Different types of intercellular junctions, including plasmodesmata, tight junctions, gap junctions, and desmosomes.
If you were building a building, what kinds of connections might you want to put between the rooms? In some cases, you’d want people to be able to walk from one room to another, in which case you’d put in a door. In other cases, you’d want to hold two adjacent walls firmly together, in which case you might put in some strong bolts. And in still other cases, you might need to ensure that the walls were sealed very tightly together – for instance, to prevent water from dripping between them.
As it turns out, cells face the same questions when they’re arranged in a tissue next to other cells. Should they put in doors that connect them directly to their neighbors? Do they need to spot-weld themselves to their neighbors to make a strong layer, or perhaps even form tight seals to prevent water from passing through the tissue? Junctions serving all of these functions can be found in cells of different types, and here, we’ll look at each of them in turn.
Plant cells, surrounded as they are by cell walls, don’t contact one another through wide stretches of plasma membrane the way animal cells can. However, they do have specialized junctions called plasmodesmata (singular, plasmodesma), places where a hole is punched in the cell wall to allow direct cytoplasmic exchange between two cells.
Image of two cells connected by a plasmodesma, showing how materials can travel from the cytoplasm of one cell to the next via the plasmodesma.
Plasmodesmata are lined with plasma membrane that is continuous with the membranes of the two cells. Each plasmodesma has a thread of cytoplasm extending through it, containing an even thinner thread of endoplasmic reticulum (not shown in the diagram above).
Molecules below a certain size (the size exclusion limit) move freely through the plasmodesmal channel by passive diffusion. The size exclusion limit varies among plants, and even among cell types within a plant. Plasmodesmata may selectively dilate (expand) to allow the passage of certain large molecules, such as proteins, although this process is poorly understood.
Functionally, gap junctions in animal cells are a lot like plasmodesmata in plant cells: they are channels between neighboring cells that allow for the transport of ions, water, and other substances. Structurally, however, gap junctions and plasmodesmata are quite different.
In vertebrates, gap junctions develop when a set of six membrane proteins called connexins form an elongated, donut-like structure called a connexon. When the pores, or “doughnut holes,” of connexons in adjacent animal cells align, a channel forms between the cells. (Invertebrates also form gap junctions in a similar way, but use a different set of proteins called innexins.)
Image of the plasma membranes of two cells held together by gap junctions. Where two connexons from the different cells meet, they can form a channel leading from one cell into the next.
Gap junctions are particularly important in cardiac muscle: the electrical signal to contract spreads rapidly between heart muscle cells as ions pass through gap junctions, allowing the cells to contract in tandem.
Not all junctions between cells produce cytoplasmic connections. Instead, tight junctions create a watertight seal between two adjacent animal cells.
At the site of a tight junction, cells are held tightly against each other by many individual groups of tight junction proteins called claudins, each of which interacts with a partner group on the opposite cell membrane. The groups are arranged into strands that form a branching network, with larger numbers of strands making for a tighter seal.
Image of the membranes of two cells held together by tight junctions. The tight junctions are like rivets, and they are arranged in multiple strands that form lines and triangles.
The purpose of tight junctions is to keep liquid from escaping between cells, allowing a layer of cells (for instance, those lining an organ) to act as an impermeable barrier. For example, the tight junctions between the epithelial cells lining your bladder prevent urine from leaking out into the extracellular space.
Animal cells may also contain junctions called desmosomes, which act like spot welds between adjacent epithelial cells. A desmosome involves a complex of proteins. Some of these proteins extend across the membrane, while others anchor the junction within the cell.
Cadherins, specialized adhesion proteins, are found on the membranes of both cells and interact in the space between them, holding the membranes together. Inside the cell, the cadherins attach to a structure called the cytoplasmic plaque (red in the image at right), which connects to the intermediate filaments and helps anchor the junction.
Desmosomes pin adjacent cells together, ensuring that cells in organs and tissues that stretch, such as skin and cardiac muscle, remain connected in an unbroken sheet.
Want to join the conversation?
- What sorts of cells does the blood/brain barrier consist of? I believe chemotherapy molecules are often too big to get through to treat the brain.(17 votes)
- Each nerve cell in the brain must be fed by blood or the cells will quickly die of oxygen deprivation. I believe you are perhaps reffering to a cappilary system that sustain these cells in a way that compartmentalise them as much as possible. It is also possible because of the late hour that my mind makes me write b.s. and I'm sorry if the answer is misleading or not helping...(5 votes)
- If the tight junction proteins create a watertight seal is there any liquid in the extracellular space between the proteins? If there isn't, does this have an effect on the plasma membranes of the cells (i.e. does it affect the hydrophilic heads)?(10 votes)
- Do desmosomes and cadherins depend on one another?(9 votes)
- Yes, the cadherins are a big part of the desmosomes(2 votes)
- Could someone thoroughly explain to me what desmosomes are?(9 votes)
- A desmosome, also known as a macula adhaerens, is type of cell-to-cell adhesion.
Desmosomes resemble plasmodesmata in plant cells because they also provide little space through which membranes of to adjacent epithelial cells are connected. Hiwever, in the case of desmosomes, intermediate filaments of cytoskeleton are connected. How?
First, desmosomes, have its connecting proteins (just liek the other types of cell-to-cell connections in animal cells). Their proteins are called cadherins.
Cadherins are attached to both cells and they protrude the cell enterior and anchor on intermediate filaments (cytoskeleton). Precisely, they attach to cytoplasmic plaque, which connects to intermediate filaments.
Is it clearer now? :)(2 votes)
- How do tight junctions differ from desmosomes?(4 votes)
- What are the functions of Cadherin ? Can you help me to answer this question by writing in points, not descriptive pattern? Thanks!(2 votes)
- cadherins are basically proteins attatched to the cytoplasmic plaque of animal cells. They act as a sort of tie that prevents cells from seperating under pressure.
This is probably a really dark way of putting things but think of it as two scenarios like this:
1) you bind two people together by their wrists or bodies with a rope.
2) you bind two people by skeletons.
its harder to seperate the two bound by their skeletons no matter how far away they stretch, ryt?. its something like that. it connects two cells, say cardiac cells, by their adjacent cytoplasmic plaques which are in turn connected to the cells' cytoskeleton, which holds them together as they stretch.
Hope you got an idea:)(7 votes)
- Can bacteria form junctions with each other in order to exchange genetic material?(3 votes)
- Bacteria can form junctions with each other to exchange genetic material. This is known as conjugation. Genetic material is exchanged between bacteria through conjugation tubes or sex pilus. One bacterium usually donates its DNA while the other receives it.
For more information check out this link:
- What is cytoplasmic plaque and what are intermediate filaments and how exactly do they work with the cadherins to anchor the junction between epithelial cells?(2 votes)
- Cytoplasmic protein plaque is protrusion of cytoplasma which from luminal side binds intermediate filaments (ctoskeleton) and from the other sides binds cadherin. Cadherin binds to another cadherin which holds cytoplasmic plaque and binds to another intermediate filament in lumen of second cell adhering to the first cell.
Cytoplasmic plaque plays scaffolding role.
- Is the "junction" between the nuclear membrane and the E.R. considered a junction?
If yes, what kind of junction is it?
If no, it is not a junction, what is it?
Many thanks!(2 votes)
- I don't think there is any junction since endoplasmic reticulum membrane is continuous with the external nuclear envelope. When something is continuous means it is attached directly, we are speaking of one bigger entity.(2 votes)
- can someone explain plasmodesmata and pits to me
- co-axial membranous channels that cross walls of adjacent plant cells, linking the cytoplasm, plasma membranes and endoplasmic reticulum (ER) of cells and allowing direct cytoplasmic cell-to-cell communication of both small molecules and macromolecules.
Pits are relatively thinner portions of the cell wall that adjacent cells can communicate or exchange fluid through.
hope this helps :](1 vote)