Desmosomes and Hemi-desmosomes
In anminal, the cell membrane cannot effectively transmit forces from one cell to another or to the extracellular matrix, making it unable to withstand strong forces. Therefore, neighboring cells form a rivet-like structure to hold them together tightly. These structures are called desmosomes, also known as macula adherens.
The most prominent feature of desmosomes is the dense disc-shaped protein plaques on the cell membrane. Outside the cell, some filamentous cadherins extend outward from these plaques, which form covalent cross-links with the cadherins of another cell. The plaques are linked to the cytoskeleton inside the cell.
Desmosomes are intercellular junctions found in animal cells, particularly in tissues subjected to mechanical stress, such as skin and muscle. They serve to provide strong adhesion between adjacent cells and contribute to the overall structural integrity of tissues.
Hemi-desmosomes have a structure similar to desmosomes. One side of the protein plaque is connected to the cytoskeleton, while the other side is linked to glycoproteins in the extracellular matrix. They firmly anchor the cell in the extracellular matrix. Hemi-desmosomes are also involved in signaling pathways, such as keratinocyte migration or cancer cell invasion.
Tight junction or occluding junction
The transmembrane proteins embedded in plasma membrane are arranged in a linear strand, and these strands intertwine into a network structure. They tightly bind to transmembrane proteins of adjacent cells and are also connected to the cytoskeleton. These structures surround the cells and allow them to be closely packed together without any gaps between neighboring cells.
The primary function of tight junctions is to establish a barrier between adjacent cells, sealing the intercellular space and preventing the leakage of water and ions between cells.
Gap Junctions
Gap junctions are widely present between cells and are also known as communicating junctions. There is a narrow gap of approximately 2-4 nanometers between two cells. Six transmembrane proteins form a 1.5 nm diameter channel that spans the cell membrane and these channels are connected together between adjacent cells. Thousands of such channels aggregate in a area to form a gap junction plaque and only allows small molecules with a molecular weight below 1000Kda, such as sucrose, water, ions, ATP, etc., to pass through. Signaling molecules like cAMP and calcium ions can also rapidly diffuse through these channels to facilitate intercellular communication. Gap junctions play a significant role in molecular signal transmission and coordination of cellular activities.
Plasmodesmata: channels of cell wall
Plasmodesmata are microscopic channels that connect plant cells to facilitate communication and transport of various molecules between neighboring cells. They are unique to plant cells and play a vital role in cell-to-cell communication, nutrient exchange, and signal transduction.