., 1994]. 

These are cells of transition between the basal and superficial cells. They constitute the thickest and biggest layer. They are polygonal cells, with a convex front side and a concave back side. They are arranged on two or three layers at the center and five to six layers on the edge. Their nucleus is active and stretched out along the big axis of the cell. Their cytoplasm contains a very developed Golgi s apparatus as well as tonofilaments (microtubules and keratin filaments) connected to the desmosomes. Their cytoplasmic membranes are only united desmosomes and gap junctions that enable both the unity of intermediate cells and the union of intermediate and basal cells (Figs. 4.3. 5). 

L, lysosome D, desmosome TJ, tight junction Mv, microvilli C, cillium... 

Cadherins are calcium-dependent and establish molecular links between adjacent cells. They include neural cadherins (NC), epithelial cadherins (EC), placental cadherins (PC), protocadherins and desmosomal cadherins. Cadherins mediate homophilic interactions and form zipper-like structures at adherens junctions and... 

---